Chapter 1: Movement and Position

This chapter introduces the language of motion: speed, velocity and acceleration, the distance–time and velocity–time graphs, and the equations of uniformly accelerated motion. You must be able to calculate values, read gradients and areas from graphs, and rearrange equations.

---

SPEED

• Speed — how fast an object is moving: the distance travelled per unit time.
• Average speed = total distance ÷ total time speed (m/s) = distance (m) ÷ time (s)
• UNITS OF SPEED: metres per second (m/s), also km/h, mph. To convert km/h → m/s, divide by 3.6 (because 1 km = 1000 m, 1 h = 3600 s).
• REARRANGING THE SPEED EQUATION (triangle): distance = speed × time; time = distance ÷ speed.
• Example: a runner covers 50 m in 3.9 s → speed = 50 ÷ 3.9 = 12.8 m/s.

THE DIFFERENCE BETWEEN SPEED AND VELOCITY

• Speed — distance per unit time (no direction).
• Velocity — speed in a particular direction (a vector: magnitude AND direction).
• Example: a car going around a roundabout at a constant speed of 20 m/s has constant speed but changing velocity (its direction changes).

ACCELERATION

• Acceleration — the rate of change of velocity. acceleration (m/s²) = change in velocity (m/s) ÷ time taken (s) a = (v − u) ÷ t where u = initial velocity, v = final velocity.
• UNITS OF ACCELERATION: metres per second squared (m/s²).
• Example 2 (book): a car travels at 20 m/s, accelerates steadily for 5 s to 30 m/s:
• a = (30 − 20) ÷ 5 = 10 ÷ 5 = 2 m/s².
• Example 3 (book): an object hits the ground at 40 m/s and is brought to rest in 0.02 s:
• a = (0 − 40) ÷ 0.02 = −40 ÷ 0.02 = −2000 m/s² → we say it is decelerating at 2000 m/s² (negative acceleration).

MEASURING ACCELERATION

• Activity 1 — TENNIS BALLS: measure the time for a ball to roll known distances down a ramp (ticker timer or light gates) → calculate velocities → acceleration.
• A MODERN VERSION OF GALILEO'S experiment: light gates + data loggers measure times accurately; acceleration = change in velocity ÷ time.

DISTANCE–TIME GRAPHS

• Axes: distance (y) vs time (x).
• Gradient (slope) = speed. A steeper line = faster.
• Horizontal line = stationary (not moving).
• Straight sloping line = constant speed.
• Curved line = changing speed (accelerating/decelerating).
• Example: gradient = distance ÷ time = 30 m ÷ 2.5 s = 12 m/s.

VELOCITY–TIME GRAPHS

• Axes: velocity (y) vs time (x).
• Gradient = acceleration.
• Horizontal line = constant velocity (no acceleration).
• Positive gradient = accelerating; negative gradient = decelerating.
• AREA UNDER A VELOCITY–TIME GRAPH = DISTANCE TRAVELLED.
• Example: constant velocity 5 m/s for 10 s → distance = area = 5 × 10 = 50 m.
• For a triangle (accelerating from 0 to 10 m/s): area = ½ × base × height.

EQUATIONS OF UNIFORMLY ACCELERATED MOTION (PHYSICS ONLY)

• The four equations (for constant acceleration): v = u + at s = ut + ½at² v² = u² + 2as s = ½(u + v)t where s = distance, u = initial velocity, v = final velocity, a = acceleration, t = time.
• Example 4: object accelerates from 0 to 10 m/s → average velocity = (0 + 10) ÷ 2 = 5 m/s; distance in 10 s = 5 × 10 = 50 m (matches the area method).
• Example 5: use v² = u² + 2as to find final speed from a known distance and acceleration.
• 💡 Exam tip: always write down u, v, a, s, t first, pick the equation containing your unknown, then substitute — and give units.

LOOKING AHEAD

• These ideas feed into Forces and Movement (Chapter 3): a resultant force causes acceleration (F = ma).

✅ Quick check

1. Write the equation for average speed. (speed = distance ÷ time.)
2. What is the difference between speed and velocity? (Velocity is speed in a given direction.)
3. A car goes from 0 to 30 m/s in 6 s. What is its acceleration? (a = 30 ÷ 6 = 5 m/s².)
4. What does the gradient of a distance–time graph show? (Speed.)
5. What does the area under a velocity–time graph show? (Distance travelled.)

Chapter 2: Forces and Shape

A force is a push or pull. Forces can change the motion of an object or change its shape. This chapter covers the different types of forces, balanced and unbalanced forces, friction, and elastic deformation (stretching, compressing, bending) — including Hooke's law.

---

ALL SORTS OF FORCES

• Force — a push, pull or twist acting on an object.
• Examples: weight/gravity, friction, air resistance/drag, upthrust, thrust, tension, compression, electrostatic force, magnetic force, normal reaction (support) force.
• UNITS OF FORCE: newtons (N).
• Forces are vectors (they have size AND direction).

WEIGHT AND MASS

• Mass — the amount of matter in an object (measured in kg; stays the same anywhere).
• Weight — the force of gravity on an object (measured in N; changes with gravity). weight (N) = mass (kg) × gravitational field strength (N/kg) W = m × g (on Earth, g ≈ 10 N/kg or 9.8 N/kg)
• Example: a 5 kg object on Earth has weight = 5 × 10 = 50 N. On the Moon (g ≈ 1.6 N/kg) it would weigh only 8 N — but its mass is still 5 kg.

MORE THAN ONE FORCE

• Forces on an object can be shown with force arrows (length = size, direction = direction of the force).
• Resultant force — the single force that has the same effect as all the forces acting together.
• To find it: add forces in the same direction, subtract forces in opposite directions.

BALANCED AND UNBALANCED FORCES

• Balanced forces (resultant = 0): the object either stays still or continues at constant velocity (Newton's 1st law — no acceleration).
• Unbalanced forces (resultant ≠ 0): the object accelerates (speeds up, slows down, or changes direction) in the direction of the resultant force.
• Examples: a book on a table — weight down, reaction force up, balanced. A car speeding up — driving force > friction.

INVESTIGATING FRICTION

• Friction — the force that opposes motion between two surfaces in contact.
• It acts in the opposite direction to motion, converting kinetic energy to heat (e.g. rubbing hands).
• Useful friction: brakes, tyres gripping the road, walking.
• Wasted friction: moving parts in machines (reduced by lubrication).
• 🔬 Practical: pull a block with a newton meter over different surfaces (e.g. wood, sandpaper) → rougher surface = more friction.

CHANGING SHAPES — ELASTIC DEFORMATION

• Forces can change the shape of objects (stretch, compress, bend).
• Elastic deformation — the object returns to its original shape when the force is removed (e.g. a rubber band, spring).
• Inelastic (plastic) deformation — the object stays deformed when the force is removed (e.g. squashing plasticine, bending a paperclip too far).
• Limit of proportionality / elastic limit — the point beyond which the object no longer returns to its original shape (stretched permanently).
• Hooke's law: for a spring, the extension is proportional to the force applied (up to the limit of proportionality): F = k × e where F = force (N), k = spring constant (N/m), e = extension (m).
• Extension = new length − original length.
• 📊 Graph (force vs extension): a straight line through the origin up to the limit of proportionality; beyond it the line curves (less extension per unit force... actually the spring stretches more easily).
• 🔬 Practical: hang masses on a spring, measure extension with a ruler, plot F against e — straight line through origin (Hooke's law), find k from the gradient.

SPRINGS AND WIRES — ELASTIC BEHAVIOUR (with practical)

• Elastic deformation: returns to original shape when the force is removed (springs, rubber bands, wires).
• Inelastic/plastic deformation: stays deformed (plasticine, bending a paperclip too far).
• Limit of proportionality: the point beyond which extension is no longer proportional to force (the spring is permanently stretched).
• 🔬 Practical — investigate how the extension of a spring changes with load: hang a spring from a clamp stand, measure its original length, add masses one at a time (record load in N), measure the new length each time, calculate extension = new length − original length. Plot force vs extension: a straight line through the origin (Hooke's law, gradient = spring constant k) up to the limit of proportionality, then it curves. Precautions: read the ruler at eye level, allow the spring to stop oscillating before measuring, don't exceed the elastic limit.

✅ Quick check

1. What is the unit of force? (Newtons, N.)
2. Distinguish mass and weight. (Mass = amount of matter, kg; weight = force of gravity, N = m × g.)
3. What happens when forces are balanced? (No acceleration — stationary or constant velocity.)
4. What is the resultant force? (The single force equivalent to all forces acting.)
5. State Hooke's law. (Extension is proportional to force, F = ke, up to the limit of proportionality.)

Chapter 3: Forces and Movement

Forces cause acceleration — Newton's second law links force, mass and acceleration (F = ma). This chapter also covers terminal velocity, Newton's first and third laws, stopping distances, and momentum (in the next chapter). You must understand how forces affect falling objects and vehicles.

---

FORCE, MASS AND ACCELERATION — NEWTON'S SECOND LAW

• The resultant (unbalanced) force on an object causes it to accelerate.
• Newton's second law: resultant force (N) = mass (kg) × acceleration (m/s²) F = m × a
• The acceleration is in the same direction as the resultant force.
• Rearrangements: a = F ÷ m; m = F ÷ a.
• Example: a force of 10 N on a 2 kg object → a = 10 ÷ 2 = 5 m/s².
• 💡 Exam tip: this equation is on the formula sheet but you must know what each letter means and how to rearrange it.

FRICTION, DRAG AND AIR RESISTANCE

• When an object moves through a fluid (air or water), it experiences drag (air resistance/water resistance) — a friction-like force opposing motion.
• Drag increases with speed (the faster you go, the more drag).
• Streamlining reduces drag (e.g. racing cars, fish, planes).

FALLING OBJECTS AND TERMINAL VELOCITY

• A falling object experiences two forces: weight (down) and air resistance (up).
• At the start: weight > air resistance → the object accelerates downward.
• As it speeds up: air resistance increases → acceleration gets smaller.
• Terminal velocity: when air resistance = weight, the forces balance → the object stops accelerating and falls at a constant maximum speed (the terminal velocity).
• Examples: skydivers (before opening the parachute); a parachute increases air resistance massively → a new, much lower terminal velocity for landing.
• 📊 Velocity–time graph for a skydiver: curve rising steeply then levelling off (terminal velocity), then a dip when the parachute opens, then levelling at the lower terminal velocity.

NEWTON'S FIRST LAW

• An object stays at rest or in constant velocity motion unless acted on by an unbalanced (resultant) force.
• In other words: no resultant force → no acceleration.

NEWTON'S THIRD LAW

• When object A exerts a force on object B, object B exerts an equal and opposite force on object A. (Forces always come in pairs, acting on different objects.)
• Examples: a rocket pushes gas down → gas pushes the rocket up; your feet push the ground back → the ground pushes you forward; a book pushes down on a table, the table pushes up on the book.

