AQA GCSE Physics Physics Paper 2 (Higher), November 2020: Question 7

✅ Correct Answer

To vary the output potential difference so that you do not need a different generator for each type of device (or to make it compatible with different devices requiring different potential differences).

💡 Key Knowledge

• Transformers only work with alternating current (a.c.).
• Changing the number of turns on the secondary coil changes the output potential difference proportionally using the transformer equation: V_s / V_p = N_s / N_p .

❌ Common Errors

Students frequently lose marks by talking vaguely about "changing power" or "saving energy". Transformers alter potential difference and current, but input power equals output power (ignoring efficiency losses). You cannot change power simply by altering turns.

📐 Step-by-Step Calculation

1. Identify knowns:
   Primary potential difference ( V_p ) = 1.5 V
   Secondary potential difference ( V_s ) = 5.0 V
   Primary turns ( N_p ) = 150
2. State the transformer equation:
   V_p / V_s = N_p / N_s (or equivalent ratio)
3. Substitute values into the equation:
   1.5 / 5.0 = 150 / N_s
4. Rearrange to solve for N_s:
   N_s = 150 / (1.5 / 5.0) or N_s = (150 × 5.0) / 1.5
5. Final Answer: 500 turns

🧠 Exam Technique

Always write out the formula before substituting numbers. Even if you make a calculator error later, showing the correct formula and substitution can secure method marks.

✅ Correct Answer (5 Marking Points)

• The coil moves through the magnetic field / cuts magnetic field lines.
• A potential difference is induced across the coil.
• There is a complete circuit, so a current is induced in the coil.
• Every half turn, the potential difference reverses direction.
• Therefore, every half turn the current changes direction (producing an alternating current).

🧠 Exam Technique

This is a high-tariff 5-mark question requiring logical sequencing. Make sure you follow the chain: Motion → Cutting field lines → Induced p.d. → Complete circuit/current → Reversal every half-turn.

✅ Correct Answer

Provides a continuous / moveable contact or connection between the rotating coil and the external circuit (transformer/brushes), OR stops the wires from twisting together.

💡 Key Knowledge

Unlike a DC motor or dynamo which uses a split-ring commutator to keep the current flowing in one direction, an alternator uses slip rings combined with carbon brushes to maintain a continuous electrical connection while allowing the coil to spin freely.

✅ Correct Answer (3 Marking Points)

• After disconnection, there is no complete circuit, so there is no induced current.
• Therefore, there is no magnetic field produced around/by the coil.
• As a result, there is no magnetic force opposing the movement of the coil (conservation of energy / Lenz's Law effect).

❌ Common Errors

Students often forget that a current-carrying conductor creates its own magnetic field which interacts with the permanent magnets. When connected, generating electricity requires mechanical work because the magnetic fields oppose the turning motion.