Electromagnetic Induction & Generators

IGCSE Edexcel Physics
6.15–6.16 Electromagnetic induction and AC generators
Key Concepts: A voltage is induced when a conductor moves through a magnetic field (or when the field through a coil changes). Greater speed, stronger field or more turns → greater induced voltage. An AC generator produces alternating current by rotating a coil in a magnetic field; slip rings maintain connection to the external circuit.

Section A — Electromagnetic Induction

1. State the conditions needed to induce a voltage in a conductor. [2]
2. State three ways to increase the magnitude of the induced voltage. [3]
3. Describe what happens to the induced voltage if the direction of the motion is reversed. [1]
4. A student pushes a bar magnet into a coil, holds it still, then pulls it out. Describe the readings on a galvanometer connected to the coil at each stage. [3]

Section B — AC Generators

5. Describe how an AC generator works. Include the role of the slip rings. [4]
6. Describe the shape of the output voltage–time graph from an AC generator as the coil rotates. [2]

Total marks: 15

Mark Scheme

1. There must be relative movement between a conductor and a magnetic field [1]; or a changing magnetic field through a coil [1] [2]
2. Move faster [1]; use a stronger magnetic field [1]; use a coil with more turns [1] [3]
3. The direction (polarity) of the induced voltage reverses [1]
4. Pushing in: galvanometer deflects in one direction (current flows) [1]; held still: no deflection (no change in flux) [1]; pulling out: galvanometer deflects in the opposite direction [1] [3]
5. A coil rotates in a magnetic field [1]; as it rotates, the coil cuts through field lines, inducing a voltage [1]; the voltage alternates in direction as different sides of the coil move through the field [1]; slip rings and brushes maintain a continuous electrical connection to the external circuit [1] [4]
6. Sinusoidal (sine wave) shape [1]; voltage alternates between positive and negative values as the coil rotates [1] [2]