IGCSE 0625 / Section 4.5.4 / Core

A force from a current.

Lay a current-carrying wire between two magnets and it jumps. Two magnetic fields, the wire and the magnets, overlap and the wire is shoved sideways. Harness that shove and you have a motor.

The key idea

A wire carrying a current in a magnetic field experiences a force, the motor effect. The force is larger with a bigger current or a stronger field, and it reverses if the current or the field is reversed. The left-hand rule relates the directions of force, field and current.

Definition · learn the exact words

When a current-carrying conductor lies in a magnetic field, it experiences a force. This is the motor effect.

force is greatest when the current is at right angles to the field

Reversing the current or the field reverses the force. Fleming’s left-hand rule gives the directions of force, field and current.

Section 01

Watch the wire jump.

Send current through the wire in the field and see which way the force pushes it.

Stage 1 · Learn

Check motor effect

Four quick checks. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

A current-carrying wire placed in a magnetic field experiences...

Quick check+10 XP

The force on the wire is increased by...

Quick check+10 XP

Reversing only the direction of the current...

Quick check+10 XP

In the left-hand rule, the first finger represents the...

Section 02

Size and direction.

The size depends on current and field strength; the direction follows the left-hand rule.

Examiner trap

The force is greatest when the current is at right angles to the field, and zero when the current runs along the field. Reversing either the current or the field reverses the force, but reversing both leaves it unchanged.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

Both the current and the magnetic field are reversed at the same time. The force on the wire...

Exam style+20 XP

A current-carrying wire lies exactly along the direction of the magnetic field. The force on it is...

Exam style+20 XP

Which change would make the force on the conductor smaller?

Skill unlocked

Motor effect, mastered.

The motor effect is mapped. Keep the chain going.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
The force on a current-carrying conductor, what changes its size and direction, and the left-hand rule. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Electricity and magnetism challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just motor effect.
Start the unit challenge →