Answer to Question #126688 in Physics for Susan Williams

Question

What do understand by the r.m.s voltage of an alternate current?

Answer 1

The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect. For example, a lamp connected to a 6V RMS AC supply will shine with the same brightness when connected to a steady 6V DC supply. However, the lamp will be dimmer if connected to a 6V peak AC supply because the RMS value of this is only 4.2V (it is equivalent to a steady 4.2V DC)

AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value when connected to varying DC provided that the DC is varying quickly; if the frequency is less than about 10Hz you will see the meter reading fluctuating.