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.

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Ohm's Law

Explore the relationship between voltage, current, and resistance

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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 is an empirical relationship — true of metals at constant temperature (ohmic devices), and honestly false elsewhere: a heating filament's resistance climbs with temperature, R(T) = R₀[1+α(T−T₀)], and a diode's current grows exponentially with voltage. The battery fixes the VOLTAGE, never the current — turn up the resistance and the current must fall. Power dissipated as heat is P = IV = I²R = V²/R.

Step-by-step

Three live sliders — Voltage, Resistance, Temperature — and three devices to push them through: Ohmic Resistor, Hot Filament, Diode.
Read V, I (auto-ranging A/mA/µA), the actual operating R, power, and device mode in LIVE DATA; watch the yellow electrons drift the loop at one shared pace and the V–I characteristic redraw live.
Tour the five presets from High Current to High Resistance.

Key formulas

$V = IR$ Ohm's Law
$I = \frac{V}{R}$ Current
$P = IV = I^2R = \frac{V^2}{R}$ Power dissipated
$R(T) = R_0\left[1 + \alpha(T - T_0)\right]$ Filament resistance rises with temperature (α = 0.004/°C)

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.