 - Also see current, voltage, and Ohm's Law.
Resistance is the opposition that a substance offers to the
flow of electric current. It is represented by the uppercase letter R. The
standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega. When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt
exists, then the resistance of that component is one ohm.
In general, when the applied voltage is held constant, the current in a
direct-current (DC) electrical circuit is inversely proportional to the resistance.
If the resistance is doubled, the current is cut in half; if the resistance is
halved, the current is doubled. This rule also holds true for most low-frequency alternating-current (AC) systems, such as household utility circuits.
In some AC
circuits, especially at high frequencies, the situation is more complex,
because some
components in these systems can store and release energy, as well as
dissipating or
converting it.
The electrical resistance per unit length, area, or volume of a
substance is known as
resistivity. Resistivity figures are often specified for copper and
aluminum wire, in ohms per kilometer.
Opposition to AC, but not to DC, is a property known as reactance.
In an AC circuit, the resistance and reactance combine vectorially to yield
impedance.
| LAST UPDATED: |
03 Dec 1999
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