1. Magnetism and electromagnetism
We say the field lines flow from north to south.
The closer together the field lines, the stronger the magnetic field.
The field is concentrated at the poles.
The only test for magnetism is repulsion.
If a piece of metal is attracted by a magnetic, that only proves that the metal is made of a magnetic material. To prove that it is a magnet, it must REPEL another magnet.
When two magnets are brought close to each other, there is a force between them.
N S
S N
attract
repel
Most materials are not magnetic.
A common mistake is to believe that metals are magnetic – only a very few are.
The magnetic materials are :
iron nickel cobalt
The Earth acts as a huge magnet. Compasses work because they are affected by the Earth’s magnetic field.
As we know, the Earth has two poles. However, the North pole of the Earth is actually a magnetic South pole!
Magnetism and electromagnetism
The Loudspeaker
Electric Bell
Inside the speaker there is a permanent magnet and an electromagnet. The strength of the electromagnet is changed by the signal, and so the force between it and the permanent magnet also changes.
One of the magnets is attached to the cone of the speaker. As the strength of the electromagnet changes, the cone vibrates and makes a sound
When the switch is closed, current flows through the coil and the iron core becomes magnetised.
This attracts the iron armature.
The top of the armature is pulled towards the electromagnet.
The armature turns on the pivot, and the bottom part moves to the right, pushing the contacts together.
The second circuit is now complete, and the large current will make the motor turn.
The shape of the field around an electromagnet is similar to the field around a bar magnet.
Relay

2. Magnetism and electromagnetism
Flemings left Hand Rule
When a current flows through a wire a magnetic field is produced around the wire.
The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.
If a magnet is moved into a coil of wire a potential difference is induced across the ends of the coil.
Force
Field N > S
current + > -
force
The motor
turns
The force on a motor can be increased by: *increasing the strength of the magnetic field *increasing the size of the current This will make it turn faster
Right Hand Rule - gives direction
Switch mode transformers use very little power when they are switched on but no load is applied.
Electromagnet:
Stronger if more turns of wire on the coil and more current through the coil.
Switch mode transformers operate at a high frequency, often between 50 kHz and 200 kHz.
In a step-down transformer the potential difference across the secondary coil is less than the potential difference across the primary coil.
In a step-up transformer the potential difference across the secondary coil is greater than the potential difference across the primary coil.
The Motor Effect:
A current carrying conductor will experience a force if it is placed cutting a magnetic field The conductor will not experience a force if it is parallel to the magnetic field.
Magnetism and electromagnetism
If an electrical conductor ‘cuts’ through a magnetic field a potential difference is induced across the ends of the conductor
The Transformer: An alternating current in the primary coil produces a changing magnetic field in the iron core and hence in the secondary coil. This induces an alternating potential difference across the ends of the secondary coil.
transformer equation: Vp / Vs = np / ns
Transformer coils are made of copper wire with and insulating covering . The core is made of iron (a magnetic material which is easily magnetised and demagnetised). An ac power supply for the primary voltage.
Electromagnets are used on cranes for lifting iron/ steel.
Switch mode transformers are much lighter and smaller than traditional transformers working from a 50 Hz mains supply. They are used for mobile phone chargers
If transformers are assumed to be 100% efficient, the electrical power output would equal the electrical power input. Vp x Ip = Vs x Is