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:

(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.