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Physics

Movement from electricity

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Electric bells and motors are commonly used devices. This bite takes you through the physics behind the operation of both of them. There are three subject areas in this bite: magnets and electromagnets, uses of electromagnets and the electric motor.

Work through them all or go to the ones that interest you. You may wish to print a hard copy for your reference.

# Magnets and electromagnets

## Magnets

• Magnets have two poles called North and South.
• Similar (like) magnetic poles repel. Unlike magnetic poles attract. A magnet attracts a piece of iron. The most important of the two properties of attraction and repulsion is repulsion. The only way to tell if an object is magnetised is to see if it repels another magnetised object.
• The strength and direction of a magnetic field is represented by magnetic field lines. Field lines by convention go from North to South. A magnetic field is three-dimensional, although this is not often seen on a drawing of magnetic field lines.

## Electromagnets

• A magnetic field exists around all wires carrying a current.

### A magnetic field exists around all wires carrying a current

The slideshow above uses ‘conventional current’ – this assumes that there is a flow of positive charges in the wire. Electron flow is in the opposite direction. If the slideshow used electron flow then the red arrow would move down.

• When there is no current the compass needles in the diagram shown line up with the Earth's magnetic field. A current through the wire produces a circular magnetic field. See what happens when there is a current in the wire.
• The magnetic field for a coil of wire is shown below. The magnetic fields from each of the turns in the coil add together, so the total magnetic field is much stronger. This produces a field which is similar to that of a bar magnet. A coil of wire like this is often called a solenoid.
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