Physics GCE 'O' Level, Cambridge International Examinations

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Electric Power Transmission

We get our electricity at our home from a power station.  The power station is usually very far from our house. The electricity is carried to our house by cables that can be hundreds of kilometres long.  Cables have resistance, and a lot of energy would be wasted for the current to overcome this resistance.

There is a way to reduce this loss of energy to a very small amount.  It is to use a transformer to step up the voltage to a very high value before sending the current through the cables.

The reason this works is because when the voltage becomes very high, the current becomes very small.  Much less energy is needed to overcome the same resistance if the current is small.  To understand how this works, lets look at a transformer.

This picture shows a transformer.  It is made up of a square ring of soft iron core.  On the left is the primary coil, which may be connected to an a.c. generator in the power station.  On the right is the secondary coil, which has a lot more turns than the primary coil.  So the voltage induced in the secondary coil is much higher.  

However, if the secondary coil is connected to circuit (like the cables leading to your house), the current in the secondary coil would be much smaller than in the primary coil.  The reason is that energy must be conserved.  So power going into the primary coil must be equal to power coming out of the secondary coil.  Since power = voltage x current, or P = VI, if voltage increases, current must decrease.  That is why current in the secondary coil must become very small in order to conserve energy.

If current is small, less energy is lost through resistance in the long cables.  This is because the rate of energy loss = current² x resistance, or P = I²R.  Note that the P here is different from the power  going into the secondary coil (P = VI above).  Of the power (VI) that goes into the secondary coil, a small amount (I²R) is lost as heat through the cable resistance R.

When the cables reach our house, the electricity cannot come into our house directly because the voltage is too high.  It must first go to another transformer near our house to step down the voltage.  This transformer would have fewer turns in the secondary coil than the primary coil. After the voltage is reduced to 100 or 240 V, depending on which country you live in, the electricity then goes into our house.