10 min readConcept sheetSection A MCQ (1 mark) and fill-in-the-blank parts; Section B very short answer (2 marks) on Fleming's left-hand rule, or short answer (3 marks) on the force and the motor idea

Force on a current-carrying conductor: Fleming's left-hand rule and the motor

A current-carrying wire in a magnetic field feels a force at right angles to both. Fleming's left-hand rule gives its direction, and electric motors use this force.

Fleming's left-hand ruleFirst finger: field; middle finger: current; thumb: force (motion)

Direction of the forceAt right angles to both the current and the field

The force increases withA larger current and a stronger magnetic field

Electric motorChanges electrical energy into mechanical energy

Activity: a wire jumps in a magnetic field

1
Hang a short piece of copper wire between the poles of a strong horseshoe magnet, at right angles to the field between the poles.
2
Join the wire to a battery through a switch.
3
Switch on. Observe: the wire is pushed upwards and becomes taut. A force acts on it.
4
Turn the magnet over so that its poles swap. Observe: the wire is now pushed downwards.
5
Reverse the current instead. Observe: the push again changes direction.
6
Use a larger current. Observe: the push is stronger.
The force depends on the directions of both the current and the field, and grows with the current.

What decides the force

A current makes its own magnetic field around the wire. This field and the magnet's field act on each other, so the wire feels a force. The force is at right angles to both the current and the magnetic field. It reverses if either the current or the field is reversed. It is larger for a larger current or a stronger magnet, and largest when the wire is at right angles to the field. Michael Faraday used this force to make the first simple electric motor.

Fleming's left-hand rule

Stretch the thumb, first finger and middle finger of your left hand so that each is at right angles to the other two.
Point the first finger along the magnetic field (from N to S).
Point the middle finger along the current.
The thumb then points in the direction of the force on the conductor, that is, the way it moves.
Memory: First finger = Field, seCond finger = Current, thuMb = Motion.

Using the left-hand rule

Problem

A vertical wire carries a current upwards. A magnetic field acts horizontally, pointing towards the north. In which direction is the force on the wire?

Working

Left hand: point the first finger towards the north, along the field.
Point the middle finger straight up, along the current.
Keep the three fingers at right angles. The thumb now points towards the west.

Answer

The force on the wire is towards the west.

The idea of an electric motor

An electric motor changes electrical energy into mechanical energy (motion).
It has a rectangular coil that can turn on an axle between the poles of a magnet.
Current flows in opposite directions in the two long sides of the coil. So one side is pushed up and the other down.
These two forces turn the coil. A split ring (commutator) reverses the current every half turn, so the coil keeps turning the same way.
Motors run fans, mixer-grinders, washing machines, water pumps, cranes and electric vehicles.

How to draw the activity

Draw a horseshoe magnet with its N pole and S pole facing each other.
Draw a short wire between the poles, hung from a stand by two connecting wires, and join them to a battery and a switch.
Draw an arrow from N to S labelled magnetic field, and an arrow along the wire labelled current.
Draw a bold arrow at right angles to both, labelled force, to show which way the wire moves.
Beside it, sketch a left hand with the first finger (field), middle finger (current) and thumb (force) labelled.

In real life

The ceiling fan, the mixer-grinder, the washing machine and the water pump on the terrace all run on electric motors. The speaker of a phone moves its cone with the same force.

Exam angle

1 marks: “Which rule gives the direction of the force on a current-carrying conductor placed in a magnetic field?” Fleming's left-hand rule.

2 marks: “State Fleming's left-hand rule.” Stretch the thumb, first finger and middle finger of the left hand at right angles to one another. If the first finger points along the field and the middle finger along the current, the thumb gives the direction of the force.

3 marks: “Describe an activity to show that a current-carrying wire feels a force in a magnetic field. What happens if the current is reversed?” Hang a copper wire between the poles of a horseshoe magnet, at right angles to the field, and pass a current: the wire is pushed up. Reverse the current (or the poles): it is pushed down. A larger current gives a larger push.

Remember it

Field, Current, Motion: First, seCond, thuMb of the left hand. Left hand for motors.