Physics GCE 'O' Level, Cambridge International Examinations

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a.c. generator

Without the electric generator, there would be no electricity and no modern life.  An electric generator generates electricity.  It works by inducing a voltage with changing magnetic field.  This picture shows a simple setup that is used in many real generators.

In this set up, there is a loop of wire in between two magnets.  The magnets produce a field in between them.  This magnetic field goes through the loop, or coil, from left to right.  The coil is forced to rotate.  This can be done by hand or by connecting the coil to a water turbine (not shown here).  As the coil rotates, the actual amount of magnetic field lines (flux) going through the coil changes.  If the coil is vertical, a lot of magnetic flux goes through.  If it is horizontal, the flux just goes past the side of the coil without going through.  Because the amount of flux linking the coil keeps changing, voltage is induced.  This follows Faraday's law of electromagnetic induction.

There are now two questions that we want to answer: (1) What is the direction of the current in the coil?  (2) How do we take the current out of the rotating coil and use this electricity?

(1) What is the direction of the current in the coil?

To answer this question, we may think about Lenz's law.  However, Lenz's is actually quite difficult to use in this situation because the coil keeps rotating.  Instead, it is much easier to use Fleming's right hand rule.  (Note that this is different from Fleming's left hand rule.)

To see how to use this, look at the right side of the coil in the picture below.  The field is to the right and motion is upward.  If you now stretch out your right hand thumb and fingers in the same way as the picture above, you will find the direction of the current as shown in the picture below.

(2) How do we take the current out of the rotating coil and use this electricity?

We do this using a clever arrangement called slip rings and carbon brushes.  As shown in the picture above, two wires lead away from the rotating coil.  Each of these wires are connected rigidly to a metal ring, called a slip ring.  As the coil rotates, the slips rings rotate together.  

Each slip ring touches a small piece of graphite, called a carbon brush.  The carbon brush is fixed and does not rotate.  So it brushes against the slip ring as it rotates.  Since graphite is a conductor, we can now connect a light bulb to the carbon brushes.  This clever device will then draw electric current from the coil as it rotates and pass the current through the light bulb.

The current through the light bulb keeps changing direction.  This is because each side of the coil moves up and down repeatedly as it rotates.  If we plot a graph of the voltage across the light bulb against time, it will look like this.

We think of one direction through the light bulb as positive and the other direction as negative.  So the graph oscillates up and down like a sine wave.