Mark Scheme

Section A — Structured Questions

1. (a) charge = current × time (Q = I t) (1); unit of charge = coulomb (C) (1). [2] (b) t = 2 min = 120 s (1); Q = I t = 3 × 120 (1) = 360 C (1). [3] (c) Power P = V I = 12 × 3 = 36 W (M1 A1); energy = P × t = 36 × 120 = 4320 J (M1 A1). [4]

2. (a) Efficiency = useful energy out ÷ total energy in = 600 ÷ 800 (M1) = 0.75 (or 75%) (A1). [2] (b) It is transferred (dissipated) to the surroundings as heat/thermal energy (and some as sound) (1). [1] (c) (up to 3) Two ways: reduce energy lost as heat through friction (lubrication) or better insulation; use more efficient components/motors; reduce sound/vibration losses. 1 + development. [3] (d) (up to 2) Advantage — renewable/does not produce carbon dioxide or air pollution; Disadvantage — only generates when it is windy (unreliable/intermittent) / visual or noise impact. 1 each. [2]

3. (a) (up to 2) Speed is how fast something moves (a scalar, magnitude only); velocity is speed in a given direction (a vector). [2] (b) a = (v − u) ÷ t = (20 − 8) ÷ 6 (M1 M1) = 12 ÷ 6 = 2 m/s² (A1). [3] (c) F = m a = 1200 × 2 = 2400 N (M1 A1); if the car travels faster, the braking distance increases (the car has more kinetic energy, so more work must be done to stop it / it travels further while braking) (M1 A1). [4]

4. (a) (up to 3) Transverse — oscillations are perpendicular to the direction of energy travel (e.g. light/water/EM waves); longitudinal — oscillations are parallel to the direction of travel (e.g. sound). Difference (1) + example each (1 + 1). [3] (b) v = f λ = 50 × 6 (M1) = 300 m/s (A1). [2] (c) (up to 3) When a wave passes into a different medium and changes speed, it usually changes direction; this is called refraction; (the wavelength changes but the frequency stays the same). [3]

5. (a) (up to 3) The atom has a small central nucleus containing protons (positive) and neutrons (neutral); electrons (negative) orbit the nucleus in shells; most of the atom is empty space. [3] (b) (up to 2) Half-life = the time taken for half the undecayed nuclei in a sample to decay (or for the count rate/activity to halve). [2] (c) (up to 3) 15 days = 3 half-lives (15 ÷ 5) (M1); 800 → 400 → 200 → 100 (halving each half-life) (M1); 100 undecayed nuclei remain (A1). [3]

Section B — Extended Response

6. (a) (up to 4) Transmitting at high voltage means a lower current for the same power (P = V I); a lower current reduces the energy lost as heat in the cables (heating loss depends on current², I²R); so transmission is more efficient and less energy is wasted. [4] (b) (up to 4) A step-up transformer increases the voltage (more turns on the secondary than the primary) and is used at the power station end to raise voltage for transmission; a step-down transformer decreases the voltage (fewer turns on the secondary) and is used near homes/consumers to reduce it to a safe, usable level. [4] (c) (up to 2) Vs/Vp = Ns/Np → Vs = Vp × (Ns/Np) = 240 × (5000/100) = 240 × 50 = 12 000 V (12 kV) (M1 A1). [2]

7. (up to 6) Indicative content, in order: a star forms from a cloud of dust and gas (nebula) pulled together by gravity, forming a protostar; when hot/dense enough, nuclear fusion of hydrogen begins and it becomes a stable main-sequence star (gravity balanced by outward pressure from fusion); when the hydrogen runs low it expands and cools to a red giant; it then sheds its outer layers, leaving a white dwarf, which cools and fades. Levels: 5–6 = full sequence, correct order, key terms; 3–4 = several stages; 1–2 = basic. [6]

Sample Answers with Examiner Commentary

Question 5(c) — Sample Answers

Grade A response. "The half-life is 5 days and the total time is 15 days, so the number of half-lives is 15 ÷ 5 = 3. After each half-life the number of undecayed nuclei halves. Starting from 800: after one half-life (5 days) there are 400, after two (10 days) there are 200, and after three (15 days) there are 100. So 100 undecayed nuclei remain after 15 days."

