Ohm's Law

Let's 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 to the current multiplied by resistance, or in standard mathematical notation,
V = I x R

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



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
ohms. Here’s the formula to calculate the current:
I = V / R
  = 12volts / 9 ohms
  =1.3 amps

If you find that your light is too bright? A lower current reduces the brightness of the light, so just add a resistor to lower the current. Originally, we had 9 ohms; adding a 5-ohm resistor to the circuit makes the total resistance 14 ohms. In this case, the formula for current is
I = V / R
  = 12volts / 14 ohms
  =0.9 amps