STOPPING DISTANCES AND SAFETY

• Stopping distance = thinking distance + braking distance.
• Thinking distance — the distance travelled while the driver reacts (reaction time ~0.7 s). Increased by: tiredness, alcohol, drugs, distractions (phones), old age.
• Braking distance — the distance travelled while braking. Increased by: higher speed (doubling speed ≈ quadruples braking distance), poor brakes/tyres, wet/icy roads.
• The faster you go, the much longer the stopping distance — hence speed limits near schools.

ROAD SAFETY FEATURES (link to F = ma)

• Seat belts, air bags, crumple zones and cycle helmets all work by increasing the time over which a collision stops you (or the distance over which you decelerate).
• A smaller deceleration → a smaller force (F = ma) → less injury.
• Momentum ideas also apply (see Chapter 4): change in momentum = force × time.

✅ Quick check

1. State Newton's second law. (F = ma — resultant force = mass × acceleration.)
2. What is terminal velocity? (The constant maximum speed when weight = air resistance.)
3. State Newton's first law. (No resultant force → stays at rest or constant velocity.)
4. State Newton's third law. (Equal and opposite forces on each other.)
5. What two distances make up the stopping distance? (Thinking distance + braking distance.)
6. Why do seat belts reduce injury? (They increase the stopping time → smaller deceleration → smaller force.)

Chapter 4: Momentum

Momentum is "mass in motion" — a measure of how hard it is to stop a moving object. This chapter covers calculating momentum, the conservation of momentum in collisions and explosions, and the link between force and change in momentum (including car safety).

---

WHAT IS MOMENTUM?

• Momentum — the product of an object's mass and its velocity. momentum (kg m/s) = mass (kg) × velocity (m/s) p = m × v
• Momentum is a vector (it has direction — the direction of the velocity).
• Examples: a large lorry moving fast has huge momentum (hard to stop); a fly has tiny momentum.

FORCE AND CHANGE IN MOMENTUM

• A force changes an object's momentum: force (N) = change in momentum (kg m/s) ÷ time taken (s) F = (mv − mu) ÷ t
• KEY IDEA: the same change in momentum causes a bigger force if it happens in a shorter time (and a smaller force if the time is increased).
• This is why:
• Car safety features (seat belts, air bags, crumple zones) increase the time of a collision → smaller force → less injury.
• Bending your knees when landing from a jump increases stopping time → smaller force.
• Boxers "ride" a punch (move with it) to increase contact time and reduce the force.
• Catching a cricket ball by moving your hands back with it.

CONSERVATION OF MOMENTUM

• Law of conservation of momentum: in a closed system (no external forces), the total momentum before an event equals the total momentum after the event.
• This applies to collisions and explosions/recoil.
• Collision example (two trolleys): momentum before = momentum after.
• Trolley A (2 kg, 3 m/s) hits stationary trolley B (2 kg) and they stick together:
  • Before: (2 × 3) + (2 × 0) = 6 kg m/s.
  • After: (2 + 2) × v = 4v.
  • 4v = 6 → v = 1.5 m/s together.
• Explosion/recoil example (gun and bullet): before, total momentum = 0; after, the bullet's momentum forward equals the gun's momentum backward (recoil). This is how rockets work (exhaust gas down, rocket up).

MOMENTUM AND IMPULSE (PHYSICS ONLY)

• Impulse = force × time = change in momentum. The area under a force–time graph equals the change in momentum.

MOMENTUM AND COLLISIONS

• In any collision, total momentum before = total momentum after (conservation of momentum).
• Elastic collision: kinetic energy is conserved (e.g. two identical snooker balls).
• Inelastic collision: kinetic energy is lost (as heat/sound/deformation) — e.g. two trolleys that stick together.
• Explosions/recoil: total momentum is zero before and after (bullet forward = gun backward).
• 🔬 Practical — investigating collisions with trolleys: use light gates to measure velocities before/after, calculate momentum each side, verify conservation.

NEWTON'S LAWS OF MOTION (PHYSICS ONLY — but know all three)

• First law: an object stays at rest or moves at constant velocity unless acted on by a resultant force.
• Second law: F = ma (resultant force = mass × acceleration).
• Third law: every action has an equal and opposite reaction (forces act on DIFFERENT objects).
• Momentum and acceleration: F = (mv − mu)/t = rate of change of momentum.

FORCE AND CHANGE IN MOMENTUM (PHYSICS ONLY)

• Impulse = force × time = change in momentum (area under a force–time graph).
• The same momentum change over a longer time = smaller force → safety features (crumple zones, airbags, seat belts) work by extending the collision time.
• F = (mv − mu) ÷ t — used to calculate forces in collisions (e.g. a 0.15 kg ball at 20 m/s stopped in 0.05 s → F = (0 − 3)/0.05 = −60 N).

✅ Quick check

1. Write the momentum equation. (p = m × v, units kg m/s.)
2. A 1000 kg car travels at 20 m/s. What is its momentum? (1000 × 20 = 20 000 kg m/s.)
3. State the law of conservation of momentum. (Total momentum before = total momentum after, in a closed system.)
4. Why do air bags reduce injury? (They increase the stopping time → smaller force for the same change in momentum.)
5. How does a rocket move forward? (Explosion/recoil: gas thrown down, rocket pushed up — momentum conserved.)
6. State Newton's three laws. (1: no resultant force → constant velocity; 2: F = ma; 3: equal and opposite forces on different objects.)

Chapter 5: The Turning Effect of Forces

A force can make an object turn (rotate) as well as move it. The turning effect is called the moment of a force. This chapter covers calculating moments, the principle of moments, levers, and the centre of mass and stability.

---

MOMENTS (THE TURNING EFFECT OF A FORCE)

• Moment of a force — the turning effect of a force about a pivot (fulcrum). moment (N m) = force (N) × perpendicular distance from the pivot (m) M = F × d
• Units: newton metres (N m).
• The moment is bigger when the force is bigger OR when it acts further from the pivot.
• 💡 Exam tip: the distance must be the perpendicular distance from the pivot to the line of action of the force.

THE PRINCIPLE OF MOMENTS

• For an object to be balanced (in equilibrium): total clockwise moments = total anticlockwise moments
• Example: a see-saw — a heavier person must sit closer to the pivot.
• Child A (300 N) sits 2 m from the pivot: moment = 600 N m anticlockwise.
• Child B (400 N) must sit at 600 ÷ 400 = 1.5 m on the other side to balance.
• If moments are unbalanced, the object rotates in the direction of the larger moment.

LEVERS

• A lever is a rigid bar that turns about a pivot, used to multiply force or distance.
• Examples: a crowbar, a spanner, a wheelbarrow, a see-saw, a fishing rod, a door handle.
• A long spanner gives a bigger moment for the same force (d is bigger) — that's why longer spanners are easier to turn bolts with.
• 💡 Exam tip: for lever questions, state the pivot, the effort (force), the load, and use the principle of moments.

CENTRE OF MASS

• Centre of mass (centre of gravity) — the single point where the whole weight of an object seems to act.
• For a symmetrical object (e.g. a ruler), it is at the centre.
• To find it for an irregular shape: hang the shape from a point, hang a plumb line, draw the line; repeat from two other points — the centre of mass is where the lines cross.
• Stability:
• An object is stable if its centre of mass is low and its base is wide (e.g. a racing car, a bus — hard to topple).
• An object topples when the line of action of its weight falls outside its base.
• Lowering the centre of mass and widening the base increase stability.

✅ Quick check

1. Write the moment equation. (Moment = force × perpendicular distance from pivot, in N m.)
2. State the principle of moments. (Clockwise moments = anticlockwise moments for balance.)
3. Why does a long spanner make it easier to turn a bolt? (Bigger distance from pivot → bigger moment for the same force.)
4. What is the centre of mass? (The point where the whole weight appears to act.)
5. How can you make an object more stable? (Lower centre of mass, wider base.)

Chapter 6: Mains Electricity

Mains electricity (230 V AC in the UK) powers homes through live, neutral and earth wires. This chapter covers the three-pin plug, fuses and circuit breakers, the heating effect of current, and the equations for electrical power and energy (E = Pt).

---

MAINS ELECTRICITY

• Mains supply: 230 volts, alternating current (a.c.) — the current constantly reverses direction (50 Hz in the UK).
• Alternating current (a.c.) — current that changes direction repeatedly (mains).
• Direct current (d.c.) — current that flows in one direction only (batteries).

THE THREE WIRES IN A CABLE

Wire	Colour	Function
Live	Brown	Carries the alternating voltage (~230 V); the dangerous one
Neutral	Blue	Completes the circuit (carries current back, ~0 V)
Earth	Green/yellow	Safety wire — carries current away if there is a fault
- The earth wire is connected to the metal casing of appliances: if a fault makes the case live, current flows to earth (instead of through a person), which also blows the fuse.

THE THREE-PIN PLUG

• A plug has: live pin, neutral pin, earth pin (longer for safety), a fuse in the live wire, and a cable grip.
• Fuse — a thin wire that melts (blows) if the current is too high, breaking the circuit (connected in the live wire). A blown fuse stops current flowing → prevents fires and electric shocks.
• Circuit breakers — switches that trip (turn the circuit off) when current is too high; they can be reset (unlike fuses). RCCBs detect earth faults.
• Double insulation: appliances with no metal casing (plastic — e.g. hairdryers, power tools) don't need an earth wire.

THE HEATING EFFECT OF CURRENT

• When current flows through a wire, the wire heats up (electrons collide with ions, transferring energy). Uses: toasters, kettles, heaters, filament lamps.
• KEY POINT: power is measured in joules per second (J/s) or watts (W).

ELECTRICAL POWER AND ENERGY (KEY EQUATIONS)

• Power — the rate of energy transfer: power (W) = energy transferred (J) ÷ time (s) P = E ÷ t
• Electrical power: power (W) = current (A) × voltage (V) P = I × V and power (W) = current² (A²) × resistance (Ω) P = I² × R
• Energy transferred: energy (J) = power (W) × time (s) E = P × t
• Worked example (from the book): an appliance uses 2 A at 230 V → P = 2 × 230 = 460 W; in 60 s it transfers E = 460 × 60 = 27 600 J.
• Kilowatt-hours (kWh) — the domestic energy unit used by electricity meters: 1 kWh = energy used by a 1 kW appliance in 1 hour. Cost = kWh × price per kWh.

SAFETY

• Never touch a live wire; damaged cables are dangerous; water and electricity don't mix (water conducts); use the correct fuse rating (slightly above the appliance's normal current).

✅ Quick check

1. What are the colours and functions of the three wires? (Live brown — carries voltage; neutral blue — completes circuit; earth green/yellow — safety.)
2. Why does a fuse blow? (Current too high → thin wire melts → circuit breaks.)
3. Write the three power equations. (P = E/t; P = I×V; P = I²R.)
4. What is the mains voltage in the UK? (230 V a.c.)
5. A 3 A, 230 V kettle runs for 120 s. What energy does it transfer? (P = 3×230 = 690 W; E = 690×120 = 82 800 J.)

Chapter 7: Current and Voltage in Circuits

Electric current is the flow of charge, measured with an ammeter in series; voltage (potential difference) is the energy given to charge, measured with a voltmeter in parallel. This chapter covers circuit symbols, series and parallel circuits, and how current/voltage behave in each.

