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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 14

Topic 4.5.4 · Force on a current-carrying conductor

The motor effect

A current-carrying conductor in a magnetic field can experience a force.

Show this with a wire between magnet poles: the wire kicks when the current is switched on. Reverse the current or reverse the field and the force reverses.

The force is greatest when the wire is at 90° to the field, and zero when the current is parallel to the field.

Current-carrying wire between the poles of a magnet being pushed sideways; reversing the current or the field reverses the force.
Diagram 1: Current-carrying wire between the poles of a magnet being pushed sideways; reversing the current or the field reverses the force.

Directions, and beams of charge

Use the relative directions of force, magnetic field and current (Fleming’s left-hand rule: thuMb = force/motion, First finger = field, seCond finger = conventional current).

Beams of charged particles in a magnetic field also experience a force (perpendicular to their velocity and to the field), so the beam curves. Reverse the particle charge and the deflection reverses.

Fleming’s left-hand rule: thumb = force (motion), first finger = field, second finger = conventional current. A beam of charged particles curving in a magnetic field.
Diagram 2: Fleming’s left-hand rule: thumb = force (motion), first finger = field, second finger = conventional current. A beam of charged particles curving in a magnetic field.

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