Electromagnets and the Motor Effect

A current-carrying wire produces a circular magnetic field around it.

Right-Hand Grip Rule

Grip the wire with your right hand, thumb pointing in the direction of conventional current (positive to negative). Your fingers curl in the direction of the magnetic field.

Solenoid

A solenoid is a coil of wire. When current flows, it produces a magnetic field similar to a bar magnet — with a clear north and south pole.

When a current-carrying conductor is placed in a magnetic field, the two fields interact and produce a force on the conductor.

This only works when the conductor is not parallel to the magnetic field. Maximum force occurs when the wire is perpendicular to the field.

The Force Equation

$\boxed{F = B \times I \times L}$

Where:

• $F$ = force (N)
• $B$ = magnetic flux density (T, tesla)
• $I$ = current (A)
• $L$ = length of conductor in the field (m)

Increasing the Force

• Increase the current ($I$)
• Use a stronger magnet (increase $B$)
• Increase the length of wire in the field ($L$)

Used to find the direction of the force on a current-carrying conductor in a magnetic field.

Hold your left hand with:

• First finger: direction of the magnetic Field (N to S)
• seCond finger: direction of Current (conventional: + to −)
• thuMb: direction of Motion (force)

All three fingers point at right angles to each other.

A DC motor converts electrical energy into kinetic energy (rotation).

How It Works

1. Current flows through a coil placed in a magnetic field
2. The motor effect produces forces on each side of the coil
3. The forces act in opposite directions (one side up, one side down) → the coil rotates
4. A split-ring commutator reverses the current direction every half turn
5. This keeps the coil rotating in the same direction

Making the Motor Spin Faster

• Increase the current
• Use a stronger magnet
• More turns on the coil
• Use an iron core in the coil

The Split-Ring Commutator

• Two half-rings connected to the coil
• Brushes (carbon contacts) connect them to the power supply
• Every half rotation, the contacts swap sides
• This reverses the current in the coil, maintaining rotation in one direction

Without the commutator, the coil would oscillate back and forth rather than rotating continuously.

Loudspeakers use the motor effect to convert electrical signals into sound.

1. Varying AC current flows through a coil attached to a cone
2. The coil sits in the magnetic field of a permanent magnet
3. The motor effect causes the coil (and cone) to vibrate back and forth
4. The vibrating cone creates pressure variations in the air = sound waves
5. The frequency of the AC determines the pitch; the amplitude determines the loudness

• Motor effect: current-carrying wire in a magnetic field experiences a force
• $F = BIL$ (force = flux density × current × length)
• Fleming's left-hand rule: Field, Current, Motion
• DC motor: coil in field + split-ring commutator → continuous rotation
• Loudspeaker: varying current → vibrating coil/cone → sound