---

ELECTRIC CURRENT

• Electric current — the flow of electric charge (electrons) around a circuit.
• Current is measured in amperes (amps, A) with an ammeter.
• An ammeter is always connected in series (in the path of the current). charge (C) = current (A) × time (s) Q = I × t

VOLTAGE (POTENTIAL DIFFERENCE)

• Voltage (potential difference) — the energy transferred per unit charge as charge moves between two points. voltage (V) = energy (J) ÷ charge (C) V = E ÷ Q
• Measured in volts (V) with a voltmeter, connected in parallel across a component.
• A battery/cell "pushes" charge around the circuit, giving it energy.

CIRCUIT SYMBOLS (memorise)

• Cell/battery, bulb/lamp, switch, ammeter, voltmeter, resistor, variable resistor, fuse, diode (lets current flow one way), LED, LDR (light-dependent resistor), thermistor, motor, buzzer, wires.

SERIES CIRCUITS

• Components are connected one after another (single loop).
• Current is the same everywhere in a series circuit.
• Voltages add up: the total voltage of the supply is shared between the components (V_total = V₁ + V₂ + ...).
• Resistance adds up: total resistance = sum of all resistances (R = R₁ + R₂ + ...).
• One break stops everything (e.g. a bulb blowing switches the whole circuit off).

PARALLEL CIRCUITS

• Components are connected on separate branches (multiple loops).
• Voltage is the same across every branch (each branch gets the full supply voltage).
• Current splits: the total current from the supply equals the sum of the branch currents (I_total = I₁ + I₂ + ...).
• Each branch works independently (one bulb blowing doesn't affect the others) — this is why household circuits are wired in parallel.
• Adding more branches reduces total resistance (more paths for current).

💡 Exam tip — series vs parallel table

	Series	Parallel
Current	Same everywhere	Splits between branches
Voltage	Shared (adds to supply)	Same across each branch
Resistance	Adds up	Less than the smallest branch
Effect of a break	Whole circuit stops	Only that branch stops

✅ Quick check

1. How is an ammeter connected? (In series.) A voltmeter? (In parallel.)
2. In a series circuit, what is the same everywhere? (Current.)
3. In a parallel circuit, what is the same across all branches? (Voltage.)
4. Why are house circuits wired in parallel? (Each appliance works independently; each gets the full voltage.)
5. Write the equations linking charge, current and time; and voltage, energy and charge. (Q = It; V = E/Q.)

Chapter 8: Electrical Resistance

Resistance opposes the flow of current — it is the ratio of voltage to current (V = IR, Ohm's law). This chapter covers resistors in series/parallel, the I–V characteristics of different components (resistor, filament lamp, diode), and special resistors (LDRs and thermistors).

---

RESISTANCE

• Resistance — a measure of how much a component opposes the flow of current. resistance (Ω) = voltage (V) ÷ current (A) R = V ÷ I
• Units: ohms (Ω).
• Ohm's law: for a metal wire at constant temperature, the current is proportional to the voltage (V = IR is constant).
• A high resistance = less current for the same voltage.

RESISTORS IN SERIES AND PARALLEL

• Series: total resistance = R₁ + R₂ + ... (resistance adds up — the current must pass through each).
• Parallel: total resistance is less than the smallest individual resistance (more paths for current = less opposition). For two equal resistors in parallel, R_total = R ÷ 2.

I–V CHARACTERISTIC GRAPHS (current vs voltage)

1. Fixed resistor (at constant temperature): a straight line through the origin — current proportional to voltage (Ohm's law). Gradient = 1/R.
2. Filament lamp (bulb): a curve — as current increases, the filament heats up, resistance increases, so the line curves over (less current for each extra volt). NOT a straight line — the lamp does not obey Ohm's law at high currents.
3. Diode: current flows in one direction only — a tiny (almost zero) current in reverse; a steep rise once the forward voltage passes a threshold. Diodes act as one-way valves. - 💡 Exam tip: be able to draw all three I–V graphs and explain the lamp's curve using "resistance increases with temperature".

SPECIAL RESISTORS

• LDR (light-dependent resistor): resistance is HIGH in the dark, LOW in bright light.
• Uses: automatic street lights (dark → high resistance → light turns on), burglar alarms, camera light meters.
• Thermistor (temperature-dependent resistor): resistance is HIGH when cold, LOW when hot.
• Uses: thermostats, car engine temperature sensors, fire alarms.
• Both are used in potential divider circuits to switch devices at a set light/temperature level.

INVESTIGATING I–V CHARACTERISTICS (practical)

• 🔬 Practical — how current varies with voltage for different components: build a series circuit (cell/battery, component, ammeter) with a voltmeter in parallel across the component; use a variable resistor (rheostat) to vary the voltage; record pairs of (V, I) readings; repeat with the polarity reversed (swap the battery) for the diode; plot I against V for each component.
• Results: resistor → straight line through origin; filament lamp → curve (resistance rises with temperature); diode → current only one way.

✅ Quick check

1. Write Ohm's law. (V = IR.)
2. What happens to resistance when resistors are added in series? (It adds up.) In parallel? (Total resistance falls below the smallest.)
3. Draw/describe the I–V graph for a filament lamp. (A curve — resistance increases as it heats up.)
4. What does an LDR do? (Resistance high in dark, low in light.)
5. What does a thermistor do? (Resistance high when cold, low when hot.)

Chapter 9: Electric Charge

Static electricity is caused by the transfer of electrons (negative charge) between materials. This chapter covers how insulators become charged, the forces between charges, electric fields, and the uses and dangers of static electricity.

---

STATIC ELECTRICITY — HOW CHARGES BUILD UP

• Atoms contain protons (+), electrons (−) and neutrons. Only electrons can move — when electrons are transferred between materials, objects become charged.
• Charging by friction: rubbing two insulators together transfers electrons:
• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged.
• Example: rubbing a balloon on a jumper — electrons transfer from the jumper to the balloon → balloon negative, jumper positive.
• Like charges repel; opposite charges attract. (This is the fundamental rule.)
• Charged objects attract neutral objects too (e.g. a charged balloon picks up small paper pieces — the charges in the paper are rearranged, inducing attraction).

ELECTRIC FIELDS

• Electric field — the region around a charged object where it exerts a force on other charges.
• Field lines show the direction of the force on a positive test charge: they point away from positive charges and towards negative charges.
• The force gets weaker with distance (field lines further apart).
• 📊 Diagram: draw field lines around a positive sphere (arrows outward) and a negative sphere (arrows inward); between two opposite charges, lines curve from + to −.

USES OF STATIC ELECTRICITY

1. Paint spraying / powder coating: the paint drops are charged and the object is given the opposite charge → paint is attracted evenly onto the object (including the back), less waste.
2. Dust precipitators (chimneys): smoke particles are charged and attracted to oppositely charged plates → removed from the air (reduces pollution).
3. Photocopiers / laser printers: use static charge to attract toner (ink powder) to the paper.
4. Defibrillators use charge to restart the heart.

DANGERS OF STATIC ELECTRICITY

1. Refuelling aircraft/vehicles: friction can build up charge; a spark could ignite fuel vapour. Safety: earthing straps/conducting pipes so charge drains away.
2. Lightning: charge builds up in clouds → huge spark to Earth (lightning). Safety: lightning conductors (tall metal rods) conduct the charge safely to the ground.
3. Electronic components: static can damage computer chips — handled with anti-static bags/wrist straps. - Earthing — connecting a charged object to the ground with a conductor, so charge flows away (prevents dangerous sparks).

✅ Quick check

1. What moves when objects become charged? (Electrons.)
2. State the rule for charges. (Like charges repel, opposite charges attract.)
3. What is an electric field? (The region where a charge experiences a force.)
4. Give two uses and two dangers of static electricity. (Uses: paint spraying, dust precipitators, photocopiers. Dangers: refuelling sparks, lightning, damaging electronics.)
5. What is earthing? (Connecting an object to the ground so charge drains away safely.)

Chapter 10: Properties of Waves

A wave transfers energy (not matter) from one place to another. This chapter covers wave terms (amplitude, wavelength, frequency, period), the wave equation, transverse vs longitudinal waves, and reflection, refraction and diffraction.

---

WHAT IS A WAVE?

• Wave — a disturbance that transfers energy from one point to another without transferring matter.
• Examples: water waves, sound waves, light waves, seismic waves, waves on a string.

WAVE TERMS (memorise)

• Amplitude — the maximum displacement of a point on the wave from its rest position (measured in m). Bigger amplitude = more energy.
• Wavelength (λ) — the distance between one point on a wave and the same point on the next wave (crest to crest), measured in m.
• Frequency (f) — the number of waves (complete oscillations) per second, measured in hertz (Hz).
• Period (T) — the time for one complete wave (seconds). period (s) = 1 ÷ frequency (Hz) T = 1/f

THE WAVE EQUATION (KEY FORMULA)

wave speed (m/s) = frequency (Hz) × wavelength (m)
v = f × λ

• Example: a wave with frequency 5 Hz and wavelength 2 m travels at 5 × 2 = 10 m/s.
• 💡 Exam tip: know how to rearrange for f = v ÷ λ and λ = v ÷ f.

TRANSVERSE AND LONGITUDINAL WAVES

• Transverse waves: the vibrations are at right angles (perpendicular) to the direction of energy transfer.
• Examples: all electromagnetic waves (light, radio, etc.), water waves, waves on a string, S-waves (seismic).
• Shape: crests and troughs.
• Longitudinal waves: the vibrations are parallel to the direction of energy transfer (along the same line).
• Examples: sound waves, P-waves (seismic), some spring waves.
• Shape: compressions (particles squashed together) and rarefactions (particles spread apart).
• 💡 Exam tip: be able to state which type each wave is and describe how the particles move.

REFLECTION

• Reflection — a wave bounces off a surface. The angle of incidence = angle of reflection (measured from the normal — the line at 90° to the surface).
• Examples: light off a mirror (see Ch 12), sound echoes, water waves off a wall.

REFRACTION

• Refraction — the change of direction of a wave when it changes speed crossing a boundary (e.g. air → glass).
• When light enters a denser medium it slows down and bends towards the normal; entering a less dense medium it speeds up and bends away from the normal.
• If the angle is large enough, total internal reflection can occur (see Ch 12).

DIFFRACTION

• Diffraction — the spreading out of waves when they pass through a gap or around an obstacle.
• The effect is biggest when the gap is about the same size as the wavelength.
• Examples: sound diffracting around corners (you can hear someone around a corner); water waves spreading through a harbour entrance.
• Light diffracts too, but only noticeably through very narrow gaps (its wavelength is tiny).

THE DOPPLER EFFECT (sound)

• Doppler effect — the change in observed frequency when the source and observer move relative to each other.
• A sound source moving towards you has its waves squashed (shorter wavelength, higher pitch); moving away → stretched (lower pitch). E.g. a siren is higher-pitched as it approaches and drops as it passes. (Same effect for light → red shift, see Ch 28.)
• 🔬 Ripple tank — a practical to study waves: a vibrating bar makes waves on water in a shallow tank; use a lamp to project shadows; measure wavelength with a ruler and frequency by counting waves — v = fλ. Barriers and gaps show reflection and diffraction.

