The appliance of science Lesson objectives: To explain how an electromagnet works. To discuss how a relay uses a small current to switch on a much larger current. Key words: Current, relay, electromagnet

Electromagnet Can you think of a type of magnet that can be turned on and off. Watch the following video clip and write instructions on how to make an electromagnet.

Magnetic effects of current

Making an electromagnet

Investigating electromagnets The strength of an electromagnet depends on whether it has a core of iron. Two experiments can be carried out to investigate the other factors that can affect the strength of an electromagnet: Investigate how the number of coils affects the number of drawing pins attracted to an electromagnet – keep the current the same in this experiment. Investigate how the size of the current affects the number of drawing pins attracted to an electromagnet – keep the number of coils the same in this experiment.

How an electromagnet works An electromagnet is made by using electricity to make a piece of iron magnetic. When a wire has an electric current running through it, the current creates a magnetic field. If you twist the wire into a coil, the magnetic field has the same shape as a bar magnet. A piece of iron in the core makes this core field stronger.

How could you use the apparatus below to investigate the effects of changing the current and the number of coils on the strength of an electromagnet? Investigating electromagnets

Investigating electromagnets – results Here are some example results number of coils number of drawing pins attracted current (A) number of drawing pins attracted Describe the trend commenting on all results. (grade C) Mention how you made it a fair test and how you can improve your investigation. (Grade B/A)

number of coils number of drawing pins attracted Investigating electromagnets – analysis How did the number of coils affect the number of drawing pins attracted to the electromagnet?

Investigating electromagnets – analysis How did the size of the current affect the number of drawing pins attracted to the electromagnet? current (A) number of drawing pins attracted 50

Using electromagnets – recycling A large electromagnet is used on a recycling plant conveyor belt to pick up and move metal cans. What advantages does an electromagnet have over a permanent magnet? Which metals would the electromagnet attract?

Uses of electromagnets – electric bell When the circuit is closed, the electromagnet pulls the armature towards it, causing the hammer to strike the bell. The movement of the armature breaks the circuit, and the hammer returns to its original position. This sequence repeats, causing the bell to sound continuously. The circuit for a door bell includes an electromagnet.

Label the diagram – electric bell

Using electromagnets – door bells

A relay is a switch that is turned on and off by an electromagnet. An electromagnet used in this way is usually called a solenoid. Relays are used when a circuit with a high current needs to be switched on or off. Using a relay to do this means that a person does not have to touch the circuit with the high current in it.

Lifts, cars and other large electrical machines use high currents. Relays use an electromagnet to allow a small current in one circuit to control a large current in another circuit. Can you predict what will happen when the switch is closed in this circuit? Using electromagnets – the relay

Label the diagram – relay

Electromagnets: summary