Harold Hall

Basic Electrical Theory AC     Harold Hall

Workshop Data

The stated voltage value of an AC supply, say that known for the voltage of the domestic supply, 240V in the UK, is 0.707 times the peak value. This is known as the "effective" or "RMS" voltage (root mean squared), "effective" because it is the voltage that produces the same heating effect in a resistor as would a DC supply of the same value.


In a very few cases the average value is also used. This is arrived at by taking instantaneous voltage values at numerous points on the waveform, summing them and then calculating their average. This results is a value of 0.636 times the peak value. I have included this for completeness just in case you should come across it. However, in all common applications it is the RMS value that is given.


Applying an AC voltage to a load results in a similar current waveform, again its quoted value will be the RMS value, that is 0.707 times its peak value.

Taking resistive first, being the easiest, here the current waveform will follow faithfully that of the voltage(A), known as being in phase. In this case, calculating the relationship between current, voltage, resistance and power will be the same as for a DC circuit, that is

    V                         V

I = ———  or V = I x R  or R = ———

    R                         I


and   Power =  V  x  I  watts


Unfortunately, beyond this point the calculations become much more complex and will be beyond what most will need to understand. However, even if you find what follows difficult to comprehend do read on as there is a very important conclusion regarding power consumed that is probably not what most will anticipate.

As the title says the content of these pages only cover the subject at a basic level. They should though help with fault finding and designing simple control systems. Also, understanding other web sites and magazine articles having an electrical content. If you would like to go beyond that I list on the last page some web sites and books that may be of help.

Elsewhere on the site I consider how various components function when connected into a circuit supplied from a DC source. Whilst not essential, I think it is preferable that you read those pages before studying these relating to AC supplies.


Sketch 1 shows how an AC supply voltage varies with time, both in magnitude and polarity. Applying such a voltage to a load will result in a similar current waveform. What then is the relationship between the peak voltage and the quoted voltage of a supply.

Unfortunately, in terms of it being easy to understand, the current waveform can occur, simultaneously(A), before(B), or after(C) the voltage, Sk 2. The factor that determines this relationship is the type of load, resistive, capacitive or inductive.