- The root-mean-square value of an alternating current is equivalent to the steady direct current that would dissipate heat at the same rate as the alternating current in a given resistor.
- For a sinusoidal source,
(a) the root mean square value of the current is given by .
(b) the mean or average power < P > absorbed by a resistive load is half the maximum power.
- An a.c. transformer is a device for increasing or decreasing an a.c. voltage. It consists of a primary coil of Np turns and voltage Vp and secondary coil of Ns turns and voltage Vs wrapped around an iron core.
- For an ideal transformer (assuming no energy is lost), the following equation is obeyed
- Power loss in the transmission lines is minimized if the power is transmitted at high voltages (i.e. low currents) since where I is the current through the cables and R is the resistance of the cables.
- The equation is often mistakenly used to suggest that power lost is high when voltage of transmission is high. In fact, V refers to the potential difference across the cables, which often have but a fraction of the overall resistance through which the current passes.