Page 3 - 3-Lesson Notes-Application of Contraction and Expansion

➢ A metal rim is heated to fix it on to a cartwheel because metal rims are made

slightly smaller.On heating, the rim expands and fits on to the wheel. Then on

cooling, the rim contracts and fits slightly on to the wheel. The metal rim is usually
fitted on the wooden wheel for smooth movement and lesser wear and tear of wood.

It is a physical process.

➢ The wires on the electric poles are not strung tightly and are kept loose due to
this expansion and contraction phenomena. The electric wires are generally made up

of copper or aluminium, which conducts the electricity. During summers, it gets
heated up and results in the expansion of electric wires. And, during winters, they

contract to their original structure. So, to avoid breakage of electric wires (which may
cause accidents), due to more contraction in winters, the electric wires are strung

loosely between the poles, left shagging.

➢ Mercury is used in the clinical thermometer due to the same phenomena of
expansion and contraction. Mercury, being a liquid metal, expands on heating and

contracts on cooling. The bulb of the thermometer contains liquid mercury, which
expands on absorbing the body heat. As a result, the level of mercury rises in the

glass tube, thereby indicating the body temperature. And, if the bulb of the
thermometer is then dipped in cold water, the level of mercury is seen falling down

(contracting).

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