Mains Electricity - Alternating Current.

We are familiar with direct current circuits
where the electrons flow from one side of the cell,
through all the components and return to the other side of the cell.

With alternating current (symbol a.c.)
the electrons first start to flow in one direction
until the current reaches a maximum value,
then the current slows down and stops.
The electrons then start to flow in the opposite direction
until the current reaches the same maximum value
(but it is now given a minus sign to show it is in the opposite direction),
then the current again slows down and stops.
This whole cycle then repeats itself over and over again.

The number of cycles in one second (called the frequency) 
is given in a unit called Hertz (symbol Hz)
where 1 Hz = 1 cycle per second.
Mains electricity in the UK is supplied at 50 Hz (50 cycles per second),
in the USA it is 60 Hz.

Continued on the next page.

All of the equations which relate to direct current circuits
also apply to alternating current circuits,
so that voltages, currents, power etc.
can be calculated in the same way.