The property of a
material that reduces the flow of current through it is called electrical resistance. Every material will have some
amount of resistance except super conductors which have zero resistance. The
materials which have very low resistance are called conductors. The inverse of resistance is called conductance. Resistors are used to control the amount of current passing through conductors. But they dissipate the energy in
form of heat and thereby efficiency of the system is decreases.
form of heat and thereby efficiency of the system is decreases.
Determination of
resistance:
The resistance of a
material generally depends on its geometry and the property of material. The
resistance of a material also depends on the temperature. The resistance of a
wire of uniform area of cross section is proportional to its length and its
resistivity and inversely proportional to the area of cross section. It is
represented mathematically as below
Where R= Resistance of
the material in Ω (ohm)
ρ= resistivity
of the material in ohm-meter
l= length of the material in meters
a=area of cross section of material
in meter2
Ohm’s law can be used
to determine the resistance of linear materials by measuring the voltage and current across it. Ohms law is given by
V=IR at constant
temperature.
Where V= voltage
across the material
R= Resistance
Not all materials obey
ohms law but is generally valid for the conductors. The materials which obey ohms law have linear v-i characteristics and are
called ohmic materials.
Materials like diodes,
nonlinear resistor etc has nonlinear v-i characteristics and so ohms law cannot
be applied to determine the resistance of such materials.
Effect of Temperature on Resistance:
At room temperature, the metals have positive temperature co-efficient
whose resistivity increases with temperature. The semiconductors have negative
temperature co-efficient for which resistivity decreases as temperature is
increased. The resistivity of insulators may increase or decrease depending on
the system.
Units of measurement
of Resistance:
The electrical
resistance is usually represented by letter R and The SI unit of electrical resistance is the ohm (Ω).
Resistor materials:
Resistors are made of
various materials of less conductivity. The following are the various materials
used to make resistors.
1. nickel-chromium alloy
2. carbon
3. Nichrome and
4. tantalum nitride etc.
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