Practical Values for a Sinusoidal Current

Alternating currents follow a sinusoidal path. That means, from the zero value, the magnitude of the current (in amps) begins to increase, reaches a peak value and then, as time passes, the value of the current begins to drop back to zero. And then, as time passes, it begins to flow in the other direction (we refer to that as negative current) - notice it begins to go below zero. After that, as time passes, it reaches a peak for the negative current, then the value begins to drop (becomes less negative); it keeps on dropping until it becomes zero, and then it begins its cycle again.

Some new students find the above confusing. It is so because so far we have learned direct current (DC) circuits - where the current is always positive - meaning, for a given circuit, it flows in one direction only. Since it has a value, it is easy to calculate quantities such as power. In the DC circuits the current value is fixed so it is easy to use the current value in the power formula.

But what do you do, if the current value is not fixed - it goes up, comes down, goes in the other direction etc.?

This is a legitimate source of confusion. One explanation is that a resistor (load, could be light bulb, etc.) will get heated irrespective of the current direction. That solves on problem, but what value should we use:

The answer is we use a formula which gives us the Root Mean Square (RMS) value for the current in various formulas. The RMS value is given by:

RMS (rms) value of current is : 0.707 x Peak value of the current

The peak value was defined in the last section - thus we are able to calculate the value of current for AC circuits.

The RMS value gives the effective value (or the equivalent value) of an ac current. The dashed line shown in the illustration shows the rms value of an ac voltage or current.