Electromagnets

Introduction

How can the magnet in a scrapyard turn off its attractive force so that it can drop a car back down? It can do this because it is an electromagnet. When current flows through a wire, a magnetic field is produced around the wire and an electromagnet is formed. The magnetic field can be turned off again by switching off the current.

Current and magnetic fields

For many years electricity and magnetism were thought to be separate phenomena. Then in 1819 the Danish physicist Hans Christian Oersted placed a magnet near a current-carrying wire. You can repeat his experiment using the compass and the wire in Fig.1. The red end of the compass is north and points in the direction of the

magnetic field

Around every magnet there is an invisible magnetic field.magnetic field. Turn on the current and drag the compass around to see if you can find a connection between electricity and magnetism.

Figure 1.	Oersted's experiment to investigate the connection between electricity and magnetism.

When there is no current, the compass always points in the same direction. This is because of the earth's magnetic field.

Which of these diagrams shows the magnetic field when the current is flowing up through the screen?

What do you think will happen if the current flows in the opposite direction?
Click on the button to change the direction of current flow. Were you right?

Which of these diagrams shows the magnetic field with the current flowing downward, into the screen?

There is an easy way to remember which way the magnetic field points. Imagine gripping a wire with your right hand. Your thumb points in the direction of the current and your fingers curl around the wire in the direction of the magnetic field.

Figure 2.	Right-hand grip rule.

Coils of wire

The magnetic field from one wire is not very large. By putting lots of wires next to each other, a much larger field is created. The easiest way to do this is by making the wire into a coil. The magnetic field around a coil is very similar to the field around a bar magnet. This kind of coil is often called a solenoid.

Use the compass to explore the magnetic field around the solenoid below. Click the button to change the current flow. When no current is flowing, there is no field from the solenoid. The only field is the earth's magnetic field.

Figure 3.	The magnetic field around a solenoid.

Set the current so that it flows from left to right. The current flows up over the loops in the solenoid at the front and down the back part of the loops.

Which of these pictures shows the correct magnetic field?

The field from a bar magnet is very similar to the field from a solenoid.

Which of these pictures shows the correct way you would place a bar magnet to generate the same field as the solenoid?

Change the current so that it flows in the other direction.

A magnet like the solenoid above, which is made by passing

electrical current

Electrical current is caused by the flow of charged particles. This is most commonly due to the flow of electrons in a metal wire, but can be due to the movement of any type of charged object.electrical current through a wire, is called an

electromagnet

An electromagnet is formed by passing a current through a coil of wire. Unlike normal magnets, electromagnets can be turned on and off.electromagnet. An electromagnet can be turned on or off.

Figure 4.	An electromagnet picking up a car.

The strength of an electromagnet can also be controlled.
These three factors all increase the strength of an electromagnet:

increasing the current flowing through the coil;
increasing the number of coils;
putting a core of a magnetic material inside the solenoid. The iron core becomes magnetized itself and makes the field stronger.

Uses of electromagnets

Electromagnets have many applications. A common use for an electromagnet is within a switch called an electromagnetic relay. This is a device which enables a large current to be controlled by a much smaller one.

Figure 5.	An electromagnetic relay.

When a small current flows through the coil, a magnetic field is created.

A much larger current can then flow through the second circuit, such as to a motor or a heater.

This is the circuit symbol for a relay:

Click on the push button in the circuit below.

Click on the figure below to interact with the model.

Figure 6. An application of the relay.

Can you see how the relay is used to drive the motor? This is how the starter motor in a car works. When you turn the ignition, a small current flows through a relay. The relay then lets a much larger current flow to the starter motor.

Summary

When a current flows through a wire a magnetic field is produced.

The magnetic field from one wire is quite small. By putting lots of wires next to each other a much larger field is created. The easiest way to do this is by making the wire into a coil.

The three factors which increase the strength of an electromagnet are:

increasing the current flowing through the coil;
increasing the number of coils;
putting an iron core inside the coil

Exercises

2. For the magnetic fields around the two wires below, decide the direction in which the current is flowing.

3. Decide which compass below shows the correct reading for each of the positions marked around the solenoid in Fig.7 below.

Figure 7.

5. An electromagnets is used to lift up cars. Decide whether the following changes would allow the electromagnet to lift heavier cars.

Increase the current
Add more turns around the core
Take out the metal core of the electromagnet

6. Decide whether each of the statements below regarding an electromagnetic relay, are true or false.

A relay is like a switch that can be opened and closed using a small current.
There are no moving parts inside a relay
The electromagnet inside a relay is always magnetized.
The size of the large current flowing through a relay is proportional to the size of the small current.