The resistor is a two-terminal passive electronic component that provides the specified amount of electrical resistance for a circuit. The resistor is used to oppose the current in the circuit.
In other words, it is used to reduce the
amount of current from the circuit or circuit element. It is also used to bias
active elements, divide voltages, and adjust signal levels.
It dissipates many watts of electrical power
in terms of heat. A resistor can be a fixed value resistor or a variable
resistor.
Unit
The value of a resistor is called
resistance. It is denoted by R and the unit of resistance is Ohms (Ω).
Symbol
The symbol of a fixed value and variable value
resistance is shown in the below figure.
Resistance (R)
The ability of a material to oppose the
flow of current is known as resistance. It is measured in ohms (Ω).
The resistance of the conductor is as small
as possible. Therefore, it can allow the flow of current. But in the case of
the insulator, it will not allow the flowing of current. Because the resistance
of these materials is very high.
For the ideal conductor, the resistance is
zero. And for the ideal insulator, the resistance is infinity. In practice, the
ideal conductor and ideal insulator do not exist.
The material like metals (copper, aluminum,
etc) allows the flow of current easily. Therefore, these materials are known as
conductors.
Certain materials like plastic, wood, glass
do not allow the current to pass through them easily. And this type of material
is known as insulators.
The mathematical expression for the
resistance of a conductor is,
R = ρ(l/a)
Where
ρ = Resistivity of material
and it is constant.
I = Length of the conductor
a = Cross sectional area
Factors Affecting the Resistance
From the above equation, it is clear that
the resistance R is directly proportional to the resistivity of the material (ρ) and length of the conductor (l) and it is inversely proportional to
the cross-sectional area (a) of the conductor.
The resistivity depends on the type of
material. For the same material, the resistivity is the same.
Now take two metal bars with the same
material and same cross-sectional area, but the length of bar-1 is L and the
length of bar-2 is 2L. In this condition, the resistance of bar-1 is half of the
resistance of bar-2.
The temperature is not included in the
above equation. But the value of resistance depends on the temperature. Due to a
change in temperature, the length of a conductor will change. And that will change
the value of R. Generally, the higher the temperature, the value of resistance
increases.
Concept of Resistivity
The resistivity is also known as the specific
resistance of that material. It is denoted by ρ (rho). Rearranging the expression for resistance R we get,
ρ = Ra/l
It depends on the resistance (R), cross-sectional
area (a), and length (l).
Substituting the units of the various
quantities we get the unit of specific resistance is Ωm (Ohm-meter).
The resistance of a piece of material with
unit cross-section area and the length of material is one meter long is defined
as a specific resistance of that material.
ρ = Ra/l
In this condition, a=1 and l=1. Therefore,
ρ = R
The material which has the lowest value of specific resistance is the best conductor. And the material which has the highest value of specific resistance is known as the best insulator.
Conductance (G)
The conductance is defined as the
reciprocal of resistance. It is denoted by G and the unit of conductance is or
Siemens.
Conductance G = 1/R
Conductivity (σ)
Conductivity is defined as the reciprocal
of resistivity (ρ) and its units are (1/Ωm) or Siemens/meter. The value conductivity is high for the conductors
and low for the insulators.
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