Answer:
Ohm's
law states that the electric current flowing through a conductor is directly proportional
to the potential difference 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.
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 below in the figure, consisting of a
nichrome wire XY, an ammeter (A). A voltmeter (V) and four cells 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 VII is obtained corresponding to
one, two, three and four cells in successive readings. Also, the V - I graph is
obtained as straight line that passes through the origin. This law is only
valid for ohmic conductor e.g. metals.
S.No.
Number of cells used in the circuit
Current through the nichrome wire, I (ampere)
Potential difference across the nichrome wire, V (volt)
V/I
(Volt/Ampere)
1.
2.
3.
4.
1
2
3
4
Thus, 
is
a constant, i.e., 
This verifies Ohm' 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 metallic
conductors only when physical conditions like temperature etc. are kepi
unchanged. It is not obeyed by a lamp filament, junction diode, thermistor etc.
These are called non-ohmic conductor.
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