Page 11 - Physics - XII

SECTION - A

Current Electricity
1 Current Electricity

EXPERIMENT - 1

Aim
To determine resistivity of two/three wires by plotting a graph for potential diff erence versus current.

Apparatus and Materials Required
Wires of diff erent materials (copper, constantan, iron, etc.), a voltmeter (0–5V), a milliammeter (0–500 mA),
a battery eliminator, a rheostat, a metre scale, a one-way key, connecting wires, and a piece of sand paper

Principle/Theory
Ohm’s law states that the current fl owing through a conductor is directly proportional to the potential
diff erence across its ends, provided the physical state of the conductor remains unchanged. If I be the
current fl owing through a conductor and V be the potential diff erence across its ends, then as per Ohm’s
Law,
V ∝ I or V = RI …(1)

where, R is the constant of proportionality. It is known as resistance of the conductor.
V
from (1), R =
I
At a given temperature, the resistance of a wire is expressed as, R L where ρ is resistivity or specifi c
A
resistance and is the characteristic of the material of wire, L is the length of the wire, and A is the area of
cross-section of the wire.
L L 4 L DR
2
Now, R =
A D 2 D 2 4 L
4
where D is the diameter of wire. Battery K
In SI units, the potential diff erence V is measured in + –
volt and the current I in ampere, the resistance R is + Rheostat

measured in ohm. mA
According to ohm's law, there exist a linear relationship – R Unknown resistance
wire
between V and I. Therefore, a graph between the
potential diff erence (V) and the current (I) must be a
straight line. The slope of the straight line gives the + V –
reciprocal of the resistance. Fig. 1.1: Circuit diagram

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