How to Ace IGCSE Physics Paper 4: Theory (Extended)

The Paper 4 problem

Here's the situation many IGCSE Physics students face: they've performed reasonably well on Paper 2 (Multiple Choice, Extended) and feel ready for the written exam. Then they open Paper 4 for the first time. The questions are longer. The calculations require three or four steps instead of one. Some topics appear that weren't on Paper 3 at all. And somewhere near the end, a 6-mark question demands a structured written response that most candidates have never properly practised.

Paper 4 is where Extended-tier physics separates itself from Core, and it requires a different approach. The good news: the problems on this paper follow patterns. Once you recognise those patterns, you can build a systematic method for tackling each question type. That's what this guide is for.

How Paper 4 differs from Paper 3

Paper 3 (Theory, Core) covers a subset of the syllabus. Paper 4 covers everything, including the Extended supplement content. Both are structured-response papers with short-answer and longer questions, but Paper 4 expects higher-order thinking: more analysis, more multi-step reasoning, and more precise use of technical language.

The duration and total marks are identical. What changes is the depth. Paper 4 questions assume you can handle the Core material quickly and push you into Extended territory where the reasoning becomes more demanding.

Extended-only content that catches people out

Several IGCSE Physics topics include Extended-only material that doesn't appear on Paper 3. These are the areas where Paper 4 candidates need extra preparation.

• Momentum: p = mv and the principle of conservation of momentum in collisions. Calculations involving two objects before and after a collision.
• Orbital speed and gravitational field strength: understanding why orbital speed changes with altitude, and the equation g = F/m for gravitational field strength.
• Thermal capacity and specific heat: E = mcΔT calculations, which often combine with energy transfer problems.
• The potential divider circuit: using V_out = V_in × R₂ / (R₁ + R₂), especially with thermistors and LDRs as variable resistors in sensor circuits.
• Electromagnetic induction details: Lenz's law and its connection to energy conservation, plus determining the direction of induced current.
• Nuclear equations: balancing alpha and beta decay equations using mass number and atomic number.

These topics regularly appear on Paper 4. Candidates who only revise Core material will find entire questions they simply cannot answer.

Multi-step calculations: a systematic approach

Single-step calculations (find the current given voltage and resistance) rarely appear alone on Paper 4. Instead, you'll face questions where the answer to part (a) feeds into part (b), which feeds into part (c). Missing one link breaks the entire chain.

Here's the method I recommend for any multi-step physics calculation:

1. Read the entire question before writing anything. Identify what you're asked to find at the end.
2. List every quantity given in the question, with units.
3. Identify which equation connects the known quantities to the unknown.
4. If the equation doesn't directly solve for the final answer, work out what intermediate value you need, and which equation gives you that.
5. Solve step by step, showing each substitution clearly.
6. Check units at every stage. If you're getting metres when you expected seconds, something went wrong.

Worked Example 1: Momentum and energy

A trolley of mass 2.0 kg travelling at 3.0 m/s collides with a stationary trolley of mass 1.0 kg. After the collision, the trolleys stick together. Calculate: (a) the velocity of the combined trolleys after the collision, (b) the kinetic energy before and after the collision, (c) whether kinetic energy is conserved.

1. Conservation of momentum.
   Total momentum before = total momentum after.
   m₁v₁ + m₂v₂ = (m₁ + m₂) × v
   (2.0 × 3.0) + (1.0 × 0) = (2.0 + 1.0) × v
   6.0 = 3.0v
   v = 2.0 m/s
2. Kinetic energy before.
   KE = 0.5 × m × v²
   KE before = 0.5 × 2.0 × 3.0² = 0.5 × 2.0 × 9.0 = 9.0 J
   (The stationary trolley contributes zero KE.)
3. Kinetic energy after.
   KE after = 0.5 × 3.0 × 2.0² = 0.5 × 3.0 × 4.0 = 6.0 J
4. Compare.
   KE before (9.0 J) does not equal KE after (6.0 J). Kinetic energy is not conserved. The missing 3.0 J has been converted to heat and sound during the inelastic collision.

Notice how each step builds on the previous one. The examiner gives marks for each stage, so even if you make an arithmetic error in part (a), using your incorrect answer correctly in parts (b) and (c) still earns follow-through marks. This is a crucial point: always show your working, and always carry your answer forward.

Worked Example 2: Transformer and power loss

A power station generates electricity at 25 000 V. A step-up transformer with 500 primary turns and 10 000 secondary turns increases the voltage for transmission. The transmission cables have a total resistance of 4.0 Ω, and the power transmitted is 500 kW.

(a) Calculate the transmission voltage. (b) Calculate the current in the cables. (c) Calculate the power lost as heat in the cables.

1. Transmission voltage.
   V_s / V_p = N_s / N_p
   V_s = V_p × N_s / N_p = 25 000 × 10 000 / 500 = 500 000 V
2. Current in cables.
   P = IV, so I = P / V = 500 000 / 500 000 = 1.0 A
3. Power lost.
   P_lost = I²R = 1.0² × 4.0 = 4.0 W

That's only 4.0 W lost out of 500 000 W transmitted. This is precisely why the grid uses high-voltage transmission: stepping up the voltage forces the current down, and since power loss depends on I², even a small reduction in current produces a dramatic drop in wasted energy.

