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| SYMBOL OF RESISTANCE |
Electrical resistance is defined as the obstruction offered by a material towards the flow of electric current. Every substance possesses some or more of resistance. None of the materials on this earth is fully conducting or superconducting. It is measured using a ohmmeter and its unit in which it is measured is $\Omega
$ (ohms).
Based on the property of resistance, materials are classified as:- Conductors
- Insulators
- Semiconductors
CONDUCTORS-
Conductors are the substances (generally metals) which provide very low resistance to current flow. Their resistivity is of the order of ${{10}^{-8}}\Omega
m$. Silver is the best conductor with resistivity of $1.5\times
{{10}^{-8}}\Omega m$.
INSULATORS-
Insulators are the substances (generally non-metals) which provide high resistance to the flow of current. Their resistivity is of the order of ${{10}^{8}}-{{10}^{16}}\Omega m$. However, they can be made conducting, but at very high voltages. This is because every material undergoes electrical breakdown at a particular voltage level.
SEMICONDUCTORS-
They are a special class of materials whose resistivity lies in between those of conductors and insulators. Their resistivity ranges from ${{10}^{-4}}-{{10}^{5}}\Omega m$. But, the resistivity of these materials varies with factors such as doping concentration and temperature variation.
ON WHAT FACTORS DOES RESISTANCE DEPEND?
Consider a wire of length l and cross-sectional area A. Then, the resistance of that wire is given by:-
\[R=\rho
\frac{l}{A}\]
where,
R - resistance of material
l - length of wire
A - area of cross section of wire
$\rho
$ - resistivity of the material
Thus, resistance R depends on:
- Length
- Area of cross-section
- Type of the material and,
- Temperature of the material
The fourth factor i.e. Temperature; determines the resistance of the material at that point of time.
TEMPERATURE COEFFICIENT OF RESISTANCE-
Generally, as the temperature of a conductor material increases, its resistance also increases.
The temperature coefficient of resistance of a material is the increase in the resistance of a 1$\Omega
$ resistor of that material when it is subjected to a rise of temperature of 1℃. Its symbol is α.
\[{{R}_{\theta
}}={{R}_{0}}(1+{{\alpha }_{0}}\theta )\]
where, ${{R}_{0}}$ - resistance at 0℃.
${{R}_{\theta
}}$ - resistance at $\theta
$℃.
${{\alpha
}_{0}}$ - temperature coefficient at 0℃.
Negative sign of $\alpha
$ indicates resistance will decrease when temperature rises (resembles an insulator).
Positive sign of $\alpha $ indicates rise of resistance with temperature (resembles a conductor).
WHAT IS THE CAUSE OF RESISTANCE IN MATERIALS?
As, voltage is applied across a material, an electric field is set up which initiates the movement of electrons. These moving electrons collide with the atoms present in the material and thus their flow is obstructed. This is what is called Resistance.The low resistance of the conductors is due to the fact that they possess free electrons in them, which is not the case with insulators.
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| INSIDE A CONDUCTOR (METAL) WHEN CURRENT FLOWS |
Particularly, taking into consideration the case of conductors, the diagram above shows the free electrons moving past the positively charged atoms (Kernels) and thus experience hindrance in their movement. Since, the number of free electrons in the metals are very large; their conductivity is high.
SUPERCONDUCTIVITY-
Seeing the needs and importance of negligible resistance in future, so as to prevent losses in transmission, it is obvious that superconductivity and not conductivity is the future! Superconductors are the material which have zero resistance or in other words infinite conductance. But, unfortunately there is no material like that , which is in existence naturally.However, certain conductors at very low temperatures exhibit the property of superconductivity. For example, if mercury is cooled below 4.1K, it loses all its electrical resistance and becomes a superconductor. But, such low temperatures are not available naturally anywhere. Efforts are being made on very large scale to develop superconductor materials which can perform well in normal environmental conditions.
