AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2021: Question 5

📐 Calculations: step-by-step

1. Use the power equation: P = I × V
2. Convert the power first: 240 mW = 0.24 W
3. Substitute values: 0.24 = I × 2.5
4. Rearrange: I = 0.24 ÷ 2.5
5. Calculate: I = 0.096 A

Maximum current = 0.096 A

✅ Correct answer

• 240 mW = 0.24 W
• 0.24 = 2.5 × I
• I = 0.24 ÷ 2.5
• I = 0.096 A

Why this gets 4 marks: one mark for converting units, one for substitution, one for rearranging, and one for the final correct current.

💡 Key knowledge

• Power is measured in watts (W).
• Current is measured in amperes (A).
• Potential difference is measured in volts (V).
• mW must be changed into W before using the equation.

🧠 Exam technique

• Always write the formula first: P = I × V .
• Show the unit conversion clearly to gain the first mark.
• Keep the answer in A, not mA, unless you convert it correctly.
• If you use the wrong power value but the method is right, you can still gain method marks.

❌ Common errors

• Forgetting to convert 240 mW into 0.24 W .
• Using I = P × V instead of dividing.
• Writing the answer as 0.096 mA instead of 0.096 A .
• Leaving out units completely.

✅ Correct answer

The potential difference is always positive.

The graph shows the voltage changing in size, but it does not change direction because it never goes below zero.

💡 Key knowledge

• Alternating potential difference must change direction.
• On a graph, that means the line should go above and below zero.
• If it stays above zero, it is pulsing DC or a positive-only signal, not true alternating p.d.

🧠 Exam technique

• Use the graph evidence directly: “it stays above zero” or “it does not cross zero”.
• The mark scheme accepts either always positive or does not change direction.

❌ Common errors

• Saying “it goes up and down” without mentioning zero or direction.
• Thinking any wavy graph is automatically alternating.
• Writing “alternating current” when the question is about potential difference.

✅ Correct answer

Potential difference is directly proportional to current.

💡 Key knowledge

• Direct proportion means if one increases, the other increases by the same factor.
• This is the usual relationship for an ohmic conductor at constant temperature.
• A straight line through the origin shows this relationship.

🧠 Exam technique

• Use the words directly proportional for full credit.
• If unsure, describe the pattern as “the more potential difference, the more current”.

❌ Common errors

• Saying “current causes voltage” without describing the relationship.
• Mixing up inverse and direct proportionality.
• Forgetting that the graph is a straight line through the origin.

✅ Correct answers

Voltmeter: the reading is the same in both circuits.

Ammeter: the reading in circuit B is twice the reading in circuit A.

💡 Key knowledge

• The voltmeter measures the p.d. across one resistor in each circuit.
• Because the resistors have the same resistance, the p.d. across each one is the same in both circuits.
• Circuit B has two resistors in parallel, so the total current is larger.
• In a parallel circuit, the total current is the sum of the branch currents.

🧠 Exam technique

• For the voltmeter, say “same” clearly: “the reading is the same in both circuits”.
• For the ammeter, state the size of the difference: “twice” is the full-mark answer.
• Marks are awarded for comparing the readings, not for describing the diagram.

❌ Common errors

• Saying the voltmeter reading is bigger in circuit B.
• Saying the ammeter reading is “higher” without explaining how much higher.
• Confusing series and parallel rules.
• Thinking the voltage is shared between parallel branches in the same way as series circuits.

✅ What distinguished top answers

• Clear unit conversion before calculation.
• Correct rearrangement of P = I × V .
• Using graph evidence, especially “always positive”.
• Precise comparison words: “same” and “twice”.

❌ Where students lost marks

• Not converting mW to W.
• Writing vague explanations about AC.
• Using “directly proportional” incorrectly or not at all.
• Giving only one circuit reading instead of comparing both.