AQA GCSE Physics Physics Paper 1 (Higher), November 2020: Question 9
10 marks · Standard Demand difficulty · Short Answer
Calculate the resistance of a person given power and potential difference, explain why an electrician needs to disconnect a circuit before replacing a switch, and use a graph of maximum let-go current versus frequency to explain why a different frequency would be safer.
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Question text
09 Figure 13 shows part of a mains electricity lighting circuit in a house.
Figure 13
09.1 A fault in the switch caused a householder to receive a mild electric shock before a
safety device switched the circuit off.
The mean power transfer to the person was 5.75 W.
The potential difference across the person was 230 V.
Calculate the resistance of the person.
[5 marks]
Resistance = Ω
09.2 An electrician replaced the switch.
The electrician would have received an electric shock unless the circuit was
disconnected from the mains supply.
Explain why.
[3 marks]
09.3 The current from an electric shock causes a person’s muscles to contract. The
person cannot let go of the electrical circuit if the current is too high.
Figure 14 shows how the maximum current at which a person can let go depends on
the frequency of the electricity supply.
Figure 14
The UK mains frequency is 50 Hz.
Explain why it would be safer if the UK mains frequency was not 50 Hz.
[2 marks]
Mark scheme
Show the mark scheme
Question 9
AO /
Question Answers Extra information Mark
Spec. Ref.
09.1 5.75 = I × 230 1 AO2
4.2.4.1
5.75 4.2.1.3
I = 1
I = 0.025 (A)
allow a correct substitution using
an incorrect value of I
230 = 0.025 × R 1
or
or
allow a correct rearrangement
230 using incorrect value of I
R =
0.025
allow a correct calculation of
resistance using an incorrect 1
R = 9200 (Ω)
value of I
alternative approach for 4th and
5th marks:
5.75 = 0.0252 × R (1)
or
5.75
R = 2
0.025
R = 9200 (Ω) (1)
alternative approach:
2302
5.75 = (3)
R
2302
R = (1)
5.75
R = 9200 (Ω) (1)
Extra information AO /
Question Answers Mark
Spec. Ref.
09.2 one wire in the switch is live allow the switch / circuit is live 1 AO1
allow one wire is at a potential of 4.2.3.2
230 V
the electrician is earthed 1
or
the electrician is at earth
potential
(so) there will be a (large) 1
potential difference between the
live wire and the electrician /
earth (if the electrician touched
the wire)
09.3 50 Hz has the lowest 1 AO3
(maximum) let-go current 4.2.3.1
a higher / lower / different allow a specific numerical 1
frequency would allow people to example as opposed to a trend
let go at a greater current
Total 10
How to answer it
Mains Electricity & Safety Study Guide
What this question tests
This question assesses your understanding of domestic mains electricity circuits, safety hazards, multi-step power and resistance calculations, and your ability to interpret safety data from line graphs.
Calculating Resistance from Power and Potential Difference
📐 Step-by-Step Calculation
- Find current (I) first: Use the power formula P = V × I .
Rearrange: I = P / V
Substitute: I = 5.75 / 230 = 0.025 A - Find resistance (R): Use Ohm's Law V = I × R .
Rearrange: R = V / I
Substitute: R = 230 / 0.025 = 9200 Ω
✅ Final Answer
Resistance = 9200 Ω (or 9,200 ohms)
❌ Common Calculation Traps
- Forgetting equations linking power, potential difference, and resistance ( P = V² / R can also be used directly in one step!).
- Failing to rearrange equations properly before substituting numbers.
🧠 Exam Technique
Always write out the unadjusted formula first before substituting numbers. If you get stuck on multi-step calculations, look for alternative equations that combine variables (e.g., combining P = VI and V = IR into P = V² / R ).
Explaining Electrical Shock Hazards
💡 Key Knowledge
- Switches are always fitted to the live wire in domestic circuits.
- The human body is mostly water and acts as a conductor, and humans standing on the ground are effectively earthed (at 0 V potential).
- A potential difference causes charge to flow through a conductor.
✅ Marking Points (3 max)
- One wire connected to the switch is the live wire (at 230 V).
- The electrician is earthed / at earth potential (0 V).
- This creates a large potential difference between the live wire and the electrician, causing current to flow through them.
Interpreting Graph Data on Frequency and Current Safety
🧠 Exam Technique & Graph Analysis
When looking at graphs with a curved minimum or maximum, locate the turning point first. Here, the minimum point of the curve occurs precisely at 50 Hz, meaning the lowest current required to trap someone occurs at the current UK mains frequency.
✅ Correct Answer & Mark Scheme
- Point 1: 50 Hz has the lowest (maximum) let-go current.
- Point 2: A higher, lower, or different frequency would allow people to let go at a greater current (making it safer).
Topics
Physics · 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.