✅ Quick check

1. What do waves transfer? (Energy, not matter.)
2. Define amplitude, wavelength, frequency. (Amplitude = max displacement; wavelength = distance between same points on adjacent waves; frequency = waves per second.)
3. Write the wave equation. (v = f × λ.)
4. Give one example each of transverse and longitudinal waves. (Transverse: light/EM, water. Longitudinal: sound.)
5. What is diffraction? (Waves spreading out through a gap or around an obstacle — biggest when gap ≈ wavelength.)

Chapter 11: The Electromagnetic Spectrum

The electromagnetic (EM) spectrum is a family of waves that all travel at the same speed in a vacuum (3 × 10⁸ m/s) but have different wavelengths and frequencies. This chapter covers the spectrum order, properties common to all EM waves, and the uses and dangers of each type.

---

WHAT ARE ELECTROMAGNETIC WAVES?

• Electromagnetic waves — transverse waves made of vibrating electric and magnetic fields.
• They all:
• Travel at 3 × 10⁸ m/s in a vacuum (the speed of light).
• Transfer energy.
• Can travel through a vacuum (unlike sound).
• Obey v = f × λ.
• Order of the spectrum (memorise — by decreasing wavelength / increasing frequency): Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma
• Mnemonic: "Rabbits Mate In Very Unusual Xciting Gardens".

PROPERTIES AND USES OF EACH TYPE

Wave	Uses	Danger
Radio (longest λ, lowest f)	Radio/TV broadcasting, communication	None
Microwave	Satellite/cell phone communication, cooking (microwave ovens heat water)	Internal heating/burns (in ovens, contained)
Infrared	Heat transfer, remote controls, thermal imaging cameras, grills	Skin burns
Visible light	Seeing, photography, fibre-optic communication	Very bright light damages eyes
Ultraviolet (UV)	Sterilising equipment/water, security pens, sunbeds	Skin damage, sunburn, skin cancer, eye damage
X-rays	Medical imaging (broken bones, teeth), airport security	Cell damage, cancer (doses limited)
Gamma (γ) (shortest λ, highest f)	Sterilising medical equipment, killing cancer cells (radiotherapy), tracing	Very dangerous — cell/DNA damage, cancer
- KEY POINT — the shorter the wavelength, the more energy each wave carries → the more dangerous it is (radio safest, gamma most dangerous).

DANGERS AND SAFETY

• UV, X-rays and gamma rays are ionising — they can knock electrons off atoms, damaging cells and DNA → mutations and cancer.
• Safety: wear sunscreen; radiographers stand behind lead screens / use film badges; X-rays only when needed; gamma sources kept in lead containers and handled with tongs.

✅ Quick check

1. List the EM spectrum in order. (Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.)
2. How fast do EM waves travel in a vacuum? (3 × 10⁸ m/s.)
3. Which EM waves are ionising? (UV, X-rays, gamma.)
4. Give two uses of microwaves and two of infrared. (Microwaves: communication, cooking. Infrared: remote controls, thermal imaging, heaters.)
5. Why are gamma rays the most dangerous? (Shortest wavelength → most energy → ionising → cell damage.)

Chapter 12: Light Waves

Light is part of the EM spectrum — it reflects off mirrors, refracts (bends) when changing speed, and can be totally internally reflected in optical fibres. Lenses use refraction to focus light. This chapter covers all of these with the ray diagrams you must draw.

---

REFLECTION OF LIGHT

• Law of reflection: angle of incidence = angle of reflection (both measured from the normal, the line at 90° to the surface).
• Plane (flat) mirrors: the image is virtual (behind the mirror), upright, same size, and the same distance behind the mirror as the object is in front — and laterally inverted (left/right swapped).
• Ray diagrams: draw the object, mirror, reflected rays (using the law), and extend the rays back behind the mirror to find the virtual image.
• Uses: mirrors, periscopes (two mirrors at 45°).

REFRACTION OF LIGHT

• Refraction — the change of direction of light when it changes speed moving between media.
• Air → glass (more dense): light slows down and bends towards the normal.
• Glass → air (less dense): light speeds up and bends away from the normal.
• If light hits the boundary along the normal (90°), it passes straight through (no bending).
• Explains: a pencil looks bent in water; a pool looks shallower than it is.

TOTAL INTERNAL REFLECTION (TIR)

• When light goes from a denser to a less dense medium (e.g. glass → air), the ray bends away from the normal.
• At the critical angle (c), the refracted ray runs along the boundary (angle of refraction = 90°).
• If the angle of incidence is greater than the critical angle, the light is totally internally reflected (100% reflected — like a perfect mirror).
• Conditions for TIR: light in the denser medium, travelling towards a less dense medium, at an angle greater than the critical angle.
• Uses:
• Optical fibres — light bounces along the fibre by repeated TIR; used for communications (fast data transfer, phone/internet cables) and endoscopes (seeing inside the body).
• Prisms in periscopes/binoculars (reflect light by TIR instead of mirrors).
• Diamonds sparkle because their critical angle is very small (lots of internal reflection).

LENSES

• Convex (converging) lens — thicker in the middle; brings parallel light rays together at the focus (focal point). Used in magnifying glasses, cameras, eyes.
• Concave (diverging) lens — thinner in the middle; spreads parallel rays outward (as if from the focus). Used for short-sight correction.
• Ray diagrams (convex lens): draw the principal axis, the lens, the focal points; use two rays: (1) parallel ray refracts through the focus, (2) ray through the centre passes straight. Where they cross = the image.
• Image depends on object distance: beyond 2F → smaller, inverted, real; between F and 2F → larger, inverted, real; inside F → larger, upright, virtual.

USES OF TOTAL INTERNAL REFLECTION (with practical)

• Bicycle and car reflectors: rows of small prisms that use TIR to send light straight back to the driver (retro-reflection).
• Prismatic periscope: two 45° prisms use TIR to turn light through 90° twice — giving a brighter image than mirrors.
• Optical fibres: light repeatedly totally internally reflected along the core — used in medicine (endoscope), communications, decorative lamps.
• 🔬 Practical — investigate the refractive index for glass: shine a ray of light (ray box/slit) into a glass block at different angles; mark the rays; measure the angle of incidence (i) and angle of refraction (r) with a protractor (measuring from the normal); calculate refractive index n = sin i ÷ sin r (average over several angles). Also observe the ray along the normal (no bending) and find the critical angle by increasing i until TIR occurs.

✅ Quick check

1. State the law of reflection. (Angle of incidence = angle of reflection.)
2. What happens to light entering glass from air? (It slows down and bends towards the normal.)
3. State the conditions for total internal reflection. (Light in denser medium going to less dense, at an angle greater than the critical angle.)
4. Give two uses of optical fibres. (Communications, endoscopes.)
5. What is a convex lens? (Thicker in the middle — converges light to a focus.)

Chapter 13: Sound

Sound waves are longitudinal waves (compressions and rarefactions) that need a medium to travel through. This chapter covers how sound travels, pitch, loudness and frequency, the speed of sound (echo method), how we hear, and ultrasound uses.

---

WHAT IS SOUND?

• Sound — a longitudinal wave: particles vibrate parallel to the direction of energy transfer, forming compressions (squashed together) and rarefactions (spread apart).
• Sound cannot travel through a vacuum (no particles to vibrate) — e.g. space is silent.
• It travels as a solid (fastest) → liquid → gas (slowest) — particles are closer together in solids.
• Speed of sound in air ≈ 340 m/s (much slower than light — why you see lightning before you hear thunder).

PITCH, LOUDNESS AND FREQUENCY

• Frequency (Hz) — the number of vibrations per second.
• Higher frequency → higher pitch (e.g. a whistle vs a bass drum).
• Amplitude — the size of the vibration (loudness).
• Bigger amplitude → louder sound.
• Human hearing range: roughly 20 Hz – 20 000 Hz (20 kHz).
• Sound below 20 Hz = infrasound; above 20 kHz = ultrasound.
• An oscilloscope displays sound waves: more waves across the screen = higher frequency; taller waves = louder.

HOW WE HEAR (the ear)

• Sound waves are funnelled by the outer ear (pinna) into the ear canal → make the eardrum vibrate → three tiny bones (ossicles) amplify the vibration → the cochlea (fluid-filled, contains tiny hairs) converts vibrations into electrical impulses → sent along the auditory nerve to the brain.
• Damage: loud sounds damage the cochlea's hair cells → permanent hearing loss. Protect ears at concerts/with loud machinery.

MEASURING THE SPEED OF SOUND — THE ECHO METHOD

1. Stand a known distance (e.g. 100 m) from a large wall.
2. Make a sharp sound (clap) and start a timer; stop it when you hear the echo.
3. The sound has travelled twice the distance (to the wall and back). speed = (2 × distance) ÷ time - Example: 200 m round trip in 0.59 s → speed ≈ 340 m/s.

ULTRASOUND (frequency above 20 kHz)

• Ultrasound — sound waves above the human hearing range, which can be focused and reflected off boundaries.
• Uses: 1. Prenatal (pre-natal) scanning — images of a fetus (safer than X-rays). 2. Medical imaging — checking organs (e.g. kidney stones, heart valves). 3. Cleaning — vibrating dirt off delicate objects (jewellery, surgical instruments). 4. Industrial testing — checking for cracks in metal/piplines (flaw detection); thickness measurement. 5. Sonar/echo sounding — ships measure depth: depth = (speed × time) ÷ 2.
• Animal use: bats and dolphins use ultrasound echoes to navigate and find prey.

✅ Quick check

1. What type of wave is sound? (Longitudinal — compressions and rarefactions.)
2. Why can't sound travel through a vacuum? (No particles to vibrate.)
3. What is the human hearing range? (About 20 Hz to 20 000 Hz.)
4. How does higher frequency affect pitch? (Higher frequency = higher pitch.)
5. Give three uses of ultrasound. (Pre-natal scanning, medical imaging, cleaning, flaw detection, sonar.)

Chapter 14: Energy Transfers

Energy can be stored in different ways and transferred between stores. This chapter covers the energy stores (kinetic, gravitational potential, elastic, chemical, thermal, etc.), energy transfers (mechanically, electrically, by heating, by radiation), the principle of conservation of energy, and sankey diagrams — plus the kinetic and GPE equations.

---

ENERGY STORES

• Energy is stored in different forms (stores):
• Chemical — in food, fuel, batteries (released in reactions).
• Kinetic — energy of a moving object.
• Gravitational potential (GPE) — energy stored by an object's height.
• Elastic — energy stored in a stretched/compressed spring.
• Thermal (internal) — energy of hot objects.
• Nuclear — in the nucleus of atoms.
• (Also: magnetic, electrostatic, light, sound — these are ways energy is transferred/radiated.)

ENERGY TRANSFERS

• Energy can be transferred:
• Mechanically (by a force doing work).
• Electrically (by a current).
• By heating (conduction, convection, radiation).
• By radiation (light, sound, EM waves).

PRINCIPLE OF CONSERVATION OF ENERGY

• Energy cannot be created or destroyed — it can only be transferred or changed from one store to another.
• The total energy before = total energy after.
• In every transfer, some energy is dissipated (wasted) — usually as heat to the surroundings (e.g. friction in a machine, sound).
• Efficiency: efficiency = useful output energy ÷ total input energy (×100%) No machine is 100% efficient — some energy always becomes heat/sound.

