V = I * R
I = V / R
R = V / I
Ohm’s Law is the most important shit you’ll ever memorize. If you would like a banana-based conceptual overview, check out the Volts, Amps, and Ohms page. Ohm’s law explains the relationship between voltage potential (V), circuit resistance (R), and current flow (I). Look at the equations above; note that they’re just different ways of saying the same thing. Download them to your fleshdrive. You’re going to use them A LOT.
Let’s do some examples. If you have a 450 ohm resistor with 9V across it and you want the current in the circuit, divide that shit up:
V / R = I
9 volts / 450 ohms = .02 amps (20 milliamps)
Fuck you, science, you don’t scare us.
Need to know the voltage across a resistor? Take the current through it and multiply by the resistance. If you have a 100 ohm resistor in series with a load drawing 50 milliamps, you’re dropping:
R * I = V
100 ohms x .05 amps = 5 volts
If you can measure the voltage across a resistor and you know the current in the circuit, you can figure out the resistor’s value. It is dropping 35 volts and there’s 100 mA in the circuit. That resistor is:
V / I = R
35 volts / .1 amps = 350 ohms
But wait, there’s more. The Watt Law is the peanut butter to the Ohm’s Law jelly. Watts are a measure of power. That can be the power sent to a speaker or the power dissipated in a resistor (or many other things). Basically, you’re usually going to use this to make sure your amplifiers don’t blow shit up.
W = V * I
W = I^2 * R
W = V^2 / R
So your amp smells like burning. You better check dem resistors. You know that one of them is 1000 ohms and is drawing about 60mA.
I^2 * R = W
.06 amps * .06 amps * 1,000 ohms = 3.6 watts
Let’s double check that figure by getting the volts, too:
I * R = V
1,000 ohms * .06 amps = 60 volts
Now, power in the resistor:
V * I = W
60 volts * .06 amps = 3.6 watts
V^2 / R = W
60 volts * 60 volts / 1,000 ohms = 3.6 watts
Survey says 3.6W in the resistor. You put a 1W resistor in the amp though because you’re lazy and/or impatient and/or stupid (I’m frequently all three). That’s the burning smell (true story).
Here’s something really useful you won’t ever remember: