# Ohms Law Explained

concept

Ohm's Law is a simple mathematical tool that relates voltage, current and resistance.
With Ohm's Law and two of the three values we can calculate the third.
Often when designing or analysing circuits we'll know voltage and current but not the resistance, or we'll know resistance and voltage but not current.
Perhaps we know what voltage our battery will give off and how much current we need to have flowing around the circuit, using Ohm's Law we can calculate how much resistance to use in order to get exactly that much current.
Or maybe you know the size of your battery and how much resistance is in a circuit, Ohm's Law will tell you how much current flows out of the battery.

fact

Ohm's law is:
\(I = \frac{V}{R}\)
and it can be rearranged to give two other forms:
\(R = \frac{V}{I}\)
\(V = I\times R\)

These math formulas are often shown graphically as a triangle:

fact

To use the triangle cover up the value you want to calculate. Then the positions of the other two remaining values resemble the above equations and show whether they should be used as a fraction (and which one should be on top) or whether they should be multiplied together.

example

In a circuit with a single battery of 9 Volts and a single resistor of 100Ω find the current.

We first need the equation for current from Ohm's law. Looking at our equations above we see that \(I = \frac{V}{R}\) so we plug in the values we've been given and get: \(I = \frac{9}{100} = 0.09A\) So 0.09 amps of current (or 90 milliamps) flow through the circuit.
practice problems