# Electrical Resistance

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Electrical Resistance

Electrical resistance is a property of an electrical circuit that

opposes the flow of current. Resistance involves collisions of the

current-carrying charged particles with fixed particles that make up

the structure of the conductor. Resistance is often considered as

localized in such devices as lamps, heaters and resistors in which it

predominates, although it is a characteristic of every part of a

circuit, including connecting wires and electric transmission lines.

In circuits where the current (I) and voltage (V) are related by a

simple proportionality constant, as in OHM'S LAW,

V = RI, the proportionality constant R is the resistance of the

circuit. This discovery was made by Georg Simon Ohm (1787-1854), a

German physicist, therefore, Ohm is the common unit of electrical

resistance.

Resistance is the property of an electric circuit or part of a circuit

that transforms electric energy into heat energy. The dissipation of

electric energy in the form of heat, even though small, affects the

amount of electromotive force, or driving voltage, required to produce

a given current through a circuit. The resistance of a circuit

element, expressed in ohms, can be calculated from the following

formula, which gives the power P, in watts, converted into heat by a

resistance of R ohms, when a current of effective value I amperes

flows through the element:

P = RI2 , R = P / I2

The resistance of a wire is directly proportional to its length and

inversely proportional to its cross sectional area, so the longer the

wire, the greater resistance to the flow of charge and as the cross

sectional area of a wire increases the resistance decreases:

R = K / A (K = constant, A = area)

Resistance also depends on the material of the conductor. For example

if you pass electricity through a wire made of a pure metal it will

have less resistance than a wire which is made up out of a metal

alloy. This is because the atoms in a pure metal are all equal in size