Understanding Ohm’s Law

Say that you’re wiring a circuit. You know the amount of current that the
component can withstand without blowing up and how much voltage the
power source applies. So you have to come up with an amount of resistance
that keeps the current below the blowing-up level.
In the early 1800s, George Ohm published an equation called Ohm’s Law that
allows you to make this calculation. Ohm’s Law states that the voltage equals
current multiplied by resistance, or in standard mathematical notation
V = I x R
Taking Ohm’s Law farther
Remember your high school algebra? Remember how if you know two things
(such as x and y) in an equation of three variables, you can calculate that
third thing? Ohm’s Law works that way; you can rearrange its elements so
that if you know any two of the three values in the equation, you can calculate
the third. So, here’s how you calculate current: current equals voltage
divided by resistance, or
I= V\R
You can also rearrange Ohm’s Law so that you can calculate resistance if you
know voltage and current. So, resistance equals voltage divided by current, or
R = V\I
So far, so good. Now, take a specific example using a circuit with a 12-volt battery
and a light bulb (basically, a big flashlight). Before installing the battery,
you measure the resistance of the circuit with a multimeter and find that it’s 9
I = V\R = 12\9 = 1.3 amps