Notes

Ohm’s Law …

Ohm’s Law is:
The electric current I flowing through a substance is proportional to the voltage V across its end, i.e.,
V ∝ I or V = RI
where the constant of proportionality R is called the resistance of the conductor. The SI unit of resistance is ohm, and is denoted by the symbol Ω. The resistance R not only depends on the material of the conductor but also on the dimensions of the conductor as below in the formula:-
1 Ω = 1 VA^-1
Although Ohm’s Law has been found valid over a large class class of materials, there do exist materials and devices used in electric circuits where the proportionality of V and I does not hold. The deviations broadly are one or more of the following types:
(1) V ceases to be proportional to I.
(2) The relation between V and I depends on the sign of V.
In other words, if I is the current for a certain V, then reversing the direction of V keeping its magnitude fixed, does not produce a current of the same magnitude as I in the opposite direction.
(3) The relation between V and I is not unique, i.e., there is more than one value of V for the same current I.