Physics

Ohm's Law: Voltage, Current, and Resistance

Learn V = IR and the power formula with two worked examples and the most common mix-up.

The law

Voltage (V) equals current (I) times resistance (R). Power (P) equals current times voltage.

Example 1

A 12 V battery drives a 4 Ω resistor. Find the current.

  1. I = V / R
  2. I = 12 / 4
I = 3 A

Example 2

How much power does that resistor use?

  1. P = I · V
  2. P = 3 · 12
P = 36 W

Common mistake

I = R / V

Current is voltage divided by resistance: I = V / R.

Check units: volts, amps, ohms, watts.

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