noun

Ohm's law

OHMZ law

Ohm's law (noun) means the rule that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance, written V = IR: voltage equals current times resistance. Example: “Using Ohm's law, a 12 V supply across 4 ohms draws 3 amps.”

noun

physics The rule that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance, written V = IR: voltage equals current times resistance.

"Using Ohm's law, a 12 V supply across 4 ohms draws 3 amps."

"Ohm's law is the first thing you learn in any electronics course."

How to Use Ohm's law

Learner’s notes

In plain EnglishVoltage equals current times resistance — the basic sum in electricity.

When to use it

Written with a capital O and an apostrophe: Ohm's law. The unit, the ohm, is lower case; its symbol is Ω.

Memory tip

Draw the triangle with V on top and I and R below: cover the quantity you want and the arrangement of the other two gives the formula.

Trace the full origin ↓

Easily confused with

ohm → Kirchhoff's laws Watt's law

Common pairings

apply Ohm's law Ohm's law triangle V = IR

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Test yourself on “Ohm's law” A quick quiz — meaning, synonyms & usage →

Fill the Gap

Can you complete this real example?

Using _____, a 12 V supply across 4 ohms draws 3 amps.

Etymology

Named after Georg Simon Ohm, the German physicist who established the relationship; the unit of resistance, the ohm, is named after him as well.

Origin: German

Frequently Asked Questions

What is the formula for Ohm's law?

V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms. Rearranged, I = V/R and R = V/I, so knowing any two values gives you the third.

What is the difference between Ohm's law and the power formula?

Ohm's law relates voltage, current and resistance. The power formula, P = VI, gives the rate at which energy is used, in watts. Combining the two yields the familiar variants P = I²R and P = V²/R.

Does Ohm's law always hold?

Only for ohmic materials, where resistance stays constant. Diodes, transistors, filament lamps and electrolytes are non-ohmic: their resistance changes with voltage, current or temperature, so a plot of current against voltage is not a straight line.