The state of an ideal resistor is completely specified by the voltage across it (call it volts) and the current passing through it ( amperes, or simply ``amps''). The ratio of voltage to current gives the value of the resistor ( resistance in Ohms). The fundamental relation between voltage and current in a resistor is called Ohm's Law:
where we have indicated also that the voltage and current may vary with time (while the resistor value normally does not).
The electrical power in watts dissipated by a resistor R is given by
where is the voltage and is the current. Thus, volts times amps gives watts. Also, volts squared over ohms equals watts, and so on.