Voltage drop describes how the energy supplied by a voltage source is reduced as electric current moves through the passive elements (elements that do not supply voltage) of an electrical circuit. The voltage drop across the internal resistance of the source, across conductors, across contacts, and across connectors is undesirable because some of the energy supplied is lost (dissipated). The voltage drop across the electrical load and across other active circuit elements is essential for supply of energy and so is not undesirable.