Structuring 6-mark extended responses

Paper 4 includes at least one question worth 6 marks that asks you to explain a process or describe how something works. These aren't calculation questions. They test whether you can construct a logical argument using correct physics terminology.

Most candidates lose marks here not because they don't know the physics, but because their answer is disorganised. The examiner works through a mark scheme with 6 specific points. Your job is to hit all 6 clearly.

Here's a decision framework for approaching these questions:

1. Does the question say "describe" or "explain"?
   Describe = state what happens, in order.
   Explain = state what happens AND give the physics reason for each step.
2. Is there a sequence of events? If yes, use chronological order. Number your points.
3. Are there cause-and-effect links? Make them explicit. Use words like "because", "which causes", "resulting in".

Before and after: a 6-mark response on how a transformer works

Weak answer (likely 2/6):

"A transformer has two coils. The current goes in one coil and comes out the other. It changes the voltage."

This answer is vague. It doesn't mention AC, doesn't mention the core, doesn't explain why the voltage changes, and uses no technical terms correctly.

Strong answer (likely 6/6):

1. An alternating current in the primary coil produces a changing magnetic field.
2. The soft iron core channels this changing magnetic field to the secondary coil.
3. The changing magnetic field through the secondary coil induces an alternating e.m.f. (electromagnetic induction).
4. The ratio of output voltage to input voltage equals the ratio of secondary turns to primary turns: V_s/V_p = N_s/N_p.
5. If the secondary coil has more turns than the primary, the output voltage is higher (step-up transformer).
6. If fewer turns, the output voltage is lower (step-down transformer).

Six distinct physics points, stated precisely, in logical order. Each sentence earns a mark.

Time management on Paper 4

You have 75 minutes for 80 marks. That works out to just under 1 minute per mark. Here's how to allocate that time:

• A 2-mark question deserves about 2 minutes. Write two clear points or show a quick calculation.
• A 4-mark calculation deserves about 4 minutes. Show working, substitute values, give the final answer with units.
• A 6-mark extended response deserves about 6-7 minutes. Plan your answer briefly (30 seconds), then write 6 clear points.

That pacing leaves roughly 5 minutes at the end to check your work. Use those 5 minutes. Specifically, check three things:

1. Units. Did you convert minutes to seconds? Centimetres to metres? Grams to kilograms?
2. Significant figures. Give your answer to the same number of significant figures as the data in the question (usually 2 or 3).
3. Reasonableness. If you calculated that a car has a speed of 5000 m/s, something went wrong. If a current came out negative, check whether that makes physical sense.

Choosing the right equation

Paper 4 questions rarely tell you which formula to use. You're expected to select it yourself. Here's a reference for the most commonly tested areas:

Don't try to memorise equations in isolation. Instead, practise selecting the right one from a scenario. Past paper questions are the best training ground for this: each one forces you to read a physical context and match it to the correct physics.

Five common traps on Paper 4

1. Forgetting the Extended content. If you revised only from Core notes, topics like momentum, potential dividers, and Lenz's law will blindside you. Use a syllabus checklist and tick off every Extended supplement topic.
2. Showing no working. Even if your final answer is correct, a calculation without working scores fewer marks than you'd expect. If the answer is wrong and there's no working, you get zero. If the answer is wrong but the method is visible, you still earn method marks.
3. Writing "it" instead of naming the quantity. "It increases" tells the examiner nothing. "The current increases" earns the mark. Always name the physical quantity explicitly.
4. Mixing up proportionality directions. "Resistance increases so current increases" is backwards for constant voltage. Double-check whether a relationship is direct or inverse before committing to your answer.
5. Leaving 6-mark questions to the end and rushing them. These questions carry the same weight as six individual 1-mark questions. Budget the time properly and answer them in full.

Your practice strategy

Working through past papers is the single most effective way to prepare for IGCSE Physics Paper 4. But doing it poorly wastes time. Here's a structured approach:

1. Do a full paper under timed conditions (75 minutes, no notes, no equation sheet).
2. Mark it against the mark scheme immediately.
3. For every mark you lost, categorise the reason: didn't know the content, knew it but made a calculation error, knew it but didn't express it precisely enough, or ran out of time.
4. Focus your next revision session on the "didn't know" category. Calculation errors and expression problems fix themselves with continued practice.
5. Redo the same paper two weeks later without looking at the mark scheme. Your score should improve. If it doesn't, you haven't actually internalised the material.

Cambridge publishes mark schemes and examiner reports for every past session. The examiner reports are especially valuable because they tell you exactly where candidates went wrong. Reading three or four of these reports honestly teaches you more about what the examiners look for than doing an extra past paper would.

Paper 4 doesn't demand brilliance. It demands precision, structure, and the discipline to show every step. Candidates who practise those three habits systematically almost always see their scores improve within two or three papers. That's not a guess. It's the logical outcome of working through the patterns that this paper tests, one mark at a time.