Direct current is a steady current – it does not change in size or direction
with time. Direct current is the sort of current that you get from a battery. The size of a direct
current from a battery obviously changes when it passes through different things but is
always flows from positive to negative and there are no rapid changes in its
size.
Direct current can be used to operate bulbs, heaters and electronic circuit and
is needed to give a voltage to accelerate electrons in a television
tube.
Alternating current is a changing current –
its direction and size vary constantly, sometimes it flows in one direction in the circuit and a
fraction of a second later it is flowing in the other.
Alternating current is used in all
transformers and it is as alternating current that electricity is transmitted long distances
across the country.
The picture shows the size of a direct current and an alternating
current as time passes
In Britain the a.c mains varies with a frequency of
50 Hz. This means that there are 50 cycles per second and so one cycle takes 1/50
s.
As we have seen before, the voltage of a.c. varies with time. Figure 1 shows this.
You can see that the voltage is sometimes positive and sometimes negative. There is as
much positive as there is negative. The maximum voltage is called the PEAK VOLTAGE
If we work out the average voltage we get
zero. However there is a special type of average that is the a.c voltage that is as effective at
heating as a d.c voltage.
For the mains in Britain this special average is 240 V while
the peak value is 340 V.
You would find that Ohm's law holds for a.c. circuits as well
as for d.c.