The electrical resistance, R, of a component in an electric circuit is given by the equation R = V / I, where V is the voltage across the component and I is the electric current flowing through it.
I = V / R is Ohm's law and can be used to determine the characteristics of a material if its resistance over a significant voltage range remains constant. The equation holds true for DC current and voltage, and it also applies to AC resistors.
However, Ohm’s law does not hold during all conditions. The resistance of a material is dependent on its resistivity — a quality that varies with temperature changes. In such cases, a derivative of the equation of resistance is often more useful.