SANKEE DIAGRAMS (sankey diagrams)

• A sankey diagram shows energy transfers as arrows: the width of each arrow is proportional to the amount of energy.
• Input arrow on the left splits into useful (horizontal) and wasted (angled down) arrows.
• Example: a light bulb — 100 J in, 10 J light (useful), 90 J heat (wasted) → efficiency 10%.

KEY ENERGY EQUATIONS

• Kinetic energy: KE (J) = ½ × mass (kg) × velocity² (m/s)² KE = ½mv²
• Gravitational potential energy: GPE (J) = mass (kg) × gravitational field strength (N/kg) × height (m) GPE = mgh
• Work done (energy transferred by a force): work done (J) = force (N) × distance (m) W = F × d
• Example: a 2 kg ball at 10 m height: GPE = 2 × 10 × 10 = 200 J. If it falls, this becomes KE: 200 = ½ × 2 × v² → v = √200 ≈ 14 m/s (ignoring air resistance).

✅ Quick check

1. Name five energy stores. (Chemical, kinetic, GPE, elastic, thermal, nuclear.)
2. State the principle of conservation of energy. (Energy cannot be created or destroyed, only transferred.)
3. Write the KE and GPE equations. (KE = ½mv²; GPE = mgh.)
4. What is efficiency? (Useful output ÷ total input, as a %.)
5. What does a sankey diagram show? (Energy transfers — arrow widths proportional to energy.)

Chapter 15: Thermal Energy

Thermal energy (heat) moves from hot to cold objects by conduction, convection and radiation. This chapter covers the three transfer methods, thermal conductors and insulators, and how homes are kept warm (and heat loss reduced).

---

TEMPERATURE AND HEAT

• Temperature — how hot something is (measured in °C), related to the average kinetic energy of particles.
• Heat (thermal energy) — the total internal energy of the particles; it always flows from a hotter to a colder object until they reach the same temperature.

THE THREE WAYS HEAT TRANSFERS

1. CONDUCTION (mainly in SOLIDS)

• Heat passes along a solid by vibrating particles passing energy to their neighbours.
• Metals are the best conductors because they have free (delocalised) electrons that move quickly and carry energy.
• Insulators (wood, plastic, air, wool) — particles not free to move; trap air.
• Example: a metal spoon handle gets hot in a hot drink; plastic handles stay cool.

2. CONVECTION (in FLUIDS — liquids and gases)

• Heat transfer by the movement of the fluid itself: 1. Fluid near the heat source warms, expands, becomes less dense → rises. 2. Cooler, denser fluid sinks to replace it. 3. This creates a convection current (circular flow).
• Examples: hot air rising above a radiator; sea breezes; hot water rising in a kettle; a fridge (cold air sinks).
• Convection CANNOT happen in solids (particles fixed).

3. RADIATION (infrared — no medium needed)

• Heat transfer by infrared waves — can travel through a vacuum (e.g. heat from the Sun).
• Dark, matt (dull) surfaces are the best emitters and absorbers of radiation.
• Light, shiny surfaces are the best reflectors (and poor emitters/absorbers).
• Examples: black car seats get hot in the sun; shiny foil blankets keep people warm by reflecting body heat back; radiators painted dark emit more.

REDUCING HEAT LOSS IN HOMES (a classic exam application)

Method	How it works
Loft insulation (fibreglass)	Traps air — stops conduction + convection
Cavity wall insulation	Traps air in wall gaps
Double glazing	Trapped air/gas between two panes insulates
Draught excluders	Stop convection of warm air out
Silver foil behind radiators	Reflects infrared back into the room
Thick curtains	Trap air; block radiation and draughts
- Less heat loss → less fuel used → cheaper bills and less pollution.

SPECIFIC HEAT CAPACITY (PHYSICS ONLY)

• Specific heat capacity (c) — the energy needed to raise the temperature of 1 kg of a substance by 1 °C (units J/kg°C). energy (J) = mass (kg) × specific heat capacity (J/kg°C) × temperature change (°C) E = mcΔθ
• Water has a high specific heat capacity (4200 J/kg°C) — it heats and cools slowly (useful for central heating and cooling engines).

✅ Quick check

1. Name the three ways heat is transferred. (Conduction, convection, radiation.)
2. Which needs particles and which doesn't? (Conduction needs solids' particles; convection needs fluid movement; radiation needs no medium.)
3. Why are metals good conductors? (Free electrons carry energy.)
4. What surfaces are best for emitting/absorbing radiation? (Dark, matt.)
5. Give three ways to insulate a home. (Loft insulation, cavity walls, double glazing, draught excluders, foil.)

Chapter 16: Work and Power

Work is done when a force moves an object — it is the same as energy transferred. Power is the rate of doing work (energy per second). This chapter covers work done, power, and gravitational potential energy in the context of lifting.

---

WORK DONE

• Work done — the energy transferred when a force moves an object through a distance. work done (J) = force (N) × distance moved in the direction of the force (m) W = F × d
• Units: joules (J) — work done = energy transferred.
• Example: lifting a 10 N weight 2 m → work done = 10 × 2 = 20 J (this becomes GPE).
• If the object doesn't move, no work is done (e.g. holding a heavy bag still — force but no distance → no work).

POWER

• Power — the rate of doing work (or transferring energy) — how much work per second. power (W) = work done (J) ÷ time taken (s) P = W ÷ t
• Units: watts (W) — 1 watt = 1 joule per second (J/s).
• Example: 200 J of work in 40 s → power = 200 ÷ 40 = 5 W.
• A more powerful machine does the same work faster (e.g. a powerful car accelerates quicker; a powerful motor lifts the same load in less time).

GPE AND WORK LINKED

• Lifting an object does work against gravity, which is stored as gravitational potential energy: GPE (J) = mass (kg) × g (N/kg) × height (m) GPE = mgh
• The work done lifting = the GPE gained: W = F × d = mgh (since F = mg).
• Example: lifting a 2 kg box 3 m (g = 10): GPE = 2 × 10 × 3 = 60 J — so the work done is also 60 J.

EFFICIENCY OF MACHINES

• Machines transfer energy but always waste some (heat from friction, sound). efficiency = (useful output energy ÷ total input energy) × 100%
• No machine is 100% efficient.

ENERGY AND WORK / KE LINKED (with practical)

• Doing work on an object transfers energy: work done (J) = energy transferred.
• Lifting does work against gravity → stored as GPE (mgh); a moving object carries kinetic energy: KE (J) = ½ × mass (kg) × velocity² (m/s)² KE = ½mv²
• Example: a 2 kg trolley at 3 m/s has KE = ½ × 2 × 9 = 9 J. If it is brought to rest by a force over 1.5 m, the braking force = KE ÷ distance = 9 ÷ 1.5 = 6 N.
• 🔬 Practical — investigate your own power output: run (or walk) up a flight of stairs of known height as fast as you can while a partner times you; work done = your weight (N) × height (m) [= mgh = GPE gained]; then power = work done ÷ time. Repeat at different speeds — faster = more power (same work, less time).

✅ Quick check

1. Write the work done equation. (W = F × d, in joules.)
2. When is no work done? (When the object doesn't move in the direction of the force.)
3. Define power and write its equation. (Rate of doing work — P = W ÷ t, in watts.)
4. A 50 N force moves an object 4 m. What work is done? (50 × 4 = 200 J.)
5. What is the GPE equation? (GPE = mgh.)

Chapter 17: Energy Resources and Electricity Generation

Electricity is generated by turning a turbine connected to a generator (electromagnetic induction). The energy to turn the turbine comes from fossil fuels, nuclear fuel or renewable resources (wind, hydroelectric, solar, waves, tides, geothermal, biomass). This chapter compares these resources and the environmental issues.

---

HOW ELECTRICITY IS GENERATED

• Basic principle (all power stations): 1. An energy source heats water → steam (or wind/water turns the turbine directly). 2. Steam/water/wind turns the turbine. 3. The turbine turns the generator → electricity (by electromagnetic induction — a coil rotating in a magnetic field). 4. A transformer steps up the voltage for the National Grid.
• Thermal power stations (fossil/nuclear): fuel → heat → steam → turbine → generator.

NON-RENEWABLE RESOURCES

• Non-renewable — will run out (cannot be replaced in a useful time).
• Fossil fuels (coal, oil, gas): burned to heat water. Advantages: reliable, established, cheap-ish. Disadvantages: run out, produce CO₂ (global warming) and sulfur dioxide (acid rain), pollution, transport/storage.
• Nuclear fuel (uranium/plutonium): nuclear fission releases huge heat. Advantages: no CO₂, huge energy from little fuel. Disadvantages: radioactive waste (dangerous, stored for thousands of years), risk of accidents (Chernobyl, Fukushima), decommissioning cost.

RENEWABLE RESOURCES

• Renewable — will not run out (replaced naturally). 1. Wind: turbines turned by wind. Clean; but unreliable (no wind), noisy, needs space, visual pollution. 2. Hydroelectric: water stored in a dam flows through turbines. Reliable, can be switched on quickly (pumped storage); but floods valleys, expensive to build, depends on rain. 3. Solar: photovoltaic cells convert light directly to electricity. Clean, no running cost; but only works in daylight, expensive panels, low output. 4. Waves and tides: wave machines / tidal barrages turn turbines. Predictable (tides); but expensive, few suitable sites, affects habitats (tidal barrages). 5. Geothermal: heat from underground rocks makes steam. Clean, reliable; but only in volcanic regions, expensive drilling. 6. Biomass: burning (or fermenting) plant material/wood. Renewable, carbon-neutral-ish (plants absorbed CO₂ while growing); but uses land/food crops, produces some CO₂.

COMPARING RESOURCES (exam style)

Resource	Renewable?	CO₂	Reliability	Main problem
Fossil fuels	No	High	Very reliable	Runs out, pollution
Nuclear	No	None	Very reliable	Radioactive waste, accidents
Wind	Yes	None	Unreliable	No wind, visual noise
Hydroelectric	Yes	None	Reliable	Floods land, expensive
Solar	Yes	None	Unreliable	Night/cloud
Geothermal	Yes	None	Reliable	Few sites
Tidal/Wave	Yes	None	Fairly	Expensive, habitats
Biomass	Yes	Some	Reliable	Land use, some CO₂

THE NATIONAL GRID

• The system of cables and transformers that distributes electricity nationwide.
• Step-up transformers raise the voltage (to ~400 kV) — high voltage, low current reduces energy loss as heat in the cables.
• Step-down transformers lower the voltage to safe levels (230 V) for homes.

✅ Quick check

1. How is electricity generated in a power station? (Heat → steam → turbine → generator → electricity.)
2. Distinguish renewable and non-renewable. (Renewable won't run out; non-renewable will.)
3. Give two disadvantages of fossil fuels and two of nuclear. (Fossil: CO₂, acid rain, runs out. Nuclear: radioactive waste, accidents.)
4. Give one advantage and one disadvantage of wind and of solar. (Wind: clean/unreliable. Solar: clean/night-time.)
5. Why does the National Grid use high voltages? (High voltage + low current = less heat loss in cables.)

