The RMS value of AC current is defined as the current that would produce the same heating as a direct current.

• A. The current value that would produce the same heating as a direct current
• B. The maximum instantaneous current
• C. The average current
• D. The frequency of the current

Answer: (A)

RMS current is about the heating an electric current produces. The power dissipated in a resistor goes with the square of the current (P = I^2R), so if you average that heating over time for an AC waveform, you get an equivalent DC current that would cause the same amount of heat. That average heating corresponds to the DC current value, which is called the RMS current. For a sine wave, this RMS value equals the peak current divided by the square root of two.

This explains why the RMS value isn’t the maximum instantaneous current (that’s the peak), and it isn’t the average current (the average current of AC can be zero, even though heating stays due to I^2). It also isn’t related to the frequency, which describes how often the current reverses, not how much heat is produced.