P3bL3 Electromagnetic Induction 2
Key Words
AC generator - a generator that makes alternating current (really?)
Coil - wire wrapped up on a frame.
Conductor - material that allows electricity to flow.
Magnetic field - a force field which affects magnetic materials.
Mutual Induction - a voltage obtained from a coil when a changing current flows from a coil next to it.
Oscilloscope - the CRO, an instrument that displays electric waveforms.
Power surge - an increase in the energy available in the national grid.
Voltage - electrical energy per unit charge.
Grade E
Like all European countries, the UK has an extensive network of electrical wires that enable electricity to be distributed nationwide. This is the national grid. There are many power stations providing the electricity we need. The demand for electricity varies throughout the day and there are power stations on stand-by that can be connected when a power surge is needed, for example during half-time at a big football match, when everyone puts the kettle on.
In North Wales there are two pumped storage power stations, where water is pumped up to a reservoir high in the hills, and falls to drive turbines to generate electricity in periods of high demand. At night, when the demand is low, the generators act as very powerful motors to pump the water.
In the last lesson we saw how we can make a simple ac generator (an alternator). Alongside you can see a simple d.c. motor.
They are similar in that they have:
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a magnet;
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an armature;
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brushes.
They are different because:
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in the motor there is a commutator (a ring of copper split by a layer of insulating material);
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in the generator there are two slip rings.
The DC motor will act as a DC generator.
Grade C
We will look at how the AC generator works.
We will now see what happens as the armature is turned in the magnetic field. The voltage produced will be displayed on both a centre-zero voltmeter and a computer programmed to show a voltage time graph. The voltage is on the vertical axis, while the time is on the horizontal axis.
Let's start with the armature vertical. It is turning all the time. There is a little red spot on the armature to show us how it turns.
The voltmeter is zero because there would be no voltage as the wire is moving parallel to the magnetic field lines.
Now let's look what happens 1/4 turn later. The little red dot has disappeared behind the brushes.
The voltage is at a maximum because the wire is cutting the magnetic field lines at 90 degrees.
Notice how the computer has plotted the rise in voltage from zero to a maximum positive value. It is positive because of the way the coil is wired.
Now look what happens 1/4 turn later:
The voltage is zero again, because the wires are moving parallel with the magnetic field lines, so none are cut.
Now 1/4 turn later, we have a maximum negative value of the voltage.
Finally, 1/4 turn later, the generator has turned a full circle. You can see a complete cycle on the computer screen.
We can show this by displaying the waveform on the oscilloscope. It is an alternating current.
The direction of the induced current is determined by Fleming's Right Hand Rule:
Yes, it's a mirror image of a previous picture!
Grade A
We have stated Faraday's Law that the voltage induced is proportional to the rate of change of flux. Flux means the number of field lines. In generators, we are cutting magnetic field lines.
However we don't need there to be movement to get a change in flux. We can do this by changing the current in a coil of wire.
An alternating current in Coil A will induce an alternating voltage in Coil B. The soft iron rod is so called because soft iron loses its magnetism instantly the current in a coil is turned off. (It is not soft to feel; it is dense and will bring tears to your eyes if you drop it on your foot.). This process is called mutual induction.
It is important to realise that the coils are NOT connected electrically.
We can even take out the soft iron rod out and there will still be a small voltage induced in Coil B.