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.

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.



Now try moving the wire upwards.



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



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.

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.



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.



The size of the induced current also depends upon the strength of the magnet.
A larger magnetic field gives a larger induced current.

To predict which way the induced current will flow, we can use Fleming's right-hand rule.

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.

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.



Now leave the magnet inside the coil without moving it.



Now pull the magnet out of the coil.



See if you can find out how the speed of the magnet affects the reading on the ammeter.



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.