Ohm's Law

Is there a relationship between the potential difference across a conductor and the current flowing through it? In 1827, German physicist Georg Simon Ohm found there is: the potential difference V across a metallic wire's ends is directly proportional to the current I flowing through it, provided the temperature stays the same. This is Ohm's law: V ∝ I.

Since V/I stays constant, that constant is called the wire's resistance (R): V = IR, or R = V/I. Resistance is the property of a conductor that resists the flow of charge through it. Its SI unit is the ohm (Ω) — a conductor has a resistance of 1 Ω if a potential difference of 1 V across it drives a current of 1 A through it.

Plotting V against I for a real wire gives a straight line through the origin — direct proof of the proportionality. The slope of that line is the resistance: a steeper line means a larger R (the same current needs a bigger push), a shallower line means a smaller R.

Rearranging Ohm's law also gives I = V/R: the current through a resistor is inversely proportional to its resistance — double the resistance, and the current halves for the same voltage. A component specially built to change resistance in a circuit without changing the voltage source is called a rheostat.