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Why does a current-carrying wire in a magnetic field feel a force?

The motor effect: the force on a current-carrying conductor in a magnetic field, the force equation, Fleming's left-hand rule, and the electric motor.

A focused answer to AQA GCSE Physics 4.7.2, covering the force on a current-carrying conductor in a magnetic field, the force equation, Fleming's left-hand rule for finding its direction, and how the electric motor works.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The motor effect
The force equation
Fleming's left-hand rule
The electric motor
Try this

What this dot point is asking

AQA wants you to explain the motor effect, use the force equation, apply Fleming's left-hand rule to find the direction of the force, and explain how an electric motor uses the motor effect. This is part of topic 4.7.2 of the AQA GCSE Physics (8463) specification.

The motor effect

The force equation

The magnetic flux density, measured in tesla, is a way of describing how concentrated the magnetic field is: a stronger field has a larger flux density and produces a larger force on the same wire carrying the same current. The equation $F = BIl$ only applies when the wire is at right angles to the field; this is the orientation AQA uses in all standard calculations. You will often need to rearrange it to find the current ( $I = F / Bl$ ), the flux density ( $B = F / Il$ ) or the length, so practise the rearrangements as well as the direct calculation.

Fleming's left-hand rule

The reason the wire is pushed is that the wire's own circular magnetic field interacts with the external field of the magnets. On one side of the wire the two fields point the same way and reinforce (a strong region), while on the other side they oppose and partly cancel (a weak region). The wire is then pushed from the strong region towards the weak region, producing the motor-effect force. Reversing either the current or the field reverses the direction of the force, which Fleming's left-hand rule lets you predict. If both the current and the field are reversed together, the force stays in the same direction.

Worked example: force on a wire

A wire carries a current of $3\,A$ and lies at right angles to a magnetic field of flux density $0.4\,T$ . The length of wire inside the field is $0.2\,m$ . We want to find the motor-effect force on the wire.

Step 1: Identify the known quantities

The motor-effect force equation is $F = BIl$ . All three quantities on the right-hand side are given, so we substitute directly:

B = 0.4\,T, \quad I = 3\,A, \quad l = 0.2\,m

Step 2: Substitute into $F = BIl$

The force is greatest when the wire is at right angles to the field, which is the case here:

F = BIl = 0.4 \times 3 \times 0.2

Step 3: Calculate

F = 0.24\,N

Final answer: the force on the wire is $0.24\,N$ , directed as given by Fleming's left-hand rule.

The electric motor

Try this

Q1. State the equation for the force on a current-carrying conductor in a magnetic field. [1 mark]

Cue. $F = BIl$ .

Q2. A wire of length $0.5\,m$ carries a current of $2\,A$ in a field of flux density $0.3\,T$ . Calculate the force on it. [2 marks]

Cue. $F = BIl = 0.3 \times 2 \times 0.5 = 0.3\,N$ .

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20205 marksA straight wire of length

0.060\,\text{m}

lies at right angles to a magnetic field of flux density

0.25\,\text{T}

. The force on the wire is

0.030\,\text{N}

. Calculate the current in the wire, and state how the force would change if the wire were turned so that it lay parallel to the field.

Show worked answer →

Use $F = BIl$ rearranged to make the current the subject: $I = \frac{F}{Bl} = \frac{0.030}{0.25 \times 0.060} = \frac{0.030}{0.015} = 2.0\,\text{A}$ (3 marks: rearrangement, substitution, value with unit). If the wire were turned to lie parallel to the field, the force would fall to zero, because the motor-effect force is greatest when the wire is at right angles to the field and zero when it is parallel (2 marks). Markers reward the correct rearrangement, the calculated current, and the recognition that a parallel wire experiences no force. A common error is to forget that the force depends on the angle between the wire and the field.

AQA 20214 marksExplain how an electric motor uses the motor effect to produce continuous rotation, and explain the purpose of the split-ring commutator.

Show worked answer →

A current-carrying coil is placed in a magnetic field, and by the motor effect each side of the coil experiences a force (1 mark). Because the current flows in opposite directions on the two sides of the coil, the forces act in opposite directions: one side is pushed up and the other down, producing a turning effect that rotates the coil (1 mark). After half a turn the sides have swapped position, so without intervention the forces would now turn the coil back the other way. The split-ring commutator reverses the direction of the current in the coil every half turn (1 mark), so the force on each side stays in the same sense relative to the rotation, keeping the coil turning continuously in one direction (1 mark). Markers reward the opposite forces producing a turning effect and the commutator reversing the current to maintain rotation.

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Sources & how we know this

AQA GCSE Physics (8463) specification — AQA (2016)