The Basics and RMS Values

The Basics and RMS Values

Cells produce currents that travel in the same direction all of the time, direct currents. However, this is not always useful - for instance, transformers will only work if the current is constantly changing.

An alternating current is constantly changing direction. It is normally sinusoidal.

Alternating Currents

The frequency of an alternating current supply, f, is the number of cycles completed per second.Measured in Hertz (Hz).

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

The peak values of current, Io, and voltage,Vo, are the maximum values at the crest or trough. They are equivalent to the amplitude of a wave. Sometimes we quote the peak-to-peak value, which is of course, double the peak value.

Wouldn't it be great if you could just use the electricity equations you are already comfortable with for d.c. circuits in your a.c. calculations? The problem is - what value do you put in, because the a.c. values are always changing.

Well, the rms values of current and voltage are the answer to your problem.

RMS values are the d.c. equivalent of an a.c. value. In other words, if you had two circuits, one d.c. and one a.c., and you wanted them to use exactly the same amount of power (energy each second) then you would choose the d.c. values of current and voltage to be the same as the rms values of current and voltage in the a.c. circuit:

RMS values

The power being used in each circuit is the same, so you can use all your old electrical equations.

For example, V = IR, P = IV, P= I2R.

For a.c. circuits, as long as you use the rms values of current and voltage, a.c. is no longer difficult!

So how do you lay your hands on this magical value?

Copyright S-cool


Copyright S-cool

Where V0 and I0 are the peak values.

Note: rms values are less than the peak values of voltage and current.

So, when given an a.c. question simply find the rms values (using the peak value and above equation) and then use your normal electrical equations.

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