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Resistance (R) is a property of a material used for describing the opposition provided to the flow of current. The higher the resistance provided by a material, the lower the flow of electrons or current through the material. It is measurable and is in contrast to the conductance property of a material, or the ease of electron flow through the substance. It can be either a desirable or undesirable property for a substance. The property of resistance is used in a wide variety of applications and appliances such as transistor radios, televisions and incandescent lamps.
The term resistance is connected with direct current, whereas in the case of alternating current, the opposition to flow of current is known as reactance. There are many factors which influence the resistance of a substance such as length of wire used, cross-sectional area of the wire, type of material used and temperature. Higher resistance is provided by longer wire, whereas a wider cross-sectional area of wire helps in decreasing the resistance. Certain materials such as metals are known to be good conductors of electricity and hence provide less resistance. Temperature influences the electron structure and with increase in temperature, most materials provide less resistance to the flow of current.
The SI unit for resistance is the ohm, symbolized by the Greek letter omega, and also sometimes represented by letter R. Resistance of a material is one ohm when a current of one ampere passes through a material with a voltage of one volt. An ohmmeter is the instrument used to measure resistance. In the case of an electric circuit, resistors are components used to provide the resistance to current flow. Resistors are provided with bands of color or stripes which denote the resistance value.