Magnetism and electromagnetism units questions

Magnetism and electromagnetism units syllabus

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

• 6.1 use the following units: ampere (A), volt (V) and watt (W)

How to answer magnetism and electromagnetism units questions

1. Write the unit symbol exactly and distinguish quantities that use similar-looking units.
2. Convert prefixes and compound units before substituting values into an equation.
3. Check that the unit of the final answer matches the quantity asked for.
4. Use the context of a diagram or calculation to reject units with the wrong dimensions.

Magnetism and electromagnetism units 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: Calculating the output current of a transformer

June 2021 · Paper 2P · Question 6(b)(ii)

Question 1

This is a question about a nuclear fusion power station.

The diagram shows how a fusion power station could supply the National Grid in the future.

The fusion power station could supply the input coil of the step-up transformer with a current of 28kA at a voltage of 21kV.

The output coil of the transformer has a voltage of 330kV.

Assume the transformer is 100% efficient.

Calculate the current in the output coil of the transformer.

current = ________ kA

Final answer

For a 100% efficient transformer, input power equals output power:

VinIin = VoutIout
28 × 21 = 330 × Iout
Iout = (28 × 21) ÷ 330 = 1.78... kA
current = 1.8 kA

Mark scheme points

1. M1 Substitute the input and output values into the power equation, V_inI_in = V_outI_out.
2. M2 Rearrange the equation to find I_out.
3. M3 Calculate the output current as 1.8 kA.

Explanation

At 100% efficiency, the transformer transfers all the input power to the output. Use the given input current and voltages in the power equation, then divide by the output voltage to find the output current.

Iout = (VinIin) ÷ Vout
            = (21 kV × 28 kA) ÷ 330 kV
            = 1.78... kA
            ≈ 1.8 kA

The voltage units cancel, leaving the current in kA.

Common mistakes

• Using the output voltage in the numerator instead of dividing the input power by 330 kV.
• Giving a current without the unit kA, or misplacing the decimal point in the calculated value.

Example 2: Finding frequency from a dynamo trace

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

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 dynamo's output voltage varies with time.

Calculate the frequency of the output voltage.

frequency = ________ Hz

Final answer

The time for one complete cycle is 0.04 s, so the frequency is f = 1/0.04 = 25 Hz.

Mark scheme points

1. M1 Read the time for one complete cycle: T = 0.04 s. This is also credited if the working shows f = 1/0.04.
2. M2 Calculate the frequency correctly: 25 Hz. Writing the given equation alone earns no mark.

Explanation

The period, T, is the time for one complete cycle—for example, from one positive peak to the next positive peak. Frequency is the number of complete cycles per second, so it is the reciprocal of the period.

T = 0.04 s
f = 1/T
  = 1/0.04
  = 25 Hz

Common mistakes

• Using half a cycle or two cycles as the period. Measure between equivalent points on consecutive cycles.
• Writing only f = 1/T. Substitute the period and evaluate the frequency to earn the marks.

Practise Magnetism and electromagnetism units questions

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