Download Direct and alternating current

Direct current (d.c) and alternating current (a.c)
Direct current
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
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
Voltage
Direct current
Alternating current
Time
One cycle
Figure 1
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.
1