Chapter 18: Density and Pressure

Density is how much mass is packed into a volume; pressure is the force spread over an area. This chapter covers the density equation (and how to measure density), pressure in solids and liquids, and hydraulics.

---

DENSITY

• Density — the mass per unit volume of a substance. density (kg/m³) = mass (kg) ÷ volume (m³) ρ = m ÷ V
• Units: kg/m³ (or g/cm³).
• Solid > liquid > gas density (usually) — particles packed more closely in solids.
• Example: 2 kg of iron occupying 0.00025 m³ → density = 2 ÷ 0.00025 = 8000 kg/m³.
• Measuring density:
• Regular solid: measure mass (balance), measure volume from dimensions (l×w×h), divide.
• Irregular solid: measure mass; find volume by displacement (submerge in a measuring cylinder of water — the rise in level = volume).
• Liquid: measure mass of a known volume (mass of liquid = mass of measuring cylinder with liquid − empty).

PRESSURE

• Pressure — the force acting per unit area (perpendicular to the surface). pressure (Pa) = force (N) ÷ area (m²) p = F ÷ A
• Units: pascals (Pa) (1 Pa = 1 N/m²).
• KEY POINT: the same force on a smaller area gives a bigger pressure.
• Examples of HIGH pressure: sharp knife, drawing pins, stiletto heels, nails.
• Examples of LOW pressure: snowshoes, caterpillar tracks on tanks, skis (spread weight over a large area).

PRESSURE IN LIQUIDS

• Pressure in a liquid:
• Increases with depth (more liquid above pressing down).
• Acts in all directions.
• Increases with density of the liquid. pressure in liquid (Pa) = density (kg/m³) × g (N/kg) × depth (m) p = ρgh
• Examples: dams are thicker at the bottom (pressure greatest there); a submarine's hull must withstand huge pressure at depth.
• Hydraulics: liquids are (almost) incompressible — a force applied to a small piston creates pressure that is transmitted equally, producing a larger force on a bigger piston (a force multiplier — used in car brakes, hydraulic jacks, diggers).

PRESSURE IN GASES (link to Ch 19)

• Gas particles bombard the container walls — this creates gas pressure.
• Pressure increases if the gas is compressed into a smaller volume (more collisions) or heated (faster particles).

✅ Quick check

1. Write the density equation. (ρ = m ÷ V.)
2. How do you find the volume of an irregular solid? (Water displacement in a measuring cylinder.)
3. Write the pressure equation. (p = F ÷ A.)
4. Why are snowshoes useful? (They spread weight over a larger area → lower pressure → don't sink.)
5. How does pressure in a liquid change with depth? (It increases with depth — p = ρgh.)

Chapter 19: Solids, Liquids and Gases

This chapter explains the particle model of solids, liquids and gases, changes of state, and the behaviour of gases — including Boyle's law (pressure × volume) and the effect of temperature on gas pressure (and the Kelvin scale, Physics only).

---

THE PARTICLE MODEL

• All matter is made of tiny particles (atoms/molecules).
• Solids: particles packed closely in a fixed pattern; strong forces; they vibrate about fixed positions. Fixed shape and volume; can't be compressed.
• Liquids: particles close together but free to move (sliding past each other); weaker forces. Fixed volume, takes the shape of its container; can't be compressed much.
• Gases: particles far apart, moving randomly at high speed; very weak forces. No fixed shape or volume — fill their container; easily compressed.
• 📊 Diagram: draw the three arrangements (closely packed regular / closely packed random / spread out random).

CHANGES OF STATE

• Melting — solid → liquid (energy gained, particles break free). Melting point.
• Boiling/evaporation — liquid → gas (energy gained). Boiling point.
• Condensing — gas → liquid (energy lost).
• Freezing/solidifying — liquid → solid (energy lost).
• Sublimation — solid → gas directly (e.g. dry ice).
• KEY POINT: during a change of state the temperature stays constant (energy is used to break/form bonds, not to raise temperature).
• Evaporation happens below the boiling point at the surface (faster at higher temperature, with a draught, larger surface area); it cools the liquid (particles with most energy escape).

PRESSURE IN GASES — BOYLE'S LAW

• Gas pressure comes from particles colliding with the container walls.
• Boyle's law (at constant temperature): for a fixed mass of gas, pressure × volume = constant (pressure is inversely proportional to volume). p₁V₁ = p₂V₂
• Squeezing a gas into half the volume doubles the pressure (particles hit the walls twice as often).
• Why: smaller volume → particles closer → more collisions per second → higher pressure.
• Example: a gas at 100 kPa in 2 m³ is compressed to 1 m³ → p = 200 kPa (at constant temperature).

TEMPERATURE AND GAS PRESSURE (PHYSICS ONLY)

• Heating a gas at constant volume: particles move faster → hit the walls harder and more often → pressure rises.
• Cooling: pressure falls.
• Absolute zero — the lowest possible temperature (−273 °C): particles have no kinetic energy.
• Kelvin scale (K): K = °C + 273 (e.g. 0 °C = 273 K).
• Pressure law (constant volume): pressure ∝ temperature (in kelvin): p₁/T₁ = p₂/T₂.
• Charles' law (constant pressure): volume ∝ temperature (in kelvin): V₁/T₁ = V₂/T₂.
• 💡 Exam tip: ALWAYS convert °C to K in gas law calculations (add 273).

MEASURING HEAT ENERGY (specific heat capacity practical)

• 🔬 Practical — measuring the specific heat capacity of a metal block: wrap an insulated metal block (with two holes) in insulation; put an immersion heater in one hole and a thermometer in the other; measure the mass of the block; run the heater with a known power (P) for a measured time (t) → energy = P × t; record the temperature rise; then: c = energy ÷ (mass × temperature rise)
• Precautions: insulate the block, use a little oil in the thermometer hole for good contact, stir. The measured value is slightly HIGHER than the true value if heat is lost to the surroundings.

✅ Quick check

1. Describe the particle arrangement of a solid, liquid and gas. (Solid: fixed, vibrating. Liquid: close, sliding. Gas: far apart, fast random motion.)
2. What happens to temperature during a change of state? (It stays constant.)
3. State Boyle's law. (At constant temperature, pressure × volume = constant for a fixed mass of gas.)
4. Why does heating a gas increase its pressure? (Faster particles → harder, more frequent collisions with walls.)
5. What is absolute zero? (−273 °C = 0 K — particles have no kinetic energy.)

Chapter 20: Magnetism and Electromagnetism

Magnets have north and south poles; like poles repel, unlike poles attract. Moving charges create magnetic fields, and current-carrying wires can become electromagnets. This chapter covers magnetic fields, electromagnets, the motor effect (F = BIl), and the uses of electromagnets.

---

MAGNETS AND MAGNETIC FIELDS

• Magnets have two poles: north (N) and south (S).
• Like poles repel; unlike poles attract.
• Magnetic field — the region around a magnet where it exerts a force on magnetic materials (iron, steel, cobalt, nickel).
• Field lines: drawn from N to S; the arrow shows the direction of the force on a north pole; closer lines = stronger field.
• The Earth has a magnetic field (a compass needle points north).
• Making a magnet: stroking with a magnet, or placing in a coil with d.c. (induced magnetism). Demagnetising: heating, dropping, hammering.

ELECTROMAGNETS

• Electromagnetic effect: a current in a wire produces a magnetic field around the wire.
• Electromagnet — a coil of wire (solenoid) wound around an iron core:
• The field is stronger with: more turns on the coil, a bigger current, and an iron core (iron is easily magnetised and loses its magnetism when the current stops).
• The field is like a bar magnet: N at one end, S at the other (reversing the current reverses the poles).
• KEY DIFFERENCE from a permanent magnet: an electromagnet's magnetism can be switched on and off (and its strength varied).

USES OF ELECTROMAGNETS

• Scrapyard cranes (pick up and drop scrap iron/steel).
• Electric bells, relays, circuit breakers.
• Speakers and headphones (coil + magnet vibrates the cone).
• Magnetic locks, MRI scanners, Maglev trains.

THE MOTOR EFFECT (PHYSICS ONLY)

• A current-carrying wire in a magnetic field experiences a force (because the wire's field interacts with the magnet's field).
• The force increases with: bigger current, stronger magnetic field, longer wire in the field.
• Fleming's left-hand rule gives the direction: thumb = force (motion), first finger = field (N→S), second finger = current (conventional + to −).
• Force equation: force (N) = magnetic flux density (T) × current (A) × length (m) F = BIl
• The motor effect is what makes electric motors turn: a coil in a magnetic field experiences forces that rotate it (see Chapter 21).

INVESTIGATING MAGNETIC FIELDS (practical)

• 🔬 Practical — magnetic field patterns of bar magnets: place a bar magnet under paper and sprinkle iron filings over it (tap gently) — the filings line up along the field lines, showing the pattern (N to S curves). Or use a plotting compass at several points, marking the needle direction each time, and join the dots to draw field lines.
• Overlapping magnetic fields: two magnets near each other distort each other's fields — like poles repel (field lines squash between them), unlike poles attract (lines join N→S across the gap).
• Creating a uniform field: two opposite poles close together with parallel faces give straight, evenly spaced field lines between them (e.g. inside a motor/generator).

✅ Quick check

1. What is the rule for magnetic poles? (Like repel, unlike attract.)
2. Which materials are magnetic? (Iron, steel, cobalt, nickel.)
3. How can you make an electromagnet stronger? (More turns, bigger current, iron core.)
4. Give two uses of electromagnets. (Scrapyard cranes, electric bells, relays, speakers.)
5. What is the motor effect? (A current-carrying wire in a magnetic field experiences a force.)

Chapter 21: Electric Motors and Electromagnetic Induction

Electric motors turn using the motor effect; generators and transformers work by electromagnetic induction — creating a current by moving a conductor in a magnetic field. This chapter covers the motor, the generator (dynamo/alternator), and the transformer used by the National Grid.

---

THE ELECTRIC MOTOR

• How it works: a coil of wire sits between the poles of a magnet. When current flows, the motor effect produces forces on the two sides of the coil in opposite directions → the coil rotates.
• The split-ring commutator reverses the current every half turn, so the coil keeps turning in the same direction.
• Making the motor more powerful: bigger current, stronger magnet, more turns on the coil.
• Uses: fans, electric cars, drills, washing machines, pumps.

ELECTROMAGNETIC INDUCTION

• Electromagnetic induction — generating a voltage (and current) in a conductor by: 1. Moving a magnet into/out of a coil, or 2. Moving a coil in a magnetic field, or 3. Changing the current in a nearby coil.
• The size of the induced voltage increases with: faster movement, stronger magnet, more turns on the coil.
• Reversing the direction of movement (or the poles) reverses the induced current.

THE GENERATOR (DYNAMO)

• Turning a coil in a magnetic field (or spinning a magnet inside a coil) induces a current.
• Alternator (a.c. generator): uses slip rings — produces alternating current (direction reverses each half turn) — this is how power stations generate electricity.
• Dynamo (d.c. generator): uses a split-ring commutator — produces direct current (one direction).
• The faster the coil turns, the bigger the voltage and frequency.
• Microphones use induction: sound waves move a coil near a magnet → induced current carries the sound.

