AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2022: Question 5

✅ Correct answer

Reverse the connections to the LED or battery.
The LED must be connected the correct way round so it is forward-biased and current can flow.

💡 Key knowledge

• An LED is a diode.
• A diode only allows current to flow in one direction.
• If it is reversed, it has a very large resistance and no current flows.

🧠 Exam technique

• To get both marks, give the change and the reason.
• Use the phrase reverse the LED/battery or reverse the potential difference.
• Link the change to current flow and the LED lighting.

❌ Common errors

• Just saying “increase the current” is too vague.
• Writing “make the battery stronger” does not explain the physics.
• Some students forgot the key idea that a diode only works in one direction.

✅ Correct answer

3.4 V (to 2 significant figures)

📐 Calculations: step by step

1. Use the power equation:
   P = IV
2. Rearrange to make voltage the subject:
   V = P ÷ I
3. Convert the current from mA to A:
   290 mA = 0.29 A
4. Substitute values:
   V = 0.98 ÷ 0.29
5. Calculate:
   V = 3.379...
6. Round to 2 significant figures:
   V = 3.4 V

💡 Key knowledge

• Power, current and voltage are linked by P = IV .
• Current must be in amps, not milliamps, when using SI units.
• 2 significant figures means the answer should match the given data.

🧠 Exam technique

• Show the equation first, then rearrange clearly.
• Always convert 290 mA to 0.29 A before substituting.
• Include units in the final answer: V.

❌ Common errors

• Using 290 instead of 0.29 gives the wrong answer by a factor of 1000.
• Using the wrong rearrangement, such as V = 0.98 × 0.29 .
• Forgetting to round to 2 significant figures.
• Mixing up power and energy.

✅ Correct answer

Electrons in the atom fall to a lower energy level.

💡 Key knowledge

• In a fluorescent tube, electrons in the atom become excited first.
• When they return to a lower energy level, energy is released as light.
• This is the key idea behind fluorescence in xenon gas.

🧠 Exam technique

• Pick the option about falling to a lower energy level.
• The mark scheme wants the precise physics idea, not a general description of “giving off light”.

❌ Common errors

• Choosing “move to a higher energy level” is the opposite of emission.
• “Electrons leave the atom” is ionisation, not light emission.
• “Electrons transfer from the electrical circuit” is not the explanation here.

✅ Correct answer

25 A

📐 Calculations: method 1 using charge

1. Use the equation:
   E = VQ
2. Rearrange to find charge:
   Q = E ÷ V
3. Substitute values:
   Q = 1.4 ÷ 200
4. Calculate charge transferred:
   Q = 0.0070 C
5. Use:
   Q = It
6. Rearrange for current:
   I = Q ÷ t
7. Substitute values:
   I = 0.0070 ÷ 2.8 × 10⁻⁴
8. Calculate:
   I = 25 A

📐 Calculations: method 2 using power

1. First find power using:
   P = E ÷ t
2. Substitute values:
   P = 1.4 ÷ 2.8 × 10⁻⁴
3. Calculate:
   P = 5000 W
4. Use:
   P = IV
5. Rearrange:
   I = P ÷ V
6. Substitute values:
   I = 5000 ÷ 200
7. Calculate:
   I = 25 A

💡 Key knowledge

• E = VQ links energy, potential difference and charge.
• Q = It links charge, current and time.
• P = E ÷ t gives power from energy transferred in time.
• Use standard units: V, C, A, s, W.

🧠 Exam technique

• Underline the given numbers and units before starting.
• Be careful with the time: 2.8 × 10⁻⁴ s is very small.
• Give the answer with the correct unit: A.
• Either method is acceptable if every step is shown clearly.

❌ Common errors

• Not using the time in seconds.
• Forgetting to divide by the time after finding charge.
• Using I = V ÷ Q or another incorrect rearrangement.
• Mixing up energy and power.

💡 Strong features

• They used the right physics idea, not just the right number.
• They converted units correctly, especially mA to A.
• They showed working in a logical order.
• They gave answers with correct units and sensible rounding.

🧠 Quick checklist for full marks

• Read the command word carefully: explain, calculate, or tick.
• Use one equation at a time.
• Check whether current must be converted.
• Look back at the question to see how many significant figures are needed.