# Electrical Power

### Electrical power defined

Electrical power (P) is defined as the product of the voltage drop across the device multiplied by the current running through it.  Power is a measure of energy per unit time and is measured in Watts (W).

P = IV

This device can be an entire circuit or one element within that circuit.  Voltage and current sources deliver power to a circuit.  In contrast, a signal flowing through a resistor experiences a drop in power, as a portion of the signal’s power is dissipated in the resistor.  Combining Ohm’s law and the equation above, it’s possible to find the power dissipated in a resistor in terms of voltage or current.

$P = IV = I * (IR) = I^2 R$

$P = IV = \frac{V}{R} * V = \frac{V^2}{R}$

### Variables in resistive circuits

Voltage, current, resistance, and power are the four variables you’ll encounter in resistive circuits.  Given two of them, you can find the other two using Ohm’s law and the equations for power.  Below is a summary of the important equations derived from these two laws.

$V = IR$               $P = IV = I^2 R = \frac{V^2}{R}$

Being able to recognize which components are in series and which are in parallel is a useful skill to have and one that makes circuit analysis much easier.  When it comes to circuit analysis, the simpler you can make a circuit, the better.  Written by Ryan Eatinger (reatinge@ksu.edu).  Kansas State University.