Electromagnetism (Physics Further)

What is it?

When we place a wire carrying an electric current in a magnetic field, it may experience a force. This is the motor effect

The force produced ia a maximum if the wire is at a 90 degree angle to the magnetic field, and the force will be at zero if the wire is parallel to the magnetic field.

This force can be increased simply by:

• Increasing the strength of the magnetic field
• Increasing the size of the current

The direction of the force produced on the wire will be reversed if either:

• The direction of the current is reversed
• The direction of the magnetic field is reversed

When a wire cuts through magnetic field lines a potential difference is induced across the ends of the wire. This therefore shows us that when a magnet is moved into a coil of wire, a P.D is induced across the ends of the coil. So what this therefore means that if the coil is part of a completed circuit, a current will pass though the circuit.

A potential difference is only induced when there is movement

The size of this P.d can be increased by increasing:

• The speed of movement
• The strength of the magnetic field
• The number of turns on the coil
• The area of the coil

A transformer consists of two coils of insulated wire, called the 'Primary' and 'Secondary' coils. These coils are wound around the same iron core.

When an alternating current (a.c) passes through the primary coil, it produces an alternating Magnetic field around the iron core, which continually expands and collapses.

The alternating magnetic field lines pass through the secondary coil and induce a p.d across its ends. If the coil is part of a circuit, an alternating current is produced.

Remember:

• The coils of wire are insulated so that current does not short across either the iron core or the adjacent turns of wire.
• The core is made of iron so it is easily magnetised

Transformers do not work with d.c current. Many students will forget this and lose valuable marks

Transformers are used in the Narional Grid to increase or decrease potential difference.

They are used to step up p.d from power stations, this is because the higher the p.d transmitted across the grid, the less energy loss in the cables. Step-down transformers are used to reduce p.d so it is safe to be used by consumers (us)

• In a step-Up transformer the p.d across the secondary is greater than the p.d on the primary
• In a Step-Down transformer the p.d across the primary is greater than the p.d across the secondary