AQA GCSE Physics Physics Paper 1 (Higher), November 2020: Question 2
11 marks · Standard Demand difficulty · Short Answer
Analyze an LED torch circuit, calculate charge flow, explain the effect of reversing a diode, and calculate power efficiency in a series of structured sub-questions.
Practise this questionQuestion
Question text
02 Figure 3 shows an LED torch.
Figure 3
02.1 The torch contains one LED, one switch and three cells.
Which diagram shows the correct circuit for the torch?
[1 mark]
Tick ( ) one box.
02.2 Write down the equation which links charge flow (Q), current (I) and time (t).
[1 mark]
02.3 The torch worked for 14 400 seconds before the cells needed replacing.
The current in the LED was 50 mA.
Calculate the total charge flow through the cells.
[3 marks]
Total charge flow = C
02.4 When replaced, the cells were put into the torch the wrong way around.
Explain why the torch did not work.
[2 marks]
02.5 Write down the equation which links efficiency, total power input and useful power
output.
[1 mark]
*0072.*6 The total power input to the LED was 0.24 W.
The efficiency of the LED was 0.75
Calculate the useful power output of the LED.
[3 marks]
Useful power output = W
Mark scheme
Show the mark scheme
Question 2
AO /
Question Answers Extra information Mark
Spec. Ref.
02.1 AO1
1 4.2.1.1
02.2 charge flow = current × time 1 AO1
or 4.2.1.2
Q = It
02.3 AO2
I = 0.050 (A) 1 4.2.1.2
Q = 0.050 × 14 400 allow a correct substitution using 1
an incorrectly/not converted
value of I
Q = 720 (C) allow a correct calculation using 1
an incorrectly/not converted
value of I
02.4 there is no current in a diode (in allow diode will not conduct 1 AO1
the reverse direction) (electric charge) 4.2.1.4
or 4.2.1.3
charge will not flow through a
diode (in the reverse direction)
do not accept the circuit is not
complete
(because) a diode has a (very) 1
high resistance (in the reverse
direction)
02.5 AO1
Useful power output 1 4.1.2.2
Efficiency =
Total power input
Question Answers Extra information Mark AO /
Spec. Ref.
02.6 AO2
Useful power output 1 4.1.2.2
0.75 =
0.24
Useful power output = 0.75 × 1
0.24 9
Useful power output = 0.18 (W) 1
Total 11
How to answer it
LED Torch Circuits and Calculations Study Guide
This question assesses your understanding of basic circuit symbols, component behaviour (specifically diodes and LEDs), charge flow calculations, and efficiency formulas. You will need to recall standard physics equations, convert units (such as milliamps to amps), and explain electrical concepts logically.
Identifying the Correct Circuit Diagram
✅ Correct Answer
The first (left-hand) diagram must be ticked. In this diagram, all three cells face the same direction (terminals aligned correctly) and the LED symbol points in the forward-biased direction to allow current to flow through the circuit when the switch is closed.
💡 Key Knowledge
- Cells in series: Must face the same way so their potential differences add up (positive terminal connected to the negative terminal of the next).
- LED Symbol: A triangle pointing in the direction of conventional current flow, with arrows pointing outwards representing emitted light.
Equation Linking Charge Flow, Current, and Time
✅ Correct Answer
charge flow = current × time (or Q = I × t )
🧠 Exam Technique
You can write either words or standard accepted symbols. Avoid ambiguous abbreviations. Memorising standard equation triangles or formula lists is essential for these guaranteed recall marks.
Calculating Total Charge Flow
📐 Step-by-Step Calculation
- Identify values from the question: Time ( t ) = 14 400 s; Current ( I ) = 50 mA.
- Convert units (Crucial!): Convert milliamperes to amperes.
50 mA ÷ 1000 = 0.050 A - Substitute into the equation: Q = I × t
Q = 0.050 × 14 400 - Calculate final answer: Q = 720 C
❌ Common Errors
- Forgetting to convert milliamperes (mA) into amperes (A), leading to an incorrect answer of 720 000 C .
- Note: The mark scheme allows ecf (error carried forward) if substitution was done using un-converted values, but full marks require correct unit conversion.
✅ Final Answer Line
Total charge flow = 720 C
Explaining Why the Torch Did Not Work
✅ Correct Answer
An LED is a diode. When the cells are put in the wrong way around, current tries to flow in the reverse direction. Diodes have a very high resistance in the reverse direction, meaning no current can flow and the LED cannot light up.
❌ Common Errors
Students often lose the second mark by vaguely stating "the circuit is not complete". Examiners strictly penalize this because the loop of wire is physically complete; the issue is electrical resistance and diode properties, not a broken wire.
Equation Linking Efficiency, Power Input, and Power Output
✅ Correct Answer
Efficiency = Useful power output ÷ Total power input
💡 Key Knowledge
Efficiency can be expressed as a ratio (between 0 and 1) or as a percentage. When using this specific equation layout, ensure output is on top (numerator) and input is on the bottom (denominator).
Calculating Useful Power Output
📐 Step-by-Step Calculation
- Identify known values: Efficiency = 0.75; Total power input = 0.24 W.
- Rearrange the efficiency formula:
Useful power output = Efficiency × Total power input - Substitute values:
Useful power output = 0.75 × 0.24 - Calculate final value:
0.18 W
🧠 Exam Technique
Always show your working clearly by writing out the rearranged equation before substituting numbers. This guarantees method marks even if a minor arithmetic slip occurs.
✅ Final Answer Line
Useful power output = 0.18 W
Topics
Physics · P1: Energy · P2: Electricity
Question and mark scheme from the AQA GCSE Physics examination, Physics Paper 1 (Higher), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.