TRANSFORMERS (PHYSICS ONLY)

• A transformer changes the voltage of an alternating current using two coils (primary and secondary) wound on an iron core.
• How it works: alternating current in the primary coil creates a changing magnetic field in the iron core, which induces an alternating voltage in the secondary coil.
• Transformers only work with a.c. (a steady d.c. current produces no change in field → no induction).
• Step-up transformer: more turns on the secondary → voltage up, current down (used at power stations).
• Step-down transformer: fewer turns on the secondary → voltage down (used at homes, 230 V). Vp ÷ Vs = Np ÷ Ns (V = voltage, N = number of turns, p = primary, s = secondary)
• Ideal transformer: power in = power out: Vp × Ip = Vs × Is.
• Why step-up: high voltage + low current → less heat loss in the cables (National Grid).

DEMONSTRATING ELECTROMAGNETIC INDUCTION (practical)

• 🔬 Practical — show induction with a coil and magnet: connect a coil of wire to a sensitive ammeter (galvanometer); push a bar magnet into the coil — the needle deflects (current induced). Pull it out — the needle deflects the other way (current reversed). Hold the magnet still — no deflection (no change = no current). Move the magnet faster or use a stronger magnet — bigger deflection (bigger voltage). This is the basis of generators and transformers.

✅ Quick check

1. Why does a motor's coil keep turning? (Split-ring commutator reverses the current each half turn.)
2. What is electromagnetic induction? (Generating a voltage by moving a conductor in a magnetic field / changing the field.)
3. What is the difference between an alternator and a dynamo? (Alternator gives a.c. via slip rings; dynamo gives d.c. via a commutator.)
4. Why do transformers only work with a.c.? (A changing field is needed to induce a voltage.)
5. What does a step-up transformer do? (Increases voltage, decreases current.)

Chapter 22: Atoms and Radioactivity

Atoms contain protons, neutrons and electrons. Some atoms are unstable (radioactive) — their nuclei decay, emitting alpha, beta or gamma radiation. This chapter covers atomic structure, isotopes, the three types of radiation, and background radiation.

---

ATOMIC STRUCTURE (recap)

• Atom — protons (+), neutrons (0) in the nucleus; electrons (−) orbiting in shells.
• Atomic number (proton number) — number of protons (defines the element).
• Mass number — protons + neutrons.
• Isotopes — atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers).
• Example: carbon-12 and carbon-14 (both 6 protons; 6 vs 8 neutrons).
• Electron shells: electrons occupy shells; an atom becomes an ion by gaining/losing electrons.

WHAT IS RADIOACTIVITY?

• Radioactivity — the spontaneous decay of unstable nuclei, emitting radiation.
• It is random (you can't predict which nucleus decays when) and spontaneous (not affected by temperature, pressure or chemical reactions).
• Background radiation — the low-level radiation always present around us (from rocks/soil — radon gas, cosmic rays from space, food, medical sources, and a tiny amount from nuclear tests/accidents).

THE THREE TYPES OF RADIATION

	Alpha (α)	Beta (β)	Gamma (γ)
What it is	2 protons + 2 neutrons (helium nucleus)	A fast-moving electron	EM wave (high energy)
Charge	+2	−1	0
Penetration	Stopped by paper/skin (a few cm of air)	Stopped by ~3 mm aluminium	Stopped by thick lead/concrete
Ionising power	Strongest	Medium	Weakest
Speed	Slow	Fast	Speed of light
- In a magnetic field: α and β are deflected (opposite directions); γ is not deflected.
- Nuclear equations: in α decay, mass number −4, atomic number −2; in β decay, mass number unchanged, atomic number +1 (a neutron becomes a proton + electron).

DANGERS

• Radiation is ionising — it can knock electrons off atoms, damaging cells and DNA → mutations, cancer, burns.
• Alpha is most dangerous INSIDE the body (strongly ionising, easily absorbed by tissue); gamma is most dangerous outside (penetrates the body).
• Safety: distance, shielding (lead), time (minimise exposure), handling with tongs, never point sources at people.

ATOMIC NOTATION AND DECAY EQUATIONS (PHYSICS ONLY)

• Atomic notation: ᴬZX — X is the element symbol, A = mass number (protons + neutrons), Z = atomic number (protons). E.g. ²³⁸₉₂U.
• Alpha decay (α): the nucleus loses 2 protons + 2 neutrons (a helium nucleus, ⁴₂He): mass number −4, atomic number −2. ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
• Beta decay (β): a neutron changes into a proton, emitting an electron (⁰₋₁e): mass number unchanged, atomic number +1. ¹⁴₆C → ¹⁴₇N + ⁰₋₁e
• Gamma decay (γ): the nucleus releases excess energy as a gamma ray — the mass and atomic numbers do NOT change (it is often emitted alongside α or β).

✅ Quick check

1. What is an isotope? (Same protons, different neutrons.)
2. What are the three types of radiation? (Alpha, beta, gamma.)
3. Which is the most ionising? (Alpha.) Which penetrates the most? (Gamma.)
4. What is background radiation? (The low-level radiation always present in the environment.)
5. Why is radiation dangerous? (Ionising — damages cells and DNA.)

Chapter 23: Radiation and Half-Life

The activity of a radioactive source falls over time as unstable nuclei decay. The half-life measures how quickly this happens. This chapter covers activity, the half-life concept and calculations, and reading decay graphs.

---

ACTIVITY AND DECAY

• Activity — the rate at which a radioactive source decays (the number of decays per second), measured in becquerels (Bq).
• Activity falls over time because there are fewer unstable nuclei left to decay.
• Half-life — the time taken for half the nuclei (or half the activity) to decay. It is constant for a given isotope (you can't speed it up or slow it down).
• Short half-life = decays quickly (high activity at first, then it drops fast).
• Long half-life = decays slowly (low activity for a very long time).

HALF-LIFE CALCULATIONS

• Method 1 (counts): if a source starts at 800 counts/min and after 2 hours it's 200 counts/min:
• 800 → 400 (1 half-life) → 200 (2 half-lives) → so half-life = 2 ÷ 2 = 1 hour.
• Method 2 (decay graph): read off the time for the activity to halve; check it's the same time for each halving (800 → 400 → 200 → 100 at equal time intervals).
• Example: a source has half-life 10 days. After 30 days (3 half-lives), the fraction remaining = ½ × ½ × ½ = 1/8 of the original.

DECAY GRAPHS

• Axes: activity (or count rate) vs time.
• Shape: a smooth curve falling steeply at first, then levelling off.
• To find the half-life: pick any activity, note the time it halves, verify the next halving takes the same time.
• 💡 Exam tip: when the source is a sample measured with a detector, subtract the background radiation from the readings first.

USES DEPEND ON HALF-LIFE

• Medical tracers need a short half-life (so the radioactivity leaves the body quickly) — e.g. technetium-99m (6 hours).
• Carbon dating uses carbon-14 (half-life 5730 years) for ancient materials.
• Nuclear waste often has a very long half-life (thousands of years) — a storage problem.

DETECTING AND MEASURING RADIATION (practicals)

• Detecting ionising radiation:
• Geiger–Müller (GM) tube + counter: clicks/reading rises when radiation enters — the standard detector.
• Photographic film: radiation fogs the film (used in film badges worn by radiation workers — the darker the film, the higher the dose).
• 🔬 Practical — measuring the half-life of a radioactive source: place a source a fixed distance from a GM tube; record the count rate every 30 seconds for several minutes; subtract the background count (measured with no source) from each reading; plot corrected count rate against time; read off the time for the count rate to halve — that is the half-life.
• Radiation in living things: small amounts of radioactive isotopes (e.g. potassium-40, carbon-14) are present naturally in our bodies — a tiny part of background radiation.

✅ Quick check

1. Define half-life. (Time for half the nuclei/activity to decay.)
2. A source falls from 1200 to 300 Bq in 6 days. What is the half-life? (1200→600→300 = 2 half-lives in 6 days → half-life = 3 days.)
3. What fraction is left after 4 half-lives? (1/16.)
4. Why do medical tracers have short half-lives? (So they leave the body quickly — less damage.)
5. What shape is a decay graph? (A curve falling steeply then levelling off.)

Chapter 24: Applications of Radioactivity

Radioactivity is used in medicine (tracers, radiotherapy, sterilisation), industry (thickness gauges, smoke detectors) and archaeology (carbon dating). The choice of isotope depends on its type of radiation and half-life.

---

MEDICAL USES

1. MEDICAL TRACERS (diagnosis)

• A radioactive isotope (e.g. technetium-99m — gamma emitter, short half-life ~6 hours) is injected or swallowed; a detector (gamma camera) tracks where it goes.
• Why gamma? Gamma penetrates the body, so it can be detected from outside.
• Why short half-life? It decays quickly → less damage, leaves the body fast.
• Uses: checking kidney function, blood flow, thyroid function (iodine-131), bone scans.

2. RADIOTHERAPY (treatment of cancer)

• Gamma rays are aimed at cancerous tumours to kill the cancer cells (they damage the rapidly dividing cells).
• The beam is rotated / shaped so healthy tissue receives the least dose; doses are carefully controlled.

3. STERILISING MEDICAL EQUIPMENT

• Gamma rays kill bacteria and other microbes on surgical instruments and bandages (sterilisation) — even inside sealed packaging.
• Also used to sterilise food (irradiation — increases shelf life).

INDUSTRIAL USES

1. THICKNESS CONTROL

• A beta source and detector on either side of a production line (e.g. paper, metal foil, plastic sheeting):
• Too much material → fewer beta particles get through → the rollers are adjusted.
• Beta is used because it is absorbed by the material but still penetrates a little (gamma would pass straight through; alpha would be stopped completely).

2. SMOKE DETECTORS

• Contain a tiny alpha source (americium-241): alpha ionises the air, creating a small current.
• Smoke particles absorb the alpha particles → current falls → alarm sounds.
• Why alpha? It is strongly ionising and is safely stopped by the detector casing (low risk).

3. LEAK DETECTION / PIPELINE TRACING

• A gamma tracer added to a fluid finds leaks in underground pipes (detected from above ground).

ARCHAEOLOGICAL AND GEOLOGICAL USES

• Carbon dating: living things take in carbon-14 (half-life 5730 years); when they die it stops being replaced and the carbon-14 decays. Measuring the remaining carbon-14 tells the age of the material (up to ~50 000 years).
• Dating rocks: uranium decays to lead with a very long half-life — used to date rocks.

CHOOSING THE RIGHT ISOTOPE (exam skill)

• Alpha: strongly ionising, short range → smoke detectors, inside-the-body therapy (if it must deliver a big dose locally).
• Beta: medium penetration → thickness gauges.
• Gamma: penetrates → tracers, radiotherapy, sterilisation (from outside).
• Half-life must suit the job: short for medical tracers; long for dating; very long is a problem for waste.