Mark: 3/3. Examiner commentary: A clear, fully-shown method. The candidate first works out the number of half-lives (3), then halves the quantity step by step, showing the value at each stage. Because every step is visible, the method marks are secure, and the final answer is correct. Setting out the successive halving (800 → 400 → 200 → 100) is exactly the approach examiners want to see, and it avoids the common slip of dividing 800 by 3.

Grade C response. "800 divided by 3 is about 267, so there are about 267 nuclei left after 15 days."

Mark: 0/3. Examiner commentary: This is the classic misunderstanding of half-life. The candidate has divided the number of nuclei by the number of half-lives, but half-life means the quantity halves each period, not that it is divided by the number of periods. The correct approach is to find the number of half-lives (3) and then halve the sample three times: 800 → 400 → 200 → 100. No marks can be awarded because neither the method nor the answer is correct, but the fix is straightforward once the meaning of half-life is understood.

Question 6(a) — Sample Answers

Grade A response. "Electricity is transmitted at high voltage because, for a given amount of power, a higher voltage means a lower current (since power = voltage × current). The energy lost as heat in the transmission cables depends on the current — it is proportional to the current squared — so reducing the current greatly reduces the energy wasted as heat in the wires. This makes transmission much more efficient over long distances, which is why the National Grid uses very high voltages and transformers to step the voltage up before transmission."

Mark: 4/4. Examiner commentary: A complete explanation that links the ideas in the correct chain: high voltage → lower current (via P = VI) → less heating loss in the cables (because loss depends on current²) → more efficient transmission. The key discriminator, which weaker answers miss, is explaining why a lower current reduces losses — the dependence of heating on the current, not the voltage. This response makes that link explicitly and earns full marks.

Grade C response. "Electricity is sent at high voltage so that less energy is lost as heat in the wires and it is more efficient."

Mark: 2/4. Examiner commentary: The candidate correctly states the outcome — less energy lost as heat and greater efficiency — which earns two marks. However, the question asks the candidate to explain, and the reasoning is missing: why does a high voltage reduce heat loss? The answer needs to make the link that a higher voltage allows a lower current for the same power, and that it is the lower current that reduces the heating loss in the cables. Adding that chain of reasoning would gain the remaining two marks.

Question 3(c) — Sample Answers

Grade A response. "To find the resultant force I use F = m × a. The mass is 1200 kg and the acceleration, from part (b), is 2 m/s², so F = 1200 × 2 = 2400 N. For the second part, if the car is travelling faster its braking distance increases. This is because a faster car has more kinetic energy (kinetic energy depends on the square of the speed), so the brakes must do more work to stop it. Since the braking force is roughly the same, the car travels a greater distance while that work is being done, so the braking distance is longer."

Mark: 4/4. Examiner commentary: Both parts are answered fully. The calculation is set out with the equation, the substitution and the answer with the correct unit, securing the calculation marks. The explanation then correctly links a higher speed to greater kinetic energy and, importantly, explains why that increases the braking distance — more work must be done by a roughly constant braking force, so the car travels further. Explaining the mechanism, not just stating "it takes longer to stop", is what earns the explanation marks.

Grade C response. "F = m × a = 1200 × 2 = 2400 N. If the car goes faster the braking distance is bigger because it is going faster and harder to stop."

Mark: 3/4. Examiner commentary: The calculation is completely correct and clearly shown, earning its marks, and the candidate knows that a higher speed means a longer braking distance. The final mark is lost because the explanation is circular — "bigger because it is going faster... harder to stop" restates the claim without giving a reason. The missing idea is that a faster car has more kinetic energy, so the brakes must do more work to stop it and the car travels further while braking. One sentence of physics reasoning would complete the answer.