Current from Magnets
Introduction

In 1819 Hans Christian Oersted discovered that an electric current creates a magnetic field.

Just over ten years later the English scientist Michael Faraday demonstrated the reverse effect. He showed how a magnetic field could be used to generate electricity.
A moving wire
The experiment in Fig.1 below lets you investigate what happens when you move a wire in a
magnetic field
Around every magnet there is an invisible magnetic field.
magnetic field
. The wire is connected to an ammeter which records any current flowing in the wire.

Try moving the wire left and right.

Does any current flow when you drag the wire left and right?
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Figure 1.   Moving a wire in a magnetic field. The arrow indicates the direction of current flow.
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Now try moving the wire upwards.

Does any current flow when you move the wire upwards?
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Watch which way the current flows when you move the wire upwards.
Now drag the wire downwards. Observe which way the current flows now.

In which direction does the current flow when you move the wire downwards?
  • Are you sure?
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When a current flows in the wire, we say that the current has been induced. A current is only induced in the wire when it cuts across the magnetic field lines. No current is induced when the wire moves along the magnetic field lines. This is an effect called electromagnetic induction.

Induction
In the experiment in Fig.2 below, the wire has been connected to an ammeter. You can use this to measure the value of the induced current.

Figure 2.   Measuring the induced current.
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Try varying the
speed
Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.
speed
with which you drag the wire about. Look for a relationship between the speed at which you drag the wire and the value of the induced current.

When is the current the greatest?
  • Are you moving the wire up and down?
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The size of the induced current also depends upon the strength of the magnet.
A larger magnetic field gives a larger induced current.

Fleming's right-hand rule
To predict which way the induced current will flow, we can use Fleming's right-hand rule.
Figure 3.   Fleming's right-hand rule.
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  1. Point your thuMb in the direction of the Motion.

  2. Rotate your hand so that your First finger points in the direction of the magnetic Field.

  3. Your seCond finger will now be pointing in the direction of the induced Current flow.



Go back to the experiment in Fig.2 to try out the right-hand rule. You should find that it works no matter what direction you move the wire in.

A moving magnet
In the experiment above, you induced a current by moving a wire around in a magnetic field. You can also induce a current by moving a magnet around near a wire. The experiment in Fig.4 lets you investigate this effect. Instead of having one wire, there is a long coil.

Move the magnet into the coil.

Does moving a magnet into the coil induce a current in the wire?
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Is the induced current positive or negative?
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Figure 4.   Moving a magnet into a coil.
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Now leave the magnet inside the coil without moving it.

Is a current induced in the wire when the magnet is not moving?
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Now pull the magnet out of the coil.

Does pulling the magnet out of the coil induce a current in the wire?
  • Are you sure?
  • Remember, a deflection to the left means a negative current.
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See if you can find out how the speed of the magnet affects the reading on the ammeter.

How must the magnet move to induce the greatest current?
  • Are you sure?
  • Try moving the magnet faster
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In general, the size of the induced current increases with these changes:

  • increasing the strength of the magnet

  • increasing the number of turns of wire on the coil

  • reducing the diameter of the coil

  • increasing the speed with which the magnet moves

Summary

A current is induced whenever a wire moves so that it cuts across magnetic field lines. The direction of the induced current can be found by using Fleming's right-hand rule.

The size of the induced current can be increased by using a stronger magnet or by moving the wire faster.

A current is also induced when the magnetic field near a wire changes, for example when a magnet moves into a coil of wire.

The size of this induced current can be increased by using a stronger magnet, moving the magnet more quickly, wrapping more turns around the coil or reducing the diameter of the coil.

Exercises
1. What is the name used to describe the current created when a wire moves in a magnetic field?
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2. A current is always induced in a wire when it …
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3. Choose the correct words to complete the sentence below:

  • When a magnet moves into a coil, the size of the induced current depends on the of the magnet, the of the movement and the on the coil.
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4. If you want to predict the direction of an induced current, which rule should you use?
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5. Use the correct rule to fill in the blanks in the table below.

  • Direction of magnetic field Direction of wire's motion Direction of induced current
    South West
    East South
    Down North
    West Up
    Up West
    North Down

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Figure 5.  
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6. The magnet in Fig.5 above is being moved slowly into the coil. What is the approximate reading on the meter?
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7. What will be the reading on the meter when the magnet in Fig.5 is stationary inside the coil?
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8. What will be the reading on the meter when the magnet in Fig.5 is pulled quickly out of the coil?
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9. What will be the reading on the meter when the magnet in Fig.5 is stationary outside the coil?
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Well done!
Try again!