State Ohm's law ? How can it be verified experimentally ? Does it hold good under all conditions ? Comment.

Explanation

Ohm's law states that the electric current flowing through a conductor is directly proportional to the potential differece applied across its ends, provided the physical conditions such as temperature remains unchanged.
If v is the potential difference applied across the ends of conductor through which current/flows, then according to Ohm's law.
`V prop I rArr V = IR`
`I = (V)/(R)`
where, R is the constant of proportionality called resistance of conductor at a given temperature. The experimental set up of Ohm's law is made as the circuit shown belown in the figure, consisting of a nichrome wire XY, an ammeter (A). A voltmeter (V) and four cell of 1.5 V each.

The reading in the ammeter A for the current and reading of the voltmeter V for the potential difference across the nichrome wire XY is measure and recorded in the table given below, first by using only one cell and then by the two, three and four cells in successive readings.

Calculating the ratio of V to I for each pair of potential difference V and current I. A graph is plotted between V and I and the nature of the graph is observed as shown here. The same value for V/I is obtained corresponding to one, two, three and four cells in successive reading. Also, the V - I graph is obtained as straight line that passes through the origin. The law is only valid for ohmic conductor e.g. metals.
Thus, V/I is a constant, i.e., `V prop I`
This verifies Ohm's law.
Ohm's law does not hold good under all conditions as it is not a fundamental law of nature like Newton's law. It is obeyed by metalic conductors only when physical conditions like temperature etc. are kept unchanged. It is not obeyed by a lamp filament, junction diode, thermistor etc. These are called non-ohmic conductor.