State laws of resistance

The electrical resistance is a property of a substance by which it resists the flow of electric current through it. Electric current is flow of charge per unit time hence it can be compared with flow of water in a pipe line. It is our general concept that for fixed water head, the rate of flow of water depends upon the length of the pipe, cross section of the pipe. If the length of the pipe increases the rate of flow of water is decreased and if the cross section of the pipe increases the rate of flow of water is decreased. As electrical current is rate of flow of charge which is caused by drift of electrons, it also depends upon the dimensions of conductor in the same way as the flow of water in pipe. In addition to that the current through a conductor also depends upon the numbers of free electrons available in it. As we know that number of free electrons depends upon internal molecular structure, temperature and other micro properties of the conductor materials, the current through the conductors for applied constant potential difference varies if the material of the conductor changes. The inter molecular vibration in a conductor increases with in in temperature. Inter molecular vibration prevents drift of free electrons, hence resistance of an conductor also depends upon temperature.

After going through this explanation, hope we get a basic clear concept of electrical resistance now we will state laws of resistance one by one as follows,

Resistance of a conductor varies directly with its length.

For example, if one meter copper wire has a resistance of 0.04 Ω then two meter of same cross section conductor will have a resistance of 0.08 Ω and 3 meter will have 0.12 Ω at same temperature.
Resistance of a conductor varies inversely with its cross section.

For example, if cross section of a conductor is doubled without changing its length, the electrical resistance of it will become half at same temperature.
The resistance of a conductor also depends upon its material. At same temperature the resistance of an aluminium conductor will be higher than that of a copper conductor of same dimension.
It also depends upon the temperature of the conductor.