Electricity is generated in power stations and
from there it is transmitted across the country by power cables to towns and villages. As you
know, all cables have resistance and so as the electricity passes through them it will lose
energy as heat in the cable.
The energy lost in a power cable that is carrying a current I
and is of resistance R is given by the formula:
As you can see the power loss depends on the
resistance of the cable and so to reduce this power loss the resistance of the cable must be
as small as possible.
The resistance of a given length of cable can be reduced by:
(a)
using thicker cables — but this will make them too heavy;
(b) using material
which is a better electrical conductor such as silver — much too expensive;
(c)
cooling the cables with a jacket of liquid hydrogen — too expensive.
So changing
the resistance is not a practical idea, but the current passing through the cable can be altered
using a transformer. An example of this is shown next.
This is an enormous saving - and for this reason electricity is transmitted at
low current and high voltage. To keep the power loss as small as possible the transmitted
voltage is very high. Electricity generated in a power station at 25 000 V (25 kV) is stepped
up to 275 kV or 400 kV for transmission across large distances. Near towns, villages and
industrial sites there are transformers that step down the voltage ready for use. The
photograph shows a transformer on a pole in farmland.
A simple model power line may
be made in the laboratory using inputs and outputs of 12 V and stepping it up for
transmission down 2 m of 28 SWG constantan wire to 120 V.
Only the commercially available apparatus must be used when carrying out this experiment and the recommended safety precations given with that apparatus MUST be followed.
(See 'The New Resourceful Physics Teacher')
The diagram shows how electricity is transmitted round the country by the national grid system. Note the step-up and step-down transformers and the voltages used.
