Magnetism, electromagnetism and electromagnetic induction

Magnetic materials

Iron, steel, nickel, cobalt

What do magnetic fields show?

• the shape of the magnetic field

• the direction of the magnetic force

• the strength of the magnetic field

Magnetism definition

A magnetic field being created around a current in a wire

Fleming’s Right-Hand Rule

With your thumb pointing in the direction of the current, your fingers will curl in the direction of the field.

Increasing electromagnetism can be done by:

• increasing the current in the wire

• wrapping the wire into a solenoid

Increasing the field around a solenoid can be done by:

• increasing the current in the solenoid

• increasing the number of turns on the solenoid

• wrapping the solenoid around a magnetically soft core such as iron

Fleming’s Left-Hand Rule

Thumb- motion

First finger- direction of field

Middle finger- current

Moving-coil loudspeaker

• electric currents from a source pass through the coils of a loudspeaker

• these represent sounds and they always change in size and direction

• the fields of the coil and the permanent magnet are therefore creating magnetic field patterns which are also always changing in strength and direction

• these cause rapidly changing forces to be applied to the wires of the coil, which cause the speaker cone to vibrate

• these cause the sound waves we hear

The electric motor

• When there is a current in the loop of wire, one side of it will experience a force pushing it upwards. The other side feels a force pushing it down, so the coil will rotate

• As it reaches vertical position, its momentum takes it past this. Then the wires are reversed- the bottom wire is pushed up and the top one is pushed down

• This is done via a split-ring commutator. Each time the wires rotate 180, the connections change and the direction of the current changes.

To increase the rate of motor turns:

1. increase the number of turns or loops of wire

2. increase the strength of the magnetic field

3. increase the current in the loop of wire

Magnetic induction demo

Moving a wire across a magnetic field at right angles will induce a voltage. If the wire is part of a complete circuit, there is current. This is electromagnetic induction.

The size of the induced voltage (and current) can be increased by:

• moving the wire more quickly

• using a stronger magnet so more field lines are ‘cut’

• wrapping the wire into a coil so that more pieces of wire move through the field.

Generators

A rotating magnet cuts through coils around it, inducing a current. This can be done on mass scale. (The coils can rotate instead)

Transformers

An alternating current in a coil will allow the magnetic field around it to constantly change in proportion. If a second coil is placed near the first, this will pass through the second coil too. The size and direction of the induced voltage changes as the voltage applied to the first coil changes.

The National Grid

Large currents in long wires lose lots of heat, so the current in wires that travel is kept to minimum to minimise this. Transformers help with this.