RESISTANCE OF MATERIALS

EFFECT OF TEMPERATURE ON RESISTANCE IN METALS, INSULATORS AND SEMICONDUCTORS.

When the temperature of a metal is increased, the amplitude of vibration of atoms in the metal also increase, this impedes the free movement of electrons in the conductors thereby increasing the resistance. Therefore, in metals, increase in temperature
will increase the resistance in metals.

Increase in temperature will increase the number of electrons that are free to move in semiconductors. Therefore, decrease the resistance. Hence, for semi-conductors, increase in temperature will decrease the resistance. This is also the case for insulators as they show decrease in resistance when the temperature is increased.

FACTORS AFFECTING THE RESISTANCE OF A CONDUCTOR (WIRE)

There are four basic factors that affects the resistance of a wire.

1. Length of the wire: The resistance of a wire is directly proportional to its length. The longer the wire the higher the resistance.

2. Area of the wire: Resistance is inversely proportional to the cross-sectional area of the wire. The higher the cross-sectional area, the lower the resistance

3. Temperature: As discussed above, resistance is directly proportional to the temperature of the conductor. The hotter wire has a larger resistance because of the increased vibration of the atomic lattice.

4. The kind of material: Difference material have conductivity hence, different resistance. For example, silver has a higher conductivity than copper. Although copper is mostly used in electrical components because it is less expensive.


RESISTIVITY 

Resistivity is a fundamental characteristics of a material that describes the natural tendency of the material to oppose to the flow of an electric current.

Example 1:

The current in a 2.50 m length of wire of diameter 1.5 mm is 0.65 A when a potential difference of 0.40 V is applied between its ends. Calculate;

•     the resistance of the wire
•     the resistivity of the material of the wire.