Page 3 - Lesson Notes

 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).

3