Effects of Heat

Effects of heat:

Heat can change the state from Solid to liquid and From Liquid to gas.

Heat can burn things which is a chemical change.

Excessive Heat kills the organisms.

Heat Causes Expansion:

When we heat a substance the energy of the molecules increases, they move away a bit more hence they occupy more space. In liquids molecules move more freely than in solids, when they are heated their movements increases more than in solids. A liquid expands more than solid. Similarly, gases expand more than liquids.

Expansion in everyday life:

The floor is laid out in rectangular pieces, sometime glass is placed between them to allow expansion.

Gaps are left between lengths of railway tracks to allow expansion in summers.

Gaps are left in the steel structure of bridges to allow expansion. Sometimes they are made to rest on rollers, so that they do not bend on expansion

Electric wire between poles sag during summer and become tight in winter because of expansion due to heat.

Temperature Rises With Heat:

As an object is heated it gains energy. The kinetic energy of the molecules increases. This increase in energy of the molecules increases the temperature of the object.

An iron rim is heated before being fitted onto wooden wheels: The iron rim is made slightly smaller than the diameter of the wooden wheel. The rim is heated and this hot iron rim is then placed on wooden wheel and cooled by pouring water over it. On cooling, the iron rim contracts and holds the wheel firmly.

Riveting of metal plates: A red-hot rivet is passed through the holes in the two plates/sheets.While still red hot it is hammered nearly flat. When the rivet cools down, it contracts, and pulls the two plates together very firmly.

To remove a stuck stopper or cap from a bottle: A metal cap stuck on a glass bottle/stuck stopper can be removed from bottle by pouring hot water on cap/neck of the bottle. When hot water is poured over the cap/neck of the bottle, it expands and cap/stopper gets loosened and can be removed easily.

Laying of rail tracks: If rails are fixed tightly, leaving no space for expansion or contraction, these will bend outward in summer. It will result in the derailment of train. To avoid such a situation, small gaps are left in between the rails.

Laying of telephone and electric transmission wires: When the telephone or telegraph wires are laid in summer they are kept slightly loose so as to allow for contraction in winter. Similarly, if they are laid in winter they are kept very tight so as to allow expansion during summer.

Laying of cement floor: Cement floor is laid in small blocks leaving a very small space in between the blocks to allow the expansion and do not crack the floor during the expansion in summer.

Illustration: When ice is added to water in a glass, why do we feel cold to touch the glass?

Solution: Heat flows from a hot body to cold body. Heat flows out of the glass into the ice. Thus the glass loses heat and hence we feel it cold.

Illustration: An empty bottle is closed tightly with a cork and then heated. After some time the cork shoots out from the mouth of the bottle. What could be the reason?

Solution: When the bottle is heated, the air in it also gets heated and air expands more. Hence it exerts a force on the stopper and causes it to shoot out of the bottle.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Heat always flows _______________ .
A. From a colder object to a hotter object		B. From a hotter object to a colder object
C. Heat never flows from one object to other		D. None of these .
Right Option : B
View Explanation

Question : 2

Which of the following are correct ? (a) Heat can burn things which is a physical change . (b) The temperature rises with heat and as an object is heated it gains energy. (c) Excessive heat kills the organisms.
A. Only (a) and (b) is correct .	B. Only (b) and (c) is correct .	C. Only (a) and (c) is correct .	D. All of these .
Right Option : B
View Explanation

Question : 3

In the transmission of electricity ______________________ between poles sag during summer and become tight in winter because of expansion due to heat.
A. Electric bulb	B. Electric current	C. Electric wire	D. None of these .
Right Option : C
View Explanation