Ordinary conductors oppose the flow of charge with an effect that resembles friction. This dissipative action is called resistance. Just as mechanical friction wastes energy as heat, current through resistance dissipates energy as heat. The unit of resistance is the ohm, named for Georg Simon Ohm. If 1 volt causes a current of 1 ampere, the circuit has 1 unit of resistance. It is useful to know that resistance is the ratio of voltage to current.
Mechanical friction can be desirable, as in automobile brakes, or undesirable, when friction creates unwanted energy loss. Resistance is always a factor in current electricity unless the circuit action involves an extraordinary low-temperature phenomenon called superconductivity. While superconducting materials exhibit absolutely no resistance, these effects are confined to temperatures so cold that it is not yet practical to use superconductivity in other than exotic applications.
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