Resistance

Resistance, property of any object or substance of resisting or opposing the flow of an electrical current. The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit. The unit of resistance is the ohm, the amount of resistance that limits the passage of current to one ampere when a voltage of one volt is applied to it. The standard abbreviation for electric resistance is R and the symbol for ohms in electric circuits is the Greek letter omega, Ω. For certain electrical calculations it is convenient to employ the reciprocal of resistance, 1/R, which is termed conductance, G. The unit of conductance is the siemens (formerly called the mho), symbol S.

The resistance of an object is determined by a property of the substance of which it is composed, known as the resistivity, and by the length and cross-sectional area of the object, and by the temperature. At a given temperature, the resistance is proportional to the object's resistivity and length, and inversely proportional to its cross-sectional area. Usually, a material's resistance increases with increases in temperature.

The term resistance is also used when the flow of a fluid or heat is impeded. The forces of friction provide the resistance to the flow of a fluid in a pipe, and insulation provides thermal resistance that reduces the flow of heat from a higher to a lower temperature.