# Electric Power (Joule’s Law)

Joule’s Law is used to calculate the power dissipate through a component. Joule’s Law states that the power of a resistive component is proportional to the product of its resistance and the square of the current flowing through the resistive component. The units for the power is Watts.

$P=R*I^{2}$

We can combine Joule’s Law with Ohm’s Law to get different equations to calculate electric power.

$P=V*I$

$P=\cfrac{V^{2}}{R}$

Example:

We want to calculate the power dissipate in the resistor of this circuit.

We could use all three equations since we have the voltage across the resistor, the current flowing through it and the resistance of the resistor. For this example, we will use $P=V*I$. The voltage across the resistor is 12V and the current flowing through it is 2.5mA or 0.0025A.

$P=V*I=12V*2.5mA=0.03W=30mW$