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Transformer

A transformer changes a voltage to a higher or lower value.  It only works for an alternating current - that is a current that keeps changing direction.  One reason why we need transformers is to save a lot of energy when transmitting electricity over long distances from power stations to your house.  To understand how a transformer works, lets start with this example.

There is an iron rod.  On the lower end is a coil around the rod that is connected to a battery and a switch.  On the higher end is another coil connected to a galvanometer that measures current in both directions.  Now imagine what happens if you close the switch.

A current flows through the lower coil.  This current produces a magnetic field.  This iron becomes magnetised, so a strong field goes along the rod through the higher coil.  This change in magnetic flux induces a voltage in the higher coil and you will see the galvanometerneedle move.  So switching on current in one coil induces a voltage in the other coil.

However, this only happens for a short time.  When the current and magnetic flux stay the same, the induced voltage in the higher coil will go back to zero.  Remember, voltage is only induced when there is a change in magnetic flux linking the coil.

Now we open the switch in the lower coil.  The current stops and the magnetic flux disappears.  This change again induces a voltage in the higher coil.  You will see the galvanometer needle move in the opposite direction.  This means that a voltage is induced in the opposite direction.  After a short time, the needle moves back to zero.  The induced voltage disappears because the flux has stopped changing.

(Why does the needle move in different directions when you switch the lower coil on or off? It is because of Lenz's law.  When you switch on the lower coil, the magnetic flux increases.  The current in the upper coil must produce a flux in the opposite direction to oppose this increase.  When you switch off, the magnetic flux decreases.  The current in the upper coil must poduce a flux in the same direction to oppose this decrease.)
Now we are readily to look at a more standard setup of an electrical transformer.  This usually consists of a metal ring (can be square or circular) with coils on two sides.  One coil is connected to an a.c. generator.  The other coil is connected to a voltmeter (or some circuit that uses the electricity).

The a.c. generating produces an alternating current in the first coil, called the primary coil.  Because this current keeps changing direction, it produces a magnetic field in the iron ring - known as iron core - that keeps changing direction.  The iron core must be made of soft iron, which is good at guiding the magnetic flux along the core through the other coil, called the secondary coil.  This changing flux induces voltage in the secondary coil.

How does a transformer change voltage?

The answer is: by changing the number of turns on the primary and secondary coils.  If we have more turns on the secondary coil and the primary coil, like in the above picture, then the induced voltage at the secondary coil will be larger.  If we have fewer turns on the secondary coil, the voltage at the secondary coil will be smaller.