S-Cool Revision Summary

S-Cool Revision Summary

Magnetic fields

A magnet has a magnetic field around it. This field is strongest at its poles.

The arrows always point from the North pole to the South pole.

  • Opposite poles will attract.

  • Similar poles will repel.

Currents and magnetic fields

All currents have a magnetic field around them.

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.

Electromagnets are far more useful than permanent magnets because:

  1. They can be switched on and off.

  2. The strength of the magnetic field can be changed, by altering the current.

  3. 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:

  1. Increasing the current in the wire.

  2. Putting more loops on the coil

  3. 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.

  • Loses its magnetism quickly when the current is switched off.

  • Harder to magnetise.

  • Stays magnetic after the current is switched off.

Most electromagnetic devices use iron as the core, because they want the magnetism to change quickly.

The motor effect

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.

To make the force bigger:

  1. Increase the size of the current.

  2. Increase the strength of the permanent magnet.

Electric motors

An electric motor uses the motor effect to spin a coil of wire inside a magnetic field.

To increase the speed of the motor:

  1. Increase the current in the coil.

  2. Increase the number of loops on the spinning coil.

  3. Increase the strength of the magnet.

Loud speakers

The alternating current that represents a sound wave flows through the coil. As the current carrying coil is inside a magnetic field a force is produced, which makes the coil move. This pulls the paper cone in the same direction. As the current changes direction, the force produced changes direction. This makes the paper cone move the opposite way. The backward and forward motion of the cone produces a sound wave in the air.

Relays

When the small current in the input circuit is switched on, the electromagnet becomes magnetic and attracts the iron armature. The armature rotates towards the electromagnet, pushing the contacts together. This switches on the large current in the output circuit.

There are two main ways to generate electricity:

  1. Moving a wire in a magnetic field.

  2. Moving a magnet in a coil of wire.

To increase the voltage or current generated:

  1. Spin the coil faster.

  2. Put more loops on the coil.

  3. Use a stronger magnetic field.

  4. Use a coil with a larger area.

Transformers

Transformers are able to change the voltage of an alternating current. This is used on the national grid. The larger the voltage, the lower the amount of wasted heat energy in the cables. However, these large voltages are too dangerous to use in the home, so transformers are used to reduce the voltage to a safe level.

Calculating the size of the output voltage

You can work out the size of the voltage using the following equation:

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This means that if there are twice as many loops on the secondary coil, then twice the voltage will be across the secondary coil, and so on.