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Why does a current-carrying wire in a magnetic field experience a force, and how does a motor use this?

Describe the force on a current-carrying conductor in a magnetic field and explain the d.c. motor

A focused answer to the O-Level Physics outcome on the motor effect. The force on a current-carrying wire in a magnetic field, Fleming's left-hand rule, factors affecting the force, and how the d.c. motor turns.

Generated by Claude Opus 4.88 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the force on a current-carrying conductor in a magnetic field (the motor effect), to use Fleming's left-hand rule to find its direction, to know what changes the size of the force, and to explain how a simple d.c. motor uses this to turn. The big idea is that a current in a magnetic field experiences a force, and a clever arrangement turns that force into continuous rotation.

The answer

The motor effect

When a current-carrying wire lies in a magnetic field, the field of the current interacts with the external field and the wire experiences a force. This is the motor effect. The force is greatest when the wire is at right angles to the field and zero when the wire is parallel to the field.

Fleming's left-hand rule

Fleming's left-hand rule gives the direction of the force. Hold the thumb and first two fingers of the left hand at right angles:

  • First finger: Field (north to south).
  • Second finger: Current (conventional, positive to negative).
  • Thumb: Thrust (the force).

What changes the size of the force

The force on the wire is larger when:

  • the current is larger,
  • the magnetic field is stronger,
  • the length of wire in the field is greater.

Reversing either the current or the field reverses the direction of the force. Reversing both leaves the direction unchanged.

The d.c. motor

A d.c. motor uses the motor effect to produce continuous rotation. A coil carrying current sits in a magnetic field. The two sides of the coil carry current in opposite directions, so one side is pushed up and the other down (by Fleming's left-hand rule). These opposite forces form a turning effect (a couple) that rotates the coil.

A split-ring commutator reverses the current in the coil every half turn. This keeps the force on each side turning the coil the same way, so it spins continuously in one direction instead of stopping after half a turn.

Examples in context

Example 1. The loudspeaker. A loudspeaker has a coil in a magnetic field attached to a paper cone. The varying audio current makes the coil feel a varying force by the motor effect, pushing the cone in and out. The vibrating cone pushes the air, producing the sound you hear, a direct everyday use of the force on a current.

Example 2. Power tools and appliances. Electric drills, fans, and washing machines all use d.c. or a.c. motors built on the motor effect. The current in the coils interacts with a magnetic field to produce a turning force, and the commutator (or its a.c. equivalent) keeps the rotation steady, turning electrical energy into useful motion.

Try this

Q1. Name the rule used to find the direction of the force on a current-carrying conductor in a magnetic field. [1 mark]

  • Cue. Fleming's left-hand rule.

Q2. State two ways to increase the force on a current-carrying wire in a magnetic field. [2 marks]

  • Cue. Increase the current, or increase the strength of the magnetic field (or use a longer wire in the field).

Q3. Explain the purpose of the split-ring commutator in a d.c. motor. [2 marks]

  • Cue. It reverses the current in the coil every half turn so the turning effect always acts the same way, keeping the coil rotating continuously in one direction.

Exam-style practice questions

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

Original4 marksA straight wire carries a current and lies at right angles to a magnetic field, so it experiences a force. (a) Name the rule used to find the direction of the force. (b) State two ways to increase the size of the force.
Show worked answer →

(a) Fleming's left-hand rule.

(b) Increase the current in the wire, or increase the strength of the magnetic field. (Using a longer length of wire in the field also increases the force.)

Markers reward Fleming's left-hand rule named correctly, and two valid ways to increase the force, typically a larger current and a stronger field.

Original5 marks(a) Explain why a current-carrying coil in a magnetic field experiences a turning effect. (b) State the purpose of the split-ring commutator in a d.c. motor.
Show worked answer →

(a) The two sides of the coil carry current in opposite directions across the magnetic field, so by the motor effect each side feels a force, one upward and one downward (Fleming's left-hand rule). These opposite forces on opposite sides form a couple that turns the coil.

(b) The split-ring commutator reverses the current direction in the coil every half turn. This keeps the forces turning the coil the same way, so the coil continues to rotate in one direction instead of stopping after half a turn.

Markers reward opposite forces on the two sides producing a turning effect, and the commutator reversing the current each half turn to keep the rotation continuous in one direction.

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