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Theory:

Thermal Expansion
The expansion of a substance on heating is called, the thermal expansion of that substance.
Uses of thermal expansion

1. Fitting the iron rim on the wooden wheel:

The iron ring is always made slightly lesser in diameter than that of the wooden wheel. Therefore, the iron ring cannot easily slip on it from the rim of the wooden wheel.

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Now the iron ring is heated to a higher temperature, it results in the expansion in size, and the hot ring is then easily slipped over to the wooden wheel's rim. Coldwater is now poured on the iron ring so that it contracts in size and holds the wooden wheel tightly.

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2. Riveting

Riveting is one of the most cost-effective and superior methods of fastening (joining two parts). Rivets is having a head on one side, a tail on the other, which are used to join two steel plates together.
 
Hot rivet is forced through the hole in the plates. One end of the rivet is hammered to form a new rivet head. When cooled, the rivet will contract and hold the two plates tightly together.
 
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3. Gaps are left in between rails while laying a railway track.
An increase in temperature during the summer days causes expansion in rails. So, in order to allow expansion at the joints of the rails while laying a railway track, a small gap is left in between the rails.
 
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If such a gap is not left at the rail's joints, the track will get deformed due to thermal expansion. It will cause derailment of trains.