AQA GCSE Physics Physics Paper 2 (Higher), June 2023: Question 9

✅ Model Answer Structure

• Point 1: The coil moves through the magnetic field or cuts magnetic field lines.
• Point 2: A potential difference is induced across the coil.
• Point 3: Because there is a complete circuit, a current is induced.
• Point 4: Every half-revolution, the two ends of the coil swap brushes (or the commutator switches contacts).
• Point 5: Therefore, the direction of the induced current / potential difference does not reverse, keeping it as a direct current (d.c.).

💡 Key Knowledge

Unlike an alternator (which uses slip rings and produces alternating current / a.c.), a simple dynamo uses a split-ring commutator. Every time the coil passes the vertical position and the potential difference would normally reverse, the commutator swaps the connection to the brushes, keeping the output in the same polarity.

🧠 Exam Technique

This is a sequential 5-mark extended response. Ensure you hit the physical cause first (movement/cutting field), the consequence (p.d. and current), and finally the structural role of the commutator in preventing direction reversal.

❌ Common Errors

Students frequently confuse the split-ring commutator (used in dynamos and DC motors for d.c.) with slip rings (used in alternators for a.c.). Describing slip rings here will lose you the final marks regarding direction switching.

✅ Correct Answer Description

You must sketch a graph with four positive "humps" or lobes resting entirely above the time axis (representing two complete revolutions, meaning two humps per full 360-degree rotation). The curve should touch or start at zero volts as the coil passes perpendicular to the magnetic field lines.

💡 Key Knowledge

Because the split-ring commutator reverses the connection every half-rotation, the negative halves of an alternating sine wave are flipped upside down into the positive region. This creates a pulsating direct current.

🧠 Exam Technique

Count carefully: "two complete revolutions". Since one full rotation produces two lobes, two full rotations must show exactly four humps. Make sure the curves touch the time axis cleanly.

❌ Common Errors

Drawing an alternating current wave (sine wave that dips below the time axis) or drawing the wrong number of cycles (e.g., only two humps for one revolution instead of four for two).

✅ Model Answer Structure

• Point 1: After disconnection, there is no complete circuit, so there is no induced current.
• Point 2: Consequently, there is no magnetic field produced around/by the coil.
• Point 3: Therefore, there is no magnetic force opposing the movement of the coil (conservation of energy / generator effect reaction).

💡 Key Knowledge

This is a brilliant application of Lenz's Law / Conservation of Energy. When a current flows, it creates its own magnetic field that opposes the original motion that caused it (making it hard to turn). Remove the current, and that opposing magnetic force disappears!

🧠 Exam Technique

Build your chain of reasoning logically: Disconnected → No current → No magnetic field around the coil → No opposing force.

❌ Common Errors

Vague answers stating "friction is removed" or "air resistance decreases". The change in turning force is entirely electromagnetic, not mechanical friction within the bearings.