Opposition offered to a flow of electric current by a substance is known as resistance represented by the capital letter R. The term ohm represents the standard unit of resistance. The term ohm represented by the three-letter word ohm may also be represented by the Greek capital letter representing the four-letter word omega.
The voltage application when held constant, the current in a direct current or DC electrical circuit is inversely proportional to the resistance. The resistance when doubled halves the current, and when halved doubles the current. This applies to many low frequency alternating current systems or AC systems. Some high frequency AC circuits are complex because these systems have components with abilities to store, release, dissipate, and convert energy.
The term resistivity represents electrical resistance per unit length, area, or volume of a substance. Copper and aluminium wires have resistivity figures specified in ohms per kilometer. The term reactance represents the property of opposition to AC, and not to DC. Vectors of resistance and reactance combine to yield impedance in an AC circuit.
Resistive components include attenuators (electronics), bleeder resistors, electrical ballasts, l pads, möbius resistors, photoresistors, potentiometers, fuses that can be reset, resistance thermometers, resistance wires, resistors, rheochords, string potentiometers, thermistors, and varistors.