So far we have looked at the effect of putting a current in a magnetic field and seeing that there is a force on the wire that carries the current. On this page we will look at a related effect known as induction.
The best way to learn about induction is to consider the results of a simple experiment. In fact if your school has a spot galvanometer, you should really take a look at these results in the flesh. Otherwise you'll just have to take my word for it.
Experimental set up to demonstrate induction
Look at the diagram below. A coil of wire (solenoid) is attached to the inputs of a spot galvanometer. The solenoid can be made by winding a piece of plastic coated wire a round a paper tube.
A spot galvanometer (also known as a mirror galvanometer) looks like a fancy piece of kit, and it is pretty fancy, delicate and expensive, but it does a very simple job. It's really just a very sensitive ammeter. It measures current just like any other ammeter but is much more sensitive than the normal sort you usually see. It can detect tiny currents.
The only other thing that is needed is an ordinary bar magnet.
So, once the apparatus has been set up as in the diagram above, we can look and see what happens.
|What was done||What was observed to happen|
|The magnet is moved into the coil.||The spot deflects to the left indicating that a small current is flowing.|
|The magnet is left inside the coil.||The spot comes back to the middle showing the that no current is flowing.|
|The magnet is pulled out of the coil.||The spot deflects to the right indicating that there is a current flowing again but this time in the opposite direction.|
|The experiment is repeated except this time the magnet is inserted into the coil very slowly.||There is still a deflection but this time it's much less than before indicating a smaller current is flowing.|
|The magnet is turned around.||The results are the same as above except the deflections are reversed.|
|The magnet is kept still but the coil is moved.||The results are as above.|
- If there is relative movement between a coil of wire and a magnet, there will be a current induced in the coil.
- The faster the movement the bigger the current.
- Reversing the magnet, or the direction of movement reverses the direction of the induced current.
Q1) What effect will reversing the magnet and the direction of movement have on any induced current ?
Theoretic explanation of what's going on (Advanced)
So now we know what happens we have to come up with an explanation. Look at the diagram below.
In this diagram the green line represents a wire that is going into the screen or page. It is at right angles to the magnetic field between the two bar magnets. The is moved downwards so that it "cuts" the field lines. This induces an e.m.f. (voltage) across the length of wire. It is this e.m.f. that causes a current to flow if a complete circuit is made. So now we know why you get a current. It's caused by the induced voltage, but why do you get a voltage?
Look at the diagram below.
This is a close up of the wire seen end on. The blue circles represent where the magnetic field lines stick out of the screen or page. They are coming straight out of the screen.
The wire is made of a metal and so has many free electrons. As the wire moves downwards each individual electron travels downwards too. They effectively form loads of little current flowing down. From the motor effect we know that if a current flows in a magnetic field there will be a force. in this case the force pushes the electrons to the right. It is the electrons all moving up to the right end of the wire that causes the voltage difference.
So you see, Induction is really the motor effect.
Q2) A student uses a stronger magnet. What effect do you think this will have on any induced current?
Q3) Imagine more than one loop of wire cutting through the magnetic field. Each loop of wire has its own e.m.f. induced on it. If the wires are joined up, it's as if they are little cells in series. Fill in the blanks " As the number of windings on the solenoid increases the value of the induced voltage ______________.
Q4) An engineer wants to uses the induction to create electrcity. He gets a couple of students to push an enormous magnet into and out of a huge coil (he uses a whip to make 'em go fast). In what way will the electricity that this set up produces be different to a the electricity produced by a battery?