It is the property of a substance which restricts the flow of electric current.
We know that current is the flow of electrons, hence resistance is an opposition to the flow of electrons. This opposition occurs due to the presence of large number atoms and molecules.
When current flows through a substance, the free electrons move through the material and collide with atoms and molecules. These collisions cause the electrons to lose some of their energy and it also offer opposition to the flow of electrons. The atomic structure of the substance decides the extent of the opposition.
Silver, copper and aluminum offer least resistance to flow of current. Tungsten, Nichrome offer very high resistance to flow of current.
The unit of Resistance is ohm (Ω), denoted by R.
Factor Affecting Resistance
Resistance of a conductor depends upon the following factors
Resistance of conductor is directly proportional to the length of the conductor. Greater the length, greater the resistance and vice versa.
R α l——————– (i)
Area of Cross Section
Resistance of conductor is inversely proportional to the area of cross section of the conductor. Greater the area of cross section, lesser the resistance and vice versa.
R α 1/a——————– (ii)
Nature of Material
Resistance of the conductor depends upon the nature of the composition of the material of which the conductor is made up. Difference substances have different atomic structures and therefore, offer different resistances for the same length and area of cross section.
The resistance of the conductor depends upon the temperature of the conductor. The resistance of the conductor increases with increase in temperature.
From equation (i) and (ii), we get
R α l
R α 1/a
R α l/a
R = ρ l/a
Where P (Rho) is constant of proportionality and it’s called resistivity or specific resistance of the material. P (Rho) refers to the nature of material.
Effect of Temperature on Resistance
- For Metals
The resistance of metals increases with increase in temperature. The graph plotted between temperature and resistance is straight line. The metals have positive temperature co-efficient of resistance.
- For Alloys
The resistance of alloys increases with the increase in temperature but the increase is very small and irregular.
For insulators, electrolytes, semiconductor etc
The resistance of insulators, electrolytes and semiconductors decreases with the rise in temperature. These materials have negative temperature co-efficient of resistance.