Using Ohms Law

Lets use Ohms Law to figure out how much current is flowing through a circuit.

Schematic Symbol	Actual Image

We want to figure out the current flowing through the light bulb, so lets use form 3 of Ohms Law:

Ohms Law, form 3: I = V / R

In this case, I is the current flowing through the light bulb, V is the voltage of the battery, and R is the resistance of the light bulb.

We can fill in V because we know that it equals +9 volts.

Current equals 9 divided by R.

If we only knew the Resistance of the light bulb, then we could calculate how much current is flowing through it. It turns out that finding the resistance of a device is pretty easy, if you have a meter to do it with. For now, I'll just tell you that the light bulb we are using here has 2 ohms of resistance. I used my ohmmeter to determine the resistance of the bulb, which we talk about in our How to Use a Multimeter guide.

Referring back to the table that says Current is measured in Amps, our answer is I = 4.5 Amps. Using Ohms law we have calculated that 4.5 Amps of current are flowing through a 2 ohm light bulb when hooked up to a 9 volt battery. By knowing the voltage of the battery and measuring the resistance of the light bulb, we are able to calculate the current flowing through it.

Ohms Law is used throughout electronics to figure out how a circuit works and why it is either functioning or not functioning. It is very important that you understand everything discussed in this guide. If you are still fuzzy at this point, then take a break and come back to it again later.