Formulas & tables/Physics/Electric current/Ohm's law

Ohm's law

I=\dfrac{U}{R}

Result unit: A (ampere)

What it describes

The current is directly proportional to the voltage and inversely proportional to the resistance.

Symbols

I

current

[A]

U

voltage

[V]

R

resistance

[Ω]

Where it comes from

At constant resistance the current grows proportionally to the voltage; a larger resistance “throttles” the current. The law applies to ohmic conductors (

R=\text{const}

When to use it

Circuits with ohmic resistors. Lets you find any of the three quantities:

I

U

R

Example

A resistor of 10 Ω has a voltage of 20 V across it. Calculate the current.

Substitution

U=20

R=10

Ω.

Result

I=\dfrac{20}{10}=2

Related formulas

Resistance of a conductor Electric power

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