Opposition that a material or electrical circuit offers to the flow of electric current. It is the property of a circuit that transforms electrical energy into heat energy as it opposes the flow of current. The resistance R, the electromotive force or voltage V, and the current I are related by Ohm's law. The resistance of an electrical conductor generally increases with increasing temperature and is utilized in devices such as lamps and heaters. The ohm () is the common unit of electrical resistance; one ohm is equal to one volt (see electromotive force) per ampere.

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