( . ) Using Ohm’s law V = IR , we get P = I R = V / R ( . ) as the power loss (“ohmic loss”) in a conductor of resistance R carrying a current I . It is this power which heats up, for example, the coil of an electric bulb to incandescence, radiating out heat and light.
Where does the power come from? As we have reasoned before, we need an external source to keep a steady current through the conductor. It is clearly this source which must supply this power. In the simple circuit shown with a cell (Fig.
. ), it is the chemical energy of the cell which supplies this power for as long as it can. The expressions for power, Eqs. ( .
) and ( . ), show the dependence of the power dissipated in a resistor R on the current through it and the voltage across it. Equation ( . ) has an important application to power transmission.
Electrical power is transmitted from power stations to homes and factories, which FIGURE . Heat is produced in the resistor R which is connected across the terminals of a cell. The energy dissipated in the resistor R comes from the chemical energy of the electrolyte.