(a) Ohm's law: It states that current flowing through a conductor is proportional to the potential difference across its two ends provided the physical conditions (temperature, mechanical strain etc.) of the conductor remain unchanged.
If V is the potential difference between two ends of a conductor and I is the current flowing through it, then
V ∝ I
or V = RI ...................(1)
where R is the constant of proportionality and is called the resistance of the conductor. Its value depends upon
(i) Shape of conductor
(ii) Length of conductor
(iii) Nature of the material
If a graph is plotted between V and I, the graph will be a straight line passing through the origin as shown in Fig.
Note: Conductors obeying Ohm's law are called ohmic conductors and those which do not obey Ohm's law are called non-ohmic conductors.
(b) Resistance: Resistance is the property of a material by virtue of which it opposes the flow of current through it and quantitatively it is given by,
R = \(\frac{V}{I}\)
= \(\frac{\text { Potential difference }}{\text { Current }}\)
The S.I. LTnit of resistance is ohm (Ω). It is a practical unit of resistance.
Definition of ohm
If V = 1 volt ; I = 1 ampere
then R = 1 ohm.
Hence a conductor has a resistance of one ohm if a current of one ampere flows through it when a potential difference of one volt is maintained across its two ends.
1 ohm = \(\frac{\text {1volt}}{\text {1ampere}}\) or 1Ω
= IV A-1
