A 12V source drives a 400Ω resistor. What current flows?

I = V/R = 12/400 = 0.03 A = 30 mA.

What power is dissipated in the resistor above?

P = I²R = (0.03)² × 400 = 0.36 W.

A light bulb's resistance increases as it heats up. Why doesn't it obey Ohm's Law?

Ohm's Law assumes constant temperature; resistance of metals increases with temperature.

Pro 🔒~15 min

Ohm's Law

Explore the relationship between voltage, current, and resistance

How it works

Ohm's Law states that the current through a conductor is proportional to the applied voltage and inversely proportional to resistance: I = V/R. It applies to ohmic conductors (metals at constant temperature). Non-ohmic devices like diodes and light bulbs deviate from this relationship. Resistance is a material property measuring opposition to current flow. Power dissipated as heat is P = I²R.

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Step-by-step

Use the voltage slider to change the battery voltage.
Use the resistance slider to change the resistor value.
The ammeter shows current and the voltmeter shows voltage.
Plot V vs I to verify the linear Ohm's Law relationship.

Key formulas

$V = IR$ Ohm's Law
$I = \frac{V}{R}$ Current
$P = IV = I^2R = \frac{V^2}{R}$ Power dissipated

Frequently asked questions

A 12V source drives a 400Ω resistor. What current flows?: I = V/R = 12/400 = 0.03 A = 30 mA.
What power is dissipated in the resistor above?: P = I²R = (0.03)² × 400 = 0.36 W.
A light bulb's resistance increases as it heats up. Why doesn't it obey Ohm's Law?: Ohm's Law assumes constant temperature; resistance of metals increases with temperature.