Electromagnetic induction questions

Electromagnetic induction syllabus

Questions on electromagnetic induction can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 6.15 know that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it and describe the factors that affect the size of the induced voltage
• 6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field, and describe the factors that affect the size of the induced voltage
• 6.17P describe the structure of a transformer and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides
• 6.18P explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy
• 6.19P know and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer: input (primary) voltage / output (secondary) voltage = primary turns / secondary turns
• 6.20P know and use the relationship: input power = output power VpIp = VsIs for 100% efficiency

How to answer electromagnetic induction questions

1. Add arrows to field lines and distinguish magnetic field direction from force direction.
2. State what changes in the field or conductor before explaining an induced voltage.
3. Apply the left-hand rule systematically and describe how reversing current or field reverses force.
4. For transformer calculations, label primary and secondary quantities before using the turns ratio.

Electromagnetic induction example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Comparing induced-voltage traces

January 2015 · Paper 1P · Question 14(b)

Question 1

A student investigates how to produce a voltage.

He hangs a magnet from a spring, above a coil that is connected to a data logger.

The student repeats the experiment using two magnets taped together.

Compared to one magnet, these two magnets take a longer time to move up and down.

The dotted line on the grid shows the original graph for one magnet.

On the same grid, sketch the graph that would be produced using two magnets.

Final answer

Sketch an alternating voltage trace whose amplitude starts larger than the original and then diminishes. Make its peaks farther apart, showing a lower frequency.

Mark scheme points

1. M1 The trace alternates and diminishes in amplitude.
2. M2 The amplitude is larger than for one magnet.
3. M3 The frequency is lower than for one magnet.

Explanation

The oscillating magnet moves back and forth through the coil, so the induced voltage alternates. As the oscillation dies away, the voltage amplitude diminishes. Two magnets produce a larger change in magnetic field through the coil, so the induced voltage has a larger amplitude. The magnets take longer to move up and down, so the oscillations have a longer period and therefore a lower frequency; show this with peaks farther apart.

Common mistakes

• Do not make the peaks closer together: a longer time for each up-and-down movement means a lower frequency.
• Show both the initially larger amplitude and the gradual decrease in amplitude; these describe different features of the trace.

Example 2: Reading the output of a bicycle dynamo

January 2016 · Paper 1P · Question 8(b)(ii)

Question 2

The student has a bicycle with a dynamo (generator) that supplies electricity for its lights.

The diagram shows the dynamo.

The friction wheel, W, presses against the bicycle tyre. When the student pedals, the friction wheel turns and causes part Y to rotate.

The graph shows how the output voltage of the dynamo varies with time as the student pedals steadily.

State the maximum output voltage of the dynamo.

maximum output voltage = ________ V

Final answer

The maximum output voltage is 1.6 V.

Mark scheme points

1. M1 State a peak voltage of 1.6 V. Either sign is accepted.

Explanation

Read the voltage at the highest point of the curve using the vertical axis. The peak is at 1.6 V above zero.

Common mistakes

• Do not measure from the negative trough to the positive peak. The maximum voltage is measured from zero to the peak.

Practise Electromagnetic induction questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.