Page 91 - Physics - XI

SECTION - B

Mechanical Properties
5 Mechanical Properties
Of Solids
Of Solids

EXPERIMENT - 1

Aim

To determine Young's modulus of elasticity of the material of a given wire.

Apparatus and Materials Required

Searle’s apparatus, two long identical pieces of wire, screw gauge, metre scale, slotted weights, and spirit level

Description of Apparatus
Searle's Apparatus

This apparatus is used to fi nd Young's modulus of the material of a wire. It consists of two metal frames M
and M' having two torsion heads N and N' at the upper side and two hooks A and B at the lower side. These
frames are held together by a cross piece and are suspended
from the torsion heads T and T by means of two identical T 1 2
1
2
wires X and Y of the same material, same length, and area X Y
of cross-section. A constant weight W is suspended through Reference wire Experimental T wire
hook A which keeps the wire X taut (Fig. 1.1). A hanger H is N N′
suspended from the hook B. The experimental wire Y can be

loaded by slotted weights on the hanger H. One end of the spirit M M′
level is pivoted to the frame M' and the other end rests on the
tip of a micrometer screw which can be worked in the frame O S O
M' along a vertical scale marked in millimetres. The screw has P Spirit Level Q
a circular disc having equal divisions along its circumference. Cross bar
The screw is adjusted so that the spirit level is in the horizontal C
position. This is so when the bubble of the spirit level stands K

exactly in the centre. On loading the hanger H, the wire Y is P S
elongated and the frame M' is lowered. The screw is raised
till the bubble again stands in the centre. The vertical distance
by which the screw is moved measures the increase in length W A B
produced in the wire Y due to the load added on the hanger. H
Fig. 1.1: Searleʼs Apparatus
Principle/Theory

Searleʼs apparatus works on Hookeʼs law.
Hookeʼs law: It states that within elastic limit the stress is proportional to the strain. Hence
Stress ∝ Strain
Stress = M × Strain

where M is a constant of proportionality and is called modulus of elasticity.

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