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Electric current
What is direct current?
Direct current (d.c.) is an
electric current that always
flows in one direction.
Direct current is produced
by cells and batteries.
Electronic circuits such as
those in computers and
stereos need direct current
electricity in order to work.
Direct current cannot be
transferred efficiently over
large distances.
What is alternating current?
Alternating current (a.c.) is
an electric current that is
constantly changing direction.
Alternating current is produced
by most generators and is
used in mains electricity.
Motors often work using
alternating current. The voltage
of alternating current is easily
changed with a transformer.
Alternating current can be
transferred efficiently over large
distances.
What is the difference between a.c. and d.c.?
d.c.
time
voltage
voltage
The difference between alternating current (a.c.) and
direct current (d.c.) can be seen using an oscilloscope.
For each current, the oscilloscope trace is a graph showing
how the voltage of an electricity supply varies with time.
peak forward
voltage
a.c.
time
peak reverse
voltage
The voltage of a d.c. supply
is steady and always in the
same direction.
The voltage of an a.c. supply
follows a repeated pattern: it
rises to a peak, returns to
zero, changes direction and
so on.
Frequency of alternating current
voltage
The frequency of a.c. electricity is the number of complete
cycles per second which is measured in hertz (Hz).
1 complete cycle
peak forward
voltage
a.c.
time
peak reverse
voltage
The a.c. frequency can be determined from an oscilloscope
by counting the number of complete waves per unit time.
If the frequency is increased, the number of complete waves
shown on the screen increases. For example, if the
frequency is doubled, the number of waves doubles.
Mains electricity in the UK
In the UK, the frequency of
mains electricity is 50 hertz:
this alternating current flows
backwards and forwards
50 times per second.
This frequency is the same
at any point in the electricity
supply system but the
voltage varies in different
parts of the national grid.
The voltage of mains electricity supplied to UK homes is 230 V.
This is an effective voltage which is equal to the voltage of
a d.c. supply that would produce the same heating effect.
The peak (maximum) voltage is higher than this.
Comparing a.c and d.c
Which type of current – a.c. or d.c.?