The alternating potential causes the current in the conductor to change in accordance with Ohm's law. Since the current varies continuously, how is it possible to calculate the heating effect? This can be done by defining effective values of the current, I, and the voltage, V.
"An alternating current is said to have an effective value of 1 ampere when it will develop the same amount of heat in a given resistance as would be produced by a direct current 1 ampere in the same resistance in the same time."
An effective value of the voltage can be defined in an analogous way.
If the effective values of the voltage and current are used, the power dissipation in an A.C. circuit may be calculated in the same way as for direct currents. The effective values are simply the root mean square values of the voltage and current.
In South Africa the effective value of the voltage is quoted as 220V.
In the case of appliances that produce light and motion, not all the electrical energy is converted into the desired form of energy, as some of the electrical energy is converted to heat. The fraction of energy converted to the desired form is the efficiency of the appliance, e, given by
- A good deal of the power is wasted in generating heat (more than 90%!)
- The resistance of the bulb will depend on the operating temperature and design of the filament.