All currents have a magnetic field around them. All the cables connecting electrical appliances to the mains in your home will have magnetic fields around them, and so do the large electricity power lines you can see on pylons outside.
A straight wire has a circular magnetic field around it. A coil of wire has a magnetic field around it, that is the same shape as a bar magnet.
If the conventional current flows the other way, the magnetic field will be in the opposite direction. As you move further away from the wire, the magnetic field gets weaker, which is why the lines are drawn further apart.
These types of magnets are called electromagnets. They are temporary magnets as they can be turned on and off with the current. Normal bar magnets are permanent magnets because it is very difficult for them to lose their magnetism.
Electromagnets are far more useful than permanent magnets because:
- They can be switched on and off.
- The strength of the magnetic field can be changed, by altering the current.
- They can easily be made into a variety of shapes and are less expensive to make.
The magnetic field around a coil electromagnet can be increased by:
- Increasing the current in the wire.
- Putting more loops on the coil
- Placing an iron or steel core inside of the coil.
Iron and steel behave slightly differently as cores, because iron is magnetically soft and steel is magnetically hard.
|Magnetically soft, for example, iron:||Magnetically hard, for example, steel:|
|- Easy to magnetise.||- Harder to magnetise.|
|- Loses its magnetism quickly when the current is switched off.||- Stays magnetic after the current is switched off.|
Most electromagnetic devices use iron as the core, because they want the magnetism to change quickly.
When two magnets are close together, they affect each other and produce a force. The same happens when any two magnetic fields are close together.
If a wire carrying a current is placed in a magnetic field a force is produced. This is called the motor effect.
The direction of the force will depend on the direction of the magnetic field and the direction of the current in the field.
Note: The current, magnetic field and force will always be at right angles to each other, so the wire will not move towards the poles.
So why does this happen?
The magnetic field from the current is affected by the magnetic field from the magnet; this produces a force.
To make the force bigger:
- Increase the size of the current.
- Increase the strength of the permanent magnet.