Magnetism and Electromagnetism: Exam Practice

This final lesson brings together everything you have learned in the Magnetism and Electromagnetism topic. You will review the key concepts, practise different question types, and learn exam techniques specific to this topic. This covers the full AQA GCSE Physics specification 4.7.

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Topic Overview

The Magnetism and Electromagnetism topic covers the following key areas:

Sub-Topic	Key Content	Tier
Poles and magnetic fields	Magnetic poles, attraction/repulsion, permanent and induced magnets, field lines	Foundation and Higher
Magnetic fields of a solenoid	Field pattern around wires and solenoids, electromagnets	Foundation and Higher
Electromagnetism	How electromagnets work, domain theory, applications	Foundation and Higher
The motor effect	Force on a current-carrying conductor, F = B x I x l	Foundation (concept) and Higher (equation)
Fleming's left-hand rule	Predicting force direction	Higher only
The electric motor	How a DC motor works, commutator, brushes	Higher only
Generator effect	Electromagnetic induction, Fleming's right-hand rule, AC generators	Higher only
Microphones and loudspeakers	Motor and generator effects in devices	Higher only
Transformers	Step-up/down, transformer equation, National Grid	Higher only

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Key Equations to Remember

Equation	Meaning	Tier
F = B x I x l	Force on a conductor (N) = magnetic flux density (T) x current (A) x length (m)	Higher
V_p / V_s = n_p / n_s	Transformer turns-ratio equation	Higher
V_p x I_p = V_s x I_s	Transformer power equation (ideal, 100% efficient)	Higher

Exam Tip: These equations are on the Physics equation sheet provided in the exam. However, you still need to know what each symbol represents and be able to rearrange and apply them confidently. Practise rearranging each equation before the exam.

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Common Question Types

Type 1: Describe Questions

These ask you to describe a magnetic field pattern, how a device works, or what happens in a particular situation.

Example: Describe the magnetic field pattern of a solenoid.

Model Answer:

• Inside the solenoid, the field lines are parallel and evenly spaced, indicating a strong, uniform magnetic field.
• Outside the solenoid, the field pattern is identical to that of a bar magnet, with curved field lines running from the north pole to the south pole.
• The field is strongest inside the solenoid and at the poles.

Type 2: Explain Questions

These require you to give reasons using scientific principles.

Example: Explain why the coil in a DC motor continues to rotate.

Model Answer:

• Current flows through the coil in a magnetic field, and by the motor effect, forces act on each side of the coil.
• The forces are in opposite directions on each side, creating a turning effect (torque).
• When the coil reaches the vertical position, the split-ring commutator reverses the current direction in the coil.
• This reverses the forces, so the coil continues to rotate in the same direction.
• The coil's momentum carries it past the vertical position, and the process repeats every half turn.

Exam Tip: In "explain" questions, always link cause to effect. Use connecting phrases like "this means that," "because," "therefore," and "as a result." Each point should logically lead to the next.

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Practice Calculations

Calculation 1: Motor Effect Force

A wire of length 0.15 m carries a current of 6.0 A perpendicular to a magnetic field of 0.50 T. Calculate the force.

F = B x I x l F = 0.50 x 6.0 x 0.15 F = 0.45 N

Calculation 2: Transformer Turns Ratio

A transformer has 500 turns on the primary coil and 50 turns on the secondary coil. The input voltage is 230 V. Calculate the output voltage.

V_p / V_s = n_p / n_s 230 / V_s = 500 / 50 230 / V_s = 10 V_s = 230 / 10 V_s = 23 V

Calculation 3: Transformer Power

An ideal transformer has a primary voltage of 400 V and a primary current of 2.5 A. The secondary voltage is 20 V. Calculate the secondary current.

V_p x I_p = V_s x I_s 400 x 2.5 = 20 x I_s 1000 = 20 x I_s I_s = 1000 / 20 I_s = 50 A

Calculation 4: Combined Question

A transformer steps down from 11,000 V to 230 V. If the secondary current is 26 A, calculate: (a) the power output, (b) the primary current (assuming 100% efficiency).

(a) P = V_s x I_s = 230 x 26 = 5980 W

(b) V_p x I_p = V_s x I_s 11000 x I_p = 5980 I_p = 5980 / 11000 = 0.544 A (to 3 significant figures)

Exam Tip: Always show your working in calculations, even if the answer seems obvious. Write out the equation, substitute the values, then solve. This way, even if your final answer is wrong, you can still earn method marks.

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Key Diagrams You Should Be Able to Draw

1. Magnetic Field of a Bar Magnet

• Field lines from N to S, curving around the magnet.
• Lines closer together at the poles (stronger field).
• Lines never cross.
• Arrows point from N to S.

2. Magnetic Field of a Solenoid

• Parallel lines inside (uniform field).
• Bar-magnet pattern outside.
• N and S poles labelled at each end.

3. The DC Motor

• Rectangular coil between magnet poles.
• Split-ring commutator at the bottom.
• Carbon brushes touching the commutator.
• Current direction shown in the coil.
• Force arrows on each side of the coil (opposite directions).

4. The AC Generator

• Rectangular coil between magnet poles.
• Slip rings (two complete rings) at the bottom.
• Carbon brushes touching the slip rings.
• Direction of rotation shown.

5. A Transformer

• Primary coil on the left side of the iron core.
• Secondary coil on the right side.
• Iron core shown as a rectangular loop.
• Number of turns clearly different for step-up/step-down.

graph LR
    subgraph "Key Devices to Know"
        A["Bar Magnet: N-to-S field lines"] --- B["Solenoid: uniform internal field"]
        B --- C["DC Motor: commutator reverses current"]
        C --- D["AC Generator: slip rings produce AC"]
        D --- E["Transformer: turns ratio changes voltage"]
    end

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Common Mistakes to Avoid