Electromagnets in everyday use
Physics Narrative for 11-14
Why use electromagnets?
Why might we want to use an electromagnet rather than a permanent bar magnet? There are two obvious advantages to using electromagnets.
Firstly they can be switched on and off. Complete an electric circuit and a current passes to produce a magnetic field. Switch off the current and the magnetism disappears (provided the iron forms a temporary magnet).
Also, their magnetic strength can be changed. The strength of the magnetic field around the solenoid can be increased by:
- Increasing the number of coils (or turns) of wire.
- Increasing the electric current through the coil.
- Placing a magnetic material inside the solenoid coil.
Car scrap-yards use huge electromagnets to lift heaps of crumpled iron and steel. Switch off the current and the object crashes to the ground.
In the home, by far the most common use of electromagnets is in electric motors. Think of all of those bits of electrical equipment with some kind of electric motor: vacuum cleaners, refrigerators, washing machines, tumble driers, food blenders, fan ovens, microwaves, dish-washers, hair driers.
The list is a long one, and when you start thinking more widely about electric motors in cars, lawn-mowers and a whole host of industrial applications, it becomes obvious that this application of electromagnets is extensive and extremely important to our daily lives. The question of how electric motors work builds on the basics of magnetism introduced here, and is usually worked on in later years.
Bells, relays and motors
Electromagnetic door bells are
make and break devices which work via an electromagnet. There is one electric circuit containing two switches. One is a conventional push-button switch. The second has two parts, a spring and an electromagnet. The alternating action of the spring and the electromagnet makes and breaks the circuit for as long as the push-button switch is pressed.
An electromagnetic relay consists of two circuits. The first circuit contains a simple electromagnet which requires a relatively small current to make it work. When the switch is closed, there is an electric current through the coil of wire and the iron rocker arm is attracted to the electromagnet. The arm rotates about the pivot and closes a switch to complete the second circuit and the motor starts up – the motor requires a much larger current. When the switch in the first circuit is opened the electromagnet releases the rocker arm and the switch springs open again. The motor circuit is now broken.
The motor is more complex than either, but included here for completeness.