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.
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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?
How would you describe this magnetic field pattern in words?
  • Try again! Move the compass around some more.
  • Try again! Move the compass around some more.
  • Try again! Are you sure the current is coming out of the screen. Click on the button to change the current.
  • Well done!
  • Click here to mark the question


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.
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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.
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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.

What happens to the magnetic field when the current flows in the opposite direction?
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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.

How would you turn off an electromagnet?
  • Try again! This would just change the direction of the field.
  • Click here to mark the question
Figure 4.   An electromagnet picking up a car.
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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.
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When a small current flows through the coil, a magnetic field is created.

What is the effect of the magnetic field on the soft iron armature?
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What happens to the second circuit when the armatures moves?
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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
1. The
conventional current
Conventional current flows from positive to negative around a circuit. However, electrons in the wire actually flow from negative to positive.
conventional current
in a wire is flowing from north to south. What is the direction of the magnetic field just above the wire?
  • Click here to mark the question
2. For the magnetic fields around the two wires below, decide the direction in which the current is flowing.
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3. Decide which compass below shows the correct reading for each of the positions marked around the solenoid in Fig.7 below.
  • Click here to mark the question
Figure 7.  
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4. The direction of the current flowing through the solenoid in Fig.7 above is reversed. What happens to the direction of the compasses?
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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
  • Click here to mark the question
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.
  • Click here to mark the question
Well done!
Try again!