Ohm's law links the voltage across a resistor, the current through it and its resistance: V = I·R. Drag the sliders below to change the voltage and the resistance, and watch the current in the loop respond in real time.
Connect a battery across a single resistor with a couple of wires, and a steady current settles into the loop within an instant. Three quantities describe what is happening: the voltage the battery supplies, the current pushed through the resistor, and the resistance that opposes that current. Ohm's law ties them together in one compact statement, V = I·R, which rearranges to I = V/R and R = V/I depending on which quantity you want to find.
Here is the crucial point for a metal resistor held at constant temperature — an ohmic conductor. Its resistance is a fixed property, set by the material, its length and thickness, and its temperature; it does not change when you alter the voltage. So double the voltage and you exactly double the current, because the ratio V/I stays put — and that constant ratio simply is the resistance. Plot voltage against current and you get a straight line through the origin whose slope is R. Contrast that with a filament lamp: as more current heats the wire, its resistance rises, so its V–I graph bends into a curve rather than a straight line. The lamp is the classic non-ohmic exception.
Explore it directly below. Raise the Voltage slider and watch the current climb in step; nudge the Resistance slider and see the same voltage push a smaller current through a stiffer resistor. For worked numbers, try the Ohm's law calculator, or browse more experiments in the interactive labs library.
Ohm's law states V = I·R: the voltage across a resistor equals the current through it times its resistance. It rearranges to I = V/R and R = V/I, so knowing any two of the three quantities gives the third.
Not for an ohmic conductor such as a metal at constant temperature. Its resistance is a fixed property of the material and its dimensions, so doubling the voltage simply doubles the current and the ratio V/I — the resistance — stays the same.
For an ohmic resistor, plotting voltage against current gives a straight line through the origin whose slope is the resistance R. A curved V–I graph means the resistance is not constant.
One that does not keep a constant resistance, so it disobeys the straight-line form of Ohm's law. A filament lamp is the classic example: as current heats the wire its resistance rises, so its voltage-current graph bends into a curve instead of a straight line.