Power
Power is the rate at which work is done. For charges this can be written as:
| P |
= |
| DU
|  |
| Dt
|
|
For an amount of charge DQ :
| P |
= |
| DU
| |
| Dt
|
|
= |
| DQ
| |
| Dt
|
|
DV |
= |
IDV |
Using Ohm's Law this can be written in three ways:
| P |
= |
I DV |
= |
I2 R |
= |
| DV2
| |
| R
|
|
Batteries and power supplies supply power to a circuit, and this power is used up by motors as well as by anything that has resistance. The power dissipated in a resistor goes into heating the resistor; this is know as Joule heating. In many cases, Joule heating is wasted energy. In some cases, however, Joule heating is exploited as a source of heat, such as in a toaster or an electric heater.