Magnetism and the Motor Effect

Magnetic Fields

A magnetic field is a region where magnetic materials and current-carrying conductors experience a force. Represented by field lines that go from north pole to south pole outside the magnet; never cross; closer together = stronger field.

Permanent magnets: made of ferromagnetic materials (iron, steel, cobalt, nickel). Steel is used for permanent magnets (harder to magnetise/demagnetise); iron for electromagnet cores (easily magnetised/demagnetised).

Induced magnetism: a magnetic material placed in a field becomes a magnet itself, always attracted to the original magnet (never repelled — this is a common mistake to avoid).

Electromagnets

A coil of wire carrying a current produces a magnetic field similar to a bar magnet — called a solenoid.

Increasing the strength of an electromagnet:

1. Increase the current.
2. Increase the number of turns on the coil.
3. Add a soft iron core.

The field direction follows the right-hand rule: curl the fingers in the direction of current → thumb points to north pole.

Uses of electromagnets: electric bells, relays, magnetic cranes, MRI scanners, maglev trains.

The Motor Effect

When a current-carrying conductor is placed in a magnetic field, it experiences a force (the motor effect).

Fleming's Left-Hand Rule (FBI rule):

• First finger → Field direction (N to S)
• seCond finger → Conventional Current direction
• thuMb → Motion (force direction)

Force on a conductor: F = BIl

Where: F = force (N), B = magnetic flux density (T, tesla), I = current A, l = length of conductor in field (m).

Force is maximum when conductor is perpendicular to the field; zero when parallel.

The DC Motor

A rectangular coil of wire in a magnetic field experiences a pair of equal and opposite forces (couple) on opposite sides, causing rotation.

Key components:

• Commutator (split ring): reverses the current direction every half turn so the coil keeps rotating in the same direction.
• Brushes: maintain electrical contact with the spinning commutator.

Increasing motor speed/force: increase current, increase number of turns, use stronger magnet.

Core Practical 7 — Investigating the motor effect

Equipment: stiff wire/aluminium strip across two rails, powerful horseshoe magnet or bar magnets, ammeter, power supply, balance/sensitive force meter.

Method (measuring force):

1. Place the wire between the poles of the magnet on a balance. Zero the balance.
2. Switch on current; measure the force from the balance reading change.
3. Vary the current (using a rheostat) and record the force.
4. Plot F vs I — should be a straight line through origin confirming F ∝ I.

Alternatively: observe the direction of force using Fleming's left-hand rule; verify by reversing current or field direction.