Physics A - AS Module 3

Alternating Current

The basics

The frequency of an alternating current supply, f, is the number of cycles completed per second.

The period, T, of an alternating supply is the time taken to complete one cycle.

The peak values of current, Ip, and voltage, Vp, are the ones at the crest or trough on the diagram. They are equivalent to the amplitude of a wave.

Average current, voltage and power

Alternating current electricity is easy to make and easy to transform so it is the most common form of electrical supply. The problem with using it is that the voltage and current oscillate, these means that in calculations we would need to use an average value. However as you can see from the diagram the average value would be zero. This means we need to find an alternate root.

So if we look at the power (energy transformed per second) over a period of time, even though it is varying, its average value is greater than zero. In fact the graph looks like this:

As you can see from the graph, the average power is half the peak power.

We can use this to calculate what is known as the rms values for current and voltage

RMS Values

If:

Power = V I

then, obviously, peak power occurs at time of peak voltage and current.

Peak power = Vp Ip

So, as we’ve said,

Average power = ½ x peak power = ½ Vp Ip

and we can say

1/2 Vp.Ip =Vp.Ip/2 = (Vp/root2)(Ip/root2)

and hence

Average power = (Vp/root2)(Ip/root2)

and we can then assume