Ohm's law (V = IR)

Voltage = current × resistance.

The common mistake

Rearranging wrong — writing I = V × R instead of I = V ÷ R.

Instead: Cover the one you want in the V/IR triangle: I = V/R, R = V/I, V = IR.

Worked example with Guide me

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

Sugar · Which do we want?
Student · Current, I.
Sugar · I = V ÷ R.
Student · 12 ÷ 4 = 3.

Answer: 3 A

Try 3 practice questions

1. Voltage for 2 A through 5 Ω?
Show answer
10 V
2. Resistance if 9 V gives 3 A?
Show answer
3 Ω
3. Total resistance of 2 Ω and 3 Ω in series?
Show answer
5 Ω

Quick answers

What is the most common mistake with ohm's law (v = ir)?: Rearranging wrong — writing I = V × R instead of I = V ÷ R.
How do I get ohm's law (v = ir) right?: Cover the one you want in the V/IR triangle: I = V/R, R = V/I, V = IR.
Can you show a worked example?: A 12 V battery drives current through a 4 Ω resistor. Find the current. 3 A

Related concepts

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Save this as a miss — practice it before your test