The term resistance means the measure of opposition made to the flow of electric current. It is defined as the ratio of the voltage applied to the electric current flowing through the material. Resistance is denoted by 'R' and measured in Ohm's (Ω).
Different materials offer different resistance to the flow of electric current which depends upon the length and cross-section area of the material, the nature of the material, the resistivity of the material, and physical conditions such as temperature.
Thus there is a significant effect on the resistance of a material due to the change in temperature. The resistance can either increase or decrease when the temperature of the material increases. In this article let us learn about the effect of temperature on the resistance of the conductor, insulator, and semiconductor.
Effect of Temperature on Resistance of Conductor :
Before knowing about the effect of temperature on the resistance of conductors. Let us first learn about how a conductor conducts electric current. Conductors are materials that have a large number of free electrons in their outermost orbit of atoms. The free electron in an atom is such a particle that is capable of being able to separate from its atom and move to an adjacent atom.
On the application of an electric field to a metallic conductor, negatively charged electrons experience a force and thus the free electrons start drifting from lower potential to higher potential. This movement of free electrons gives rise to electric current.
The free electrons while moving collide with each other as well as with the positive ions present in the metallic conductor. These collisions result in a slow down of electron speed i.e., oppose the flow of electric current, and is the basic cause of resistance in a conductor.
We know that the resistance of a conductor depends upon various factors such as length and area of cross-section of the conductor, resistivity, and physical conditions such as temperature. The effect of temperature on the resistance of a conductor totally depends upon the above process.
When the temperature of a conductor raises, its atoms begin to vibrate violently due to an increase in thermal energy. This vibration of atoms increases the collision probability of moving electrons, due to which electrons find it difficult to their movement. Thus opposition to the flow of electric current increases and thus the resistance of the conductor increases.
Therefore, the effect of temperature on the resistance of a conductor is directly proportional to each other i.e., when the temperature of a conductor increases its resistance increases and vice-versa which refers to Positive Temperature Coefficient of Resistance. Different conductors have different effects of temperature on their resistance. For pure metals, resistance increases with a rise in temperature, while for alloys resistance increases slightly with temperature.
Effect of Temperature on Resistance of Insulator :
A material that does not conduct electricity is considered an insulator. The atoms of insulators have tightly bound electrons. These electrons cannot easily flow from one atom to another and be shared by neighboring atoms. Thus due to a very small amount of free electrons present in their atoms, it is difficult to conduct electric current for insulators at normal conditions.
Unlike conductors whose resistance increases with temperature, the situation is different in insulators. When the temperature of an insulator is increased, the vibration of atoms increases. When there is sufficient increase in the temperature, the atoms vibrate violently and this vibration decreases the bond strength between the nucleus and the electrons.
When the bond between the nucleus and the electrons loosens, the forbidden energy gap between the valance and conduction band minimizes. Hence when the temperature of an insulator is increased, there is a probability of creating free electrons and thus conduction of electric current.
Thus, at high temperatures insulators behave as the conductor. Therefore, an increase in the temperature of an insulator decreases its resistance and vice-versa which refers to the Negative Temperature Coefficient of Resistance.
Effect of Temperature on Resistance of Semiconductor :
A material whose properties lie between conductor and insulator is said to be known as a semiconductor. A semiconductor is neither a good conductor nor a good insulator. The energy between the valance band and conduction band of a semiconductor is small and the flow of electrons to the conduction band can be controlled by adding impurities to the material.
When the temperature of a semiconductor is increased, the vibrations of molecules do increase just like in conductors but however, it doesn't increase resistivity like conductors. The vibration energy due to temperature increase breaks covalent bonds and causes additional free electrons which are able to move freely throughout the material structure.
Thus the number of electrons that transmit charge increases, thereby increasing the conductance and decreasing the resistance. However similar to conductors, vibrations of atoms increases resistance whose effect is negligible when compared to the effect due to an increase in the number of free electrons. Therefore, the resistance of a semiconductor decreases when the temperature is increased (i.e., negative temperature coefficient of resistance).
The below figure shows the curve between resistance and temperature for conductors, insulators and semiconductors.
Above we have seen how temperature affects the resistance of a conductor, insulator, and semiconductor. Resistivity measures the resistance of a material per unit of length and cross-section. Resistivity is sometimes also called specific electrical resistance or volume resistivity. The electrical resistivity is measured in ohm-meter (Ω-m).
Resistivity of Different Materials :
Due to the availability of free electrons the resistivity of conductors at normal conditions is very low. The resistivity of conductors lies in the range of 10–8 ohm-meter to 10–6 ohm-meter. Below shows the resistivity of some conductor materials.
| Material | Resistivity |
|---|---|
| Silver | 1.59 x 10–8 |
| Copper | 1.68 x 10–8 |
| Aluminum | 2.65 x 10–8 |
| Tungsten | 5.6 x 10–8 |
| Mercury | 98 x 10–8 |
The property of a semiconductor neither completely allowing current flow nor opposing current flow makes to have a resistivity lies between conductor and insulator. The resistivity of semiconductors strongly depends upon the impurities added to the material, generally, it lies in the range of 10–5 ohm-meter to 106 ohm-meter.
| Material | Resistivity |
|---|---|
| Germanium | 1-500 x 10–3 |
| Silicon | 0.1-60 |
Insulators have high resistivity due to the availability of less number of free electrons. The resistivity of insulators lies in the range of 108 ohm-meter to 1020 ohm-meter.
| Material | Resistivity |
|---|---|
| Wood | 1014 - 1016 |
| Glass | 1-10000 x 109 |
| Hard rubber | 1-100 x 1013 |
| Quartz (fused) | 7.5 x 1017 |
Conclusion :
Therefore, conductors have positive temperature co-efficient whose resistance increases with an increase in temperature and vice-versa. Whereas the temperature coefficient of resistance for semiconductors and insulators is negative i.e., an increase in temperature decreases resistance, and vice-versa.