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Correct option is C)

where R is a constant called resistance for a given metallic wire at a given temperature.

The circuit is set up as shown in the diagram below. Firstly, one cell is used and the current (I) is noted in the ammeter and the potential difference (V) in the voltmeter across the wire AB. Now, two cells, three cells and four cells are used and the observation is repeated. The readings in the ammeter and voltmeter are noted down for each repetition.

A graph is plotted between the current (I) and potential difference (V). The graph will be a straight line. This shows that the current flowing through a conductor is directly proportional to the potential difference across its ends.

That is, $V∝IorI∝VandR=IV $ where R is the resistance of the metallic wire.

Conditions for which Ohm's law does not hold good:

- Some materials have a temperature dependent specific resistance. So there are materials with a positive temperature coefficient (PTC) which increase resistance with increasing temperature and those with a negative temperature coefficient (NTC) which decrease resistance with increasing temperature. Hence, ohm's law does not hold for varying temperatures.
- Ohm's law is all well and good for DC currents, but in AC circuits it needs a little expansion. This expansion is called impedance. It is a complex number in the form of $Z=R+j×X$, where the real part of the impedance is the ohmic resistance, and the imaginary part is called "reactance". If the reactance is positive, then you have coil-like, that is, inductive behaviour and if the reactance is negative you have capacitive behaviour.
- Ohm's law cannot be applied to unilateral networks. A unilateral network has unilateral elements like diode, transistors, etc., which do not have same voltage current relation for both directions of current.
- Ohms law is also not applicable for non linear elements. They are those which do not give current through it, is not exactly proportional to the voltage applied, that means the resistance value of those elements changes for different values of voltage and current. Examples of non linear elements are thyristor, electric arc, etc.

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