Electromagnetism questions

Electromagnetism syllabus

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

• 6.8 know that an electric current in a conductor produces a magnetic field around it
• 6.9P describe the construction of electromagnets
• 6.10P draw magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current
• 6.11P know that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field
• 6.12 understand why a force is exerted on a current-carrying wire in a magnetic field and how this effect is applied in simple d.c. electric motors and loudspeakers
• 6.13 use the left-hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field
• 6.14 describe how the force on a current-carrying conductor in a magnetic field changes with the magnitude and direction of the field and current

How to answer electromagnetism 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.

Electromagnetism 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: How a metal detector produces sound

June 2023 · Paper 1P · Question 12(c)

Question 1

A device called a metal detector can be used to find metal buried underground.

The metal detector has two circuits, each containing a coil of copper wire.

Diagram 1 shows the circuit for the transmitter coil.

Diagram 3 shows the circuit for the receiver coil.

As a result of the alternating current in the gold ring, there is an alternating current in the receiving coil.

Explain how an alternating current in the receiving coil causes a sound to be emitted from the loudspeaker.

Final answer

The alternating current in the receiving coil produces an alternating current in the loudspeaker. The magnetic fields in the loudspeaker interact, producing a force on the cone. Because the current is alternating, the force repeatedly changes direction, so the cone vibrates back and forth and emits sound.

Mark scheme points

Any four of the following five points are needed.

1. M1 There is an alternating current in the loudspeaker.
2. M2 Magnetic fields in the loudspeaker interact.
3. M3 The interaction causes a force on the loudspeaker cone (or coil).
4. M4 The alternating current makes the force change direction.
5. M5 The loudspeaker cone (or coil) vibrates, for example back and forth.

Explanation

The alternating current from the receiving coil flows in the loudspeaker. The current produces a magnetic field that interacts with the loudspeaker’s other magnetic field, creating a force on the cone. As the current alternates, the force changes direction, moving the cone back and forth. The vibrating cone produces sound.

Common mistakes

• The examiner report noted that many answers confused the receiver coil’s field with how the loudspeaker works, or only mentioned a vibrating part. Trace the full link from current, through magnetic interaction and force, to cone vibration.
• Do not say only that the cone vibrates: explain what force makes it move.
• Saying only that magnetic fields “cut” is not enough; state that the fields interact and cause a force.

Example 2: Current needed to support a load

2017 specimen · Paper 2P · Question 5(b)(i)

Question 2

A student investigates how the minimum current required to support a load using an electromagnet varies as the load is increased.

He uses metal discs to increase the load and changes the current using a variable resistor.

The student’s results are given in the table.

Number of metal discs	Minimum current / mA
0	30
2	48
5	75
6	78
7	93
10	120

On the grid, draw a bar chart of current against number of metal discs.

Final answer

Label the horizontal axis Number of metal discs and the vertical axis Current / mA. Use a uniform vertical scale from 0 to 120 mA, marked at equal intervals. Draw separate bars for these values:

0 discs: 30 mA; 2 discs: 48 mA; 5 discs: 75 mA; 6 discs: 78 mA; 7 discs: 93 mA; 10 discs: 120 mA.

Mark scheme points

1. M1 Show a suitable, uniform scale on the current axis.
2. M2 Label both axes with the variables; give the current unit, mA.
3. M3 Plot the six data values at the correct numbers of discs and current values.
4. M4 Draw bars for the plotted values.

Explanation

The number of discs is the independent variable, so it goes on the horizontal axis. The measured current goes on the vertical axis, with its unit shown. A uniform scale lets each current value be placed at the correct height. Because the question asks for a bar chart, show the six results as separate bars rather than joining them with lines.

Common mistakes

• Leaving the current unit off the vertical-axis label: include mA.
• Using unequal intervals on the current scale, which makes the bar heights inaccurate.
• Joining the results with a line instead of drawing separate bars.

Practise Electromagnetism questions

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