Double Award - Science for GCSE

Chapter 29

Electromagnetism

1.	Magnetic poles and magnetic fields The ends or parts of a magnet where the magnetic effects are the strongest are called poles. Magnets will only attract metals and alloys of iron, cobalt and nickel. The pole of a magnet which points north is called the north pole. The pole which points south is called the south pole. Like poles repel and unlike poles attract one another. The space around a magnet, where its magnetic force acts, is called a magnetic field. The magnetic field around a magnet can be shown as a series of field lines called lines of magnetic force. Lines of magnetic force point away from a north pole and towards a south pole.
2.	Electromagnets An electromagnet can be made by passing an electric current through a coil of wire. An electromagnet will be stronger if: – the current in the coil is larger; – there are more turns in the coil; – a soft iron rod (core) is placed inside the coil. A relay is an electromagnetic switch. A small current in one circuit is used to switch on a larger current in another circuit.
3.	Motors In a motor: You can use the left hand motor rule to predict the direction of movement. – Hold the first finger, second finger and thumb of your left hand at right angles to each other. – Point your First finger in the direction of the Field and your seCond finger in the direction of the Current. Your thuMb now points in the direction of Movement.

4.	Generators (dynamos) If you are studying for the higher tier, you should understand and be able to use the following equations. In a generator: Electric currents generated in this way are called induced currents and the effect is known as electromagnetic induction. The size of the induced current is greater if: - the coil or the magnet moves faster; - the magnet is stronger; - the coil has more turns. An alternating current is generated if a coil rotates in a magnetic field or if a magnet rotates inside a coil.
5.	Transformers and the National Grid A transformer consists of two coils of wire wound on an iron core. An induced current is obtained in the secondary coil if an alternating current passes through the nearby primary coil. Step-up transformers raise the voltage in the secondary coil, but lower the current. Step-down transformers lower the voltage in the secondary coil, but increase the current. Step-up transformers are used to raise the voltage generated in power stations to either 275 000 or 400 000 volts for the National Grid. Step-down transformers later reduce the voltage to 230 volts for use in our homes and work places. Equations for transformers