Force on a Current-Carrying Wire

When an electric current flows through a wire placed in a magnetic field, the wire feels a sideways push. This is called the motor effect. It is how electric motors begin to turn.

What causes the force?

Electric current is moving charge in the wire. A magnetic field (from a magnet) acts on moving charges and pushes them sideways. The wire is pushed because the charges inside it are pushed.

Direction: Fleming’s Left-Hand Rule

Use your left hand to find the force direction:

• First finger: magnetic Field direction (from North to South).
• seCond finger: Current direction (conventional current: + to −).
• Thumb: Force on the wire.

If you reverse the current or reverse the magnetic field, the force reverses. If you reverse both, the force stays the same direction.

When is the force zero?

If the wire is parallel to the magnetic field lines, there is no sideways force. The force is greatest when the wire is at right angles to the field.

What affects the size of the force?

• Stronger magnetic field → bigger force.
• Bigger current → bigger force.
• Longer length of wire in the field → bigger force.

Simple experiment to show the force

Place a straight wire between the flat poles of a strong magnet (field is across the gap). Connect the wire to a low-voltage power supply with a switch. When the switch is closed, the wire jumps sideways. Reverse the current or flip the magnet: the wire jumps the other way. Open the switch: the wire stops moving.

Real-world connection

In motors, a current-carrying coil sits in a magnetic field. Each side of the coil feels a force in opposite directions, creating a turning effect that makes the motor spin.

Common misconceptions

• Current direction means conventional current (from positive to negative), not electron flow.
• No force if the wire is parallel to the field lines.
• Reversing current or field reverses the force; reversing both does not.