Alternating Current

ACDC Definitions (Also seen in GCSE Physics 2)

Direct Current

Cells and batteries are suppliers of direct current; they supply an emf in one direction.

In the graph below we can see that the current and voltage are constant. The bottom line shows that when the battery or cell is reversed the voltage and current are constants in the other direction

Alternating Current

The Mains electricity supplies an alternating current; it supplies an emf that alternates from maximum in one direction to maximum in the other direction.

In the graph below we see the voltage and current start at zero, increase to a maximum in the positive direction, then fall to zero, reach a maximum in the negative direction and return to zero. This is one cycle.

Alternating Current Definitions

Peak Value

The peak value of either the current or the potential difference is the maximum in either direction. It can be measured from the wave as the amplitude, the distance from 0 to the top (or bottom) of the wave. We denote peak current with I0 and peak p.d. with V0.

Peak-to-Peak Value

The peak-to-peak value of either the current or potential difference is the range of the values. This is literally the distance from the peak above the zero line to the peak below the line.

Time Period

In an a.c. current or p.d. this is the time taken for one complete cycle (or wave).


As with its use at GCSE, frequency is a measure of how many complete cycles that occur per second.

Frequency is measured in Hertz, Hz.

Root Mean Squared, r.m.s.

Since the current and p.d. is constantly changing it is impossible to assign them a fixed value over a period of time, the average would be zero. The r.m.s. current produces the same heating effect in a resistor as the equivalent d.c. for example 12V dc = 12Vrms ac