AQA GCSE Combined Science: Trilogy Physics Paper 1 (Foundation), November 2020: Question 5

✅ Correct Answer

Swap the blue wire and the green-and-yellow wire.

Also accepted: "Connect the blue wire to the neutral pin and the green-and-yellow wire to the earth pin."

💡 Key Knowledge

Remember positions using standard mnemonics:

• B L ue: Goes to the Left (Neutral pin, 0 V).
• B R own: Goes to the Right (Live pin, 230 V, connected through fuse).
• Green & Yellow stripes: Goes to the Top (Earth pin, 0 V).

🧠 Exam Technique

The question asks: "What should be done to connect the wires correctly?"

Always give an action (e.g., "swap wire X with wire Y") rather than merely stating that it is wrong.

❌ Common Errors

Merely writing "the earth wire and neutral wire are wrongly connected" receives 0 marks because it repeats the premise rather than describing the fix.

✅ Correct Answer

• The potential difference is 230 V [1 mark]
• The frequency is 50 Hz [1 mark]

💡 Key Knowledge

UK domestic electricity is an alternating current (a.c.) supply:

• Current changes direction 50 times per second ( 50 Hz ).
• Average effective potential difference is 230 V .

❌ Common Errors

• Confusing UK values with US mains ( 110 V / 60 Hz ).
• Guessing 240 V or incorrect frequencies like 100 Hz or 230 Hz .

✅ Correct Answer

Potential difference = 0 V

💡 Key Knowledge

Both wires reside at ground/earth potential:

• Earth wire: Connected directly to ground ( 0 V ).
• Neutral wire: Completes the circuit and is grounded back at the local substation ( ~0 V ).
• Difference: 0 V - 0 V = 0 V .

🧠 Exam Technique

Do not confuse the potential difference between live and neutral (which is 230 V) with the potential difference between neutral and earth (which is 0 V).

✅ Correct Answer & Symbol Spec

❌ Common Errors

• Drawing a standard resistor: A rectangle with connecting lines that stop at the outside boundary (wire does not go through).
• Drawing a thermistor or LDR: Adding diagonal hockey sticks or arrows.
• Freehand sketches with loose, disjointed gaps in the wire line.

✅ Correct Answer (Mark Scheme Breakdown)

Must make two distinct points:

1. The person could receive an electric shock / be electrocuted. [1 mark]
2. Because a current flows through the person / the person provides a connection to earth / there is a potential difference across the person. [1 mark]

💡 The Science Behind the Mark

• The live wire touches the metal casing, raising the casing to 230 V.
• A person standing on the floor is at 0 V (earth potential).
• Touching the casing creates a large potential difference ( 230 V ) across the person, driving dangerous current through their body into the ground.

❌ Common Errors

Just stating "the person will be hurt" is too vague for Mark 1 (use "shock" or "electrocuted"). Saying "electricity will jump" without mentioning current flowing through the person misses Mark 2.

🧠 Top Tip

Always connect the consequence (shock) directly to the physics mechanism (potential difference causes a current to pass through the body).

✅ Correct Answer (Mark Scheme Breakdown)

1. The charge / current flows through the earth wire (instead of the person). [1 mark]
2. Because the resistance of the earth wire is much lower than that of a person. [1 mark]
   Alternative 2nd mark: "so the casing does not become live" OR "causes a large surge that melts/blows the fuse".

💡 Step-by-Step Protection Mechanism

1. Low Resistance Path: The copper earth wire has a tiny resistance compared to human skin/body resistance.
2. Current Diverts: Current takes the path of least resistance through the earth wire straight to ground.
3. Fuse Melts: The very low resistance causes a huge surge in current, which rapidly overheats and melts the fuse, completely isolating the appliance from the 230 V live supply.

🧠 Exam Technique

When comparing pathways, make explicit comparisons using comparative words: "resistance of earth wire is lower / much less than the person".

❌ Common Errors

• Claiming that the earth wire "absorbs all the electricity" (unscientific phrasing).
• Stating the fuse "turns off" rather than melts, blows, or breaks the circuit.