✅ Quick check

1. Why is gamma used for medical tracers? (It penetrates the body so it can be detected outside.)
2. Why do tracers need a short half-life? (Less radiation damage; the body clears it quickly.)
3. What is radiotherapy? (Using gamma rays to kill cancer cells.)
4. Why is beta used in thickness gauges? (It is partly absorbed by the material, so the reading changes with thickness.)
5. How does a smoke detector work? (Alpha ionises air creating a current; smoke stops the alpha → current falls → alarm.)
6. What is carbon dating? (Measuring remaining carbon-14 (half-life 5730 years) to find the age of once-living material.)

Chapter 25: Fission and Fusion

Nuclear fission — splitting a large unstable nucleus (uranium/plutonium) to release energy — powers nuclear reactors and weapons. Nuclear fusion — joining small nuclei (hydrogen) — powers the Sun and stars. This chapter covers both and their pros and cons.

---

NUCLEAR FISSION

• Fission — the splitting of a large, unstable nucleus (e.g. uranium-235 or plutonium) into two smaller nuclei, releasing energy.
• How it happens: the uranium nucleus absorbs a neutron → becomes unstable → splits into two smaller nuclei (fission fragments) + 2 or 3 neutrons + energy (heat and gamma).
• Chain reaction: the neutrons released go on to split more nuclei → a self-sustaining chain reaction: neutron + U-235 → fission → 2 smaller nuclei + 2–3 neutrons + energy
• Controlled (nuclear reactor): control rods (boron/cadmium) absorb neutrons to keep the reaction steady; a moderator (water/graphite) slows the neutrons down (slow neutrons are more likely to cause fission). The heat boils water → steam → turbine → generator.
• Uncontrolled (nuclear bomb/accident): the chain reaction runs away — huge energy in an instant.

NUCLEAR FUSION

• Fusion — the joining together of two small nuclei (e.g. hydrogen isotopes deuterium and tritium) to form a larger nucleus (helium), releasing enormous energy.
• Conditions: extremely high temperature (millions of °C — for nuclei to overcome their mutual repulsion) and high pressure.
• Fusion is the process that powers the Sun and other stars.
• On Earth: fusion is being researched (e.g. ITER) but is very hard to sustain — no working fusion power stations yet. Hydrogen bombs use fission to trigger fusion.

COMPARING FISSION AND FUSION

	Fission	Fusion
Process	Big nucleus splits	Small nuclei join
Fuel	Uranium/plutonium	Hydrogen isotopes
Energy per kg	Huge	Even bigger
Waste	Radioactive waste (dangerous)	Very little (helium)
Status	Working reactors today	Still experimental
Where	Nuclear power stations	The Sun/stars

ADVANTAGES AND DISADVANTAGES OF NUCLEAR POWER (fission)

• Advantages: no CO₂/greenhouse gases (helps climate change); huge energy from small fuel; reliable.
• Disadvantages: radioactive waste (dangerous for thousands of years); risk of accidents (Chernobyl 1986, Fukushima 2011); high build/decommissioning costs; uranium is non-renewable.

✅ Quick check

1. What is nuclear fission? (Splitting a large unstable nucleus into two smaller ones, releasing neutrons and energy.)
2. What is a chain reaction? (The neutrons released split more nuclei — self-sustaining.)
3. What do control rods and the moderator do in a reactor? (Control rods absorb neutrons; the moderator slows them down.)
4. What is nuclear fusion? (Joining two small nuclei into a larger one, releasing huge energy — powers the Sun.)
5. Give one advantage and one disadvantage of nuclear power. (Adv: no CO₂, huge energy. Disadv: radioactive waste, accident risk.)

Chapter 26: Motion in the Universe

The Solar System — the Sun and everything orbiting it — is part of the Milky Way galaxy, one of billions of galaxies in the Universe. This chapter covers the structure of the Solar System, gravity's role, and the orbits of planets, moons, comets and artificial satellites.

---

THE SOLAR SYSTEM

• The Sun — a star at the centre (a huge ball of gas, mainly hydrogen, powered by nuclear fusion).
• Orbiting the Sun: eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — "My Very Educated Mother Just Served Us Noodles"), dwarf planets (e.g. Pluto), moons (orbit planets), asteroids (rocky, in the asteroid belt between Mars and Jupiter), and comets (icy bodies with elliptical orbits).
• Planets orbit the Sun in (roughly) circular orbits, all in the same direction.

GRAVITY AND ORBITS

• Gravity — the force of attraction between all masses. The Sun's gravity keeps the planets in orbit; a planet's gravity keeps its moons in orbit; the Earth's gravity keeps satellites in orbit.
• Why objects stay in orbit: an object moving sideways with the right speed keeps "falling" toward the planet but never reaches it — it follows a curved path (circular orbit). Gravity provides the centripetal force.
• Orbital speed: the closer an object is to the Sun/planet, the stronger the gravity and the faster it must move to stay in orbit (inner planets orbit faster than outer ones).

COMETS

• Comets have highly elliptical (elongated) orbits.
• Near the Sun: they heat up — gas and dust stream out (the tail points away from the Sun).
• Far from the Sun: they are cold and dark.

ARTIFICIAL SATELLITES

• Man-made satellites are put into orbit around the Earth for:
• Communications (geostationary satellites — orbit once per day above the equator, so they stay above one point; used for TV/phone).
• Weather monitoring (weather satellites).
• Navigation (GPS).
• Observation (spy/mapping satellites).
• The ISS — the International Space Station (low orbit, astronauts aboard).
• A satellite's speed must exactly match its orbital height (higher = slower).

THE UNIVERSE

• Milky Way — our galaxy: a huge collection of stars (including our Sun) held together by gravity.
• Galaxies — billions of stars, gas and dust; there are billions of galaxies in the observable Universe.
• The Universe contains everything — all galaxies and space. The Sun is one star among ~100 billion in the Milky Way; the Milky Way is one of billions of galaxies.

✅ Quick check

1. Name the eight planets in order. (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.)
2. What keeps planets in orbit? (The Sun's gravity — as the centripetal force.)
3. Why do inner planets orbit faster? (Stronger gravity closer in — they need more speed to stay in orbit.)
4. What is a geostationary satellite used for? (Communications — stays above one point.)
5. What is the Milky Way? (Our galaxy.)

Chapter 27: Stellar Evolution (The Life Cycle of Stars)

Stars are born from clouds of gas and dust (nebulas), shine by nuclear fusion, and eventually run out of fuel and die — becoming white dwarfs, neutron stars or black holes, depending on their mass.

---

HOW A STAR IS BORN

1. Nebula — a cloud of gas and dust (mainly hydrogen) in space.
2. Gravity pulls the cloud together → it contracts and gets denser and hotter.
3. Protostar — the hot, dense core forms. If it gets hot enough (millions of °C), nuclear fusion begins (hydrogen → helium), releasing huge energy.
4. Main sequence star — a stable star (like our Sun), where the outward pressure from fusion balances the inward pull of gravity. Stars spend most of their lives here.

WHAT HAPPENS WHEN THE HYDROGEN RUNS OUT

For SMALL/MEDIUM stars (like the Sun):

1. Red giant — the star swells up and cools (fusion of helium and heavier elements begins).
2. The outer layers drift away (a planetary nebula — a glowing shell of gas).
3. The core collapses into a white dwarf — a small, very dense, hot core that gradually cools and fades.
4. Eventually it becomes a black dwarf (cold, dead).

For LARGE/MASSIVE stars (much bigger than the Sun):

1. Red supergiant — swells even more.
2. Supernova — a massive explosion (the star blasts its outer layers into space; elements heavier than iron are made here and scattered — "we are made of star dust").
3. The core left behind becomes either: - Neutron star — an incredibly dense, tiny star (a teaspoon would weigh billions of tonnes), or - Black hole — if the core is massive enough, gravity is so strong that not even light can escape.

THE LIFE CYCLE DIAGRAM (draw this)

Nebula → Protostar → Main sequence star
                          ├── (small/medium) Red giant → Planetary nebula → White dwarf → Black dwarf
                          └── (large) Red supergiant → Supernova → Neutron star / Black hole

KEY POINTS

• Stars shine because of nuclear fusion (hydrogen → helium, then heavier elements).
• The balance between fusion pressure (out) and gravity (in) keeps a main sequence star stable.
• Our Sun is a medium star — it will end as a white dwarf.
• Heavy elements (including the ones in our bodies) were made in stars and supernovas.

✅ Quick check

1. What is a protostar? (The hot, dense core formed when a nebula contracts before fusion begins.)
2. What keeps a main sequence star stable? (Fusion pressure out balances gravity in.)
3. Describe the life cycle of a Sun-like star. (Nebula → protostar → main sequence → red giant → planetary nebula → white dwarf → black dwarf.)
4. What happens at a supernova? (A massive star explodes, making heavy elements and leaving a neutron star or black hole.)
5. What is a black hole? (A collapsed core so dense that not even light escapes.)

Chapter 28: Cosmology

Cosmology is the study of the origin and structure of the Universe. Evidence from red shift (Doppler effect) shows that galaxies are moving away from us — the Universe is expanding — which led to the Big Bang theory.

---

THE DOPPLER EFFECT

• Doppler effect — the change in the observed frequency (and wavelength) of waves when the source and observer are moving relative to each other.
• Sound example: an ambulance siren sounds higher-pitched as it approaches (waves squashed — shorter wavelength, higher frequency) and lower-pitched as it moves away (waves stretched — longer wavelength, lower frequency).

RED SHIFT (of light from galaxies)

• The same effect happens with light from distant galaxies:
• Galaxy moving away → its light waves are stretched → wavelength increases → the light shifts toward the red end of the spectrum → red shift.
• Galaxy moving towards us → wavelength decreases → blue shift (rare; e.g. Andromeda).
• KEY OBSERVATION: the light from almost all distant galaxies shows red shift — they are all moving away from us.
• The further away a galaxy is, the greater its red shift (the faster it is moving away) — this is Hubble's law.

THE EXPANDING UNIVERSE

• Red shift shows the Universe is expanding — galaxies are rushing apart.
• Imagine raisins in a rising loaf of bread: every raisin (galaxy) moves away from every other — there is no "centre".
• Because space itself is stretching, distant galaxies move away faster (more red shift).

THE BIG BANG THEORY

• If the Universe is expanding, then going backwards in time, everything was once together at a single point.
• Big Bang theory — the Universe began about 13.8 billion years ago from an incredibly hot, dense point, and has been expanding and cooling ever since.
• Evidence: 1. Red shift (expansion). 2. Cosmic microwave background radiation (CMB) — faint microwave radiation left over from the Big Bang, coming from all directions. 3. The abundance of light elements (hydrogen and helium) matches Big Bang predictions.
• Steady State theory (an alternative: Universe always existed, same density) was rejected because it can't explain the CMB.

✅ Quick check

1. What is the Doppler effect? (Change in observed frequency/wavelength when source and observer move relative to each other.)
2. What is red shift? (Light from a galaxy moving away is stretched to longer (redder) wavelengths.)
3. What does red shift tell us? (That galaxies are moving away — the Universe is expanding.)
4. What is the Big Bang theory? (The Universe began ~13.8 billion years ago from a hot, dense point and has expanded ever since.)
5. Give two pieces of evidence for the Big Bang. (Red shift/expansion; cosmic microwave background radiation; light element abundances.)