Almost all circuits have some resistance, either from a resistor or from the internal resistance of the wires. Whenever a voltage is applied across such a circuit, some power is dissipated as thermal energy. This occurs when the charge collides with atoms in the resistor. The rate at which the charge loses energy is the power.
Power is measured in watts. One watt is equal to one joule per second, or one ampere-volt.
We can calculate the power consumed quite quickly using the following equation:
where P is power in watts, I is current in amperes, V is voltage in volts, and R is resistance in ohms.
An example problem:
Q: Say we have a light bulb rated 120 volts, and 70 watts.
What is the current flowing through the lightbulb?
What is the resistance of the lightbulb?
A: The current is calculated from the equation P=IV. I is therefore equal to P/V, I=70W/120V, I is approximately .58 amperes.
The resistance can now be calculated from the equation V=IR. R=V/I, R=120V/.58A, R is approximately 207 ohms.