The 6-Mark Extended Response: A Repeatable Technique

Long explain questions on Papers 3 and 4 are worth 4-6 marks each, and they decide more grades than any other question type because answers feel right while scoring 1 or 2. The fix is a named, repeatable structure. We teach one called Point-Physics-Link (PPL), and this page shows it working on two full model answers.

How does Cambridge actually mark extended responses?

One mark per correct, linked physics point on the examiner’s list, marked B1 style. There is no general impression mark and, unlike some UK boards’ level-marked answers, no credit for fluent writing. Six marks means the scheme holds roughly six creditable statements, and your job is to hit them one sentence at a time.

This has a liberating consequence. You are not writing an essay. You are laying a chain of short physics statements, each one creditable on its own. Bullet points are fine. Beautiful prose earns nothing extra; a missing link in the chain costs a mark every time.

The Point-Physics-Link method

Every PPL sentence has three jobs, and a full answer repeats the cycle until the chain reaches the thing the question asked about:

1. Point. State what happens, one event only. “The molecules move faster.”
2. Physics. Name the principle behind it. “Because their average kinetic energy increases with temperature.”
3. Link. Connect to the next step or the question’s target. “So they collide with the container walls more often and with more force.”

Then a four-step routine for the whole question:

• Step 1: Decode the command word. “Explain” wants causes and mechanisms; “describe” wants observations; “explain why” wants a chain ending at the stated effect.
• Step 2: Count the marks, plan the points. Six marks, so jot six one-word stepping stones in the margin before writing. Ten seconds of planning prevents the classic three-mark answer to a six-mark question.
• Step 3: Write the chain in PPL sentences, one point per sentence, shortest words that are still precise. Use the syllabus terms: “average kinetic energy”, not “energy stuff”; “rate of evaporation”, not “it dries faster”.
• Step 4: Audit the ends. Does sentence one start from the cause in the question? Does the last sentence state the exact effect asked about? Chains that stop one link early lose the final mark constantly.

Model answer 1: thermal physics (6 marks)

Question: A sealed metal can contains air. The can is left in strong sunshine. Explain, using the kinetic particle model, why the pressure of the air inside the can increases. [6]

Margin plan: radiation → temperature → KE → speed → collisions → pressure.

Model answer:

1. The can absorbs infrared radiation from the Sun, so the air inside heats up. (B1)
2. The temperature of the air increases, so the average kinetic energy of the air molecules increases. (B1)
3. The molecules therefore move faster on average. (B1)
4. Faster molecules collide with the can walls more frequently. (B1)
5. Each collision also exerts a greater force, because the change in momentum per impact is larger. (B1)
6. The volume is constant, so a greater total force on the same wall area means the pressure increases. (B1)

Six sentences, six marks. Notice what is absent: no repetition, no “as we all know”, no hedging. Each sentence names one mechanism and hands over to the next. Sentence 5 is the one most students miss; they say “more collisions” and stop, leaving the force-per-collision mark uncollected.

Model answer 2: electromagnetism (6 marks)

Question: Explain how a step-up transformer increases voltage, and why electrical power is transmitted at high voltage. [6]

Margin plan: a.c. → changing field → core → induced e.m.f. → turns ratio → $I^2R$ losses.

Model answer:

1. Alternating current in the primary coil produces a continuously changing magnetic field. (B1)
2. The soft-iron core carries this changing field through the secondary coil. (B1)
3. The changing field induces an alternating e.m.f. in the secondary coil (electromagnetic induction). (B1)
4. The secondary coil has more turns than the primary, so the induced e.m.f. is larger than the input voltage, in the ratio $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$. (B1)
5. For the same power, a higher transmission voltage means a smaller current in the cables. (B1)
6. Power wasted as heat in the cables equals $I^2R$, so a smaller current cuts the losses sharply. (B1)

The split question is a classic trap: students write four good sentences about transformers, zero about transmission, and cap themselves at 4. The margin plan catches it because the plan visibly covers both halves before writing starts.

Why do students who know the physics still score 2 out of 6?

Three reasons, all structural rather than conceptual. They compress three points into one sentence, so the examiner can credit only what is explicit. They skip “obvious” links, like stating that temperature raises average kinetic energy. And they answer a neighbouring question, describing transformers when asked to explain transmission voltage. PPL attacks all three by forcing one explicit point per sentence and a plan that mirrors the question.

A quick self-test you can run today: take any past-paper 5- or 6-mark question, write your answer, then number every distinct physics point in it. Students are routinely shocked to find four sentences containing two creditable points. The mark scheme then shows the missing links, and they are nearly always the “too obvious to write” ones.

“Isn’t this formulaic? Won’t every answer sound the same?” Yes, and that is the point. Examiners credit content, not originality, and a formula that reliably surfaces six creditable statements beats inspiration that surfaces three. Cambridge mark schemes are themselves formulaic; PPL simply writes in their format.

Extended responses are the skill our tutors drill hardest in 1-to-1 classes, because they respond fastest to feedback: a student writes the chain, the tutor marks it against the real Cambridge scheme on screen, and the missing links become visible within minutes. Founder Rig built this into our standard 1.5-hour lesson structure, and the free trial hour usually includes one PPL question so parents can watch the before-and-after on the same day.

Building the habit

Train on volume: one extended-response question per day for three weeks, always with the margin plan, always self-marked against the official scheme. Keep a running list of your personally skipped links; most students have three favourites, such as omitting “average kinetic energy” or stopping before the final effect. By week three the plan takes ten seconds and the chain writes itself.

Plan the stepping stones, write one point per sentence, end on the exact thing asked. Six marks stop being a lottery and become the most predictable marks on the theory paper.