English: Drawing illustrating flow of power in the operation of an ordinary positive resistor. Electric current enters the positive voltage terminal of the resistor and leaves through the negative terminal. Therefore the electric charges move in the direction of lower potential energy, doing work on the resistor. Electric power from the circuit is converted into heat by the resistor, which is dissipated in the surrounding environment, raising its temperature. This is an irreversible process, increasing entropy. A negative static resistor (illustrated in companion image Thermodynamics and negative resistance.svg) cannot function like this in reverse, converting heat from the environment into electric power, because it would violate the second law of thermodynamics, which says that heat cannot be converted into work from a single heat reservoir; a temperature difference is required. So a negative static resistance must be an active device, with a source of power.