Page 142 - Physics - XI

of 2 cm each. Fix one end of the bi-metallic strip rigidly in a clamp stand so that it lies almost horizontally
as shown in Fig. 2.1.

Principle/Theory

On heating, the length of the bar changes. It is called as linear thermal expansion. Let L and L be the
1 2
lengths of bar of a metal at temperatures t °C and t °C such that
1 2 Brass
t > t . The change in length (L – L ) is directly proportional to B
2 1 2 1
the original length L and the rise in temperature (t – t ). Clamp stand A
2
1
1
So, (L – L ) = αL (t – t ) Rivets Iron
2 1 1 2 1
L − L
⇒ α = 2 1 Fig. 2.1: Bi-metallic strip
Lt −( t )
1 2 1

Where, α is the coeffi cient of linear thermal expansion of the material of the bar.
Diff erent solids expand or contract diff erently for the same change in temperature. A bi-metallic strip is
prepared by joining a strip of brass with a strip of iron using rivets. At room temperature, the bi-metallic
strip is straight [Fig. 2. 2 (a)]. On heating, the strip bends with brass portion on the outer side [Fig. 2.2 (b)].
It shows that as compared to iron, brass expands more on heating, i.e. when a bi-metallic strip is heated,
the metal with larger coeffi cient of linear expansion remains on the outer side.

Procedure
1. Heat the bi-metallic strip to a high temperature using a burner and observe the bending of the bi-
metallic strip (Fig. 2.2). Brass Rivets
2. Fix one end of the brass-iron bi-metallic strip rigidly in

a clamp stand, keeping brass strip on the upper side and
the bimetallic strip is horizontal. (Fig. 2.1)
3. It is observed that the free end of the bi-metallic strip is Iron
bent in the form of an arc. (Fig. 2.3) (a) At room temperature
4. It is observed that the metal which forms the upper part Brass
is the metal which has larger linear expansion.
5. Let the bi-metallic strip cool.

6. Invert the bi-metallic strip by turning through 180°, so Iron (b) On heating
that the metals interchange their sides, i.e. now iron Fig. 2.2: Bi-metallic strip on heating
strip forms the upper part and brass strip forms the and at room temperature
lower part. B (Brass) Upper part
7. Heat it again and observe the bending of the strip. It
is observed that the bending is opposite to that of the A (iron)

previous case. Lower part
8. Put all the observations in the observation table. Fig. 2.3: Bending of bi-metallic strip
on heating

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