Resistance, property of any object
or substance of resisting or opposing the flow of an electrical
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, according to Ohm's law.
The commonly used unit of resistance
is the ohm, which is the resistance of a conductor in which a
potential difference of 1 volt causes a current flow of 1
The standard abbreviation for
electric resistance is R and the symbol for ohms in electric
circuits is the Greek letter omega[W].
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 mho (ohm spelled backwards) and the
symbol is an inverted omega.
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.
Resistivity is expressed in terms of
the ohms resistance per cubic centimeter of the substance at
20° C (68° F).
Usually, a material's resistance
increases with if the temperature is higher because electron
agitation increases also.
Take a look to the following
fig.3 - Resistance
Imagine that the white disc is the
current flow and the other discs are the electrons inside the
If electrons increase speed then the collision force increases
also determining the amount of resistance that the material