Page 38 - Science Class 07

allow the flow of heat as well. The property flow to the outer side except upward along the
which makes insulators different from thin glass walls of the inner bo le and through
conductors is its resis vity. the cork cover. Both the glass and the cork are

Wood, cloth, glass, mica and quartz are some poor conductor of heat.
good examples of insulators. Insulators are also Heat cannot be transferred from the
protectors as they give protec on against heat, inner bo le to the outer one by
sound and of course passage of electricity. convec on, since all the air has
Insulators don’t have any free electrons and it been removed from the space
is the main reason why they don’t conduct between the wall of the two bo les.

electricity. This produces a vacuum and the
Examples of Insulators vacuum is not able to transfer heat
by convec on as air does. Thermos Flask
v Glass is the best insulator as it has the
highest resis vity. Finally, the transfer of heat from the
inner bo le to outer one is greatly reduced by
v Plas c is a good insulator and it is used in
silvering the outer surface of the inner bo le
making a number of things.
and the inner surface to the outer bo le. This
v Rubber is a common component used in silver coa ng reduces the radia on of heat
making tyres, fire-resistant clothes and from the inner bo le. Any heat that penetrates
slippers. This is because it is a very good the glass of the inner bo le is bound to be
insulator. reflected from the silver–coated surface of the

THERMOS FLASK outer bo le. Because of this three fold
A thermos flask is a device designed to prevent protec on, the contents of a thermos flask can
loss of heat through conduc on and convec on be kept hot or cold for longer period of me.

as well as through radia on.
Construc on and Working

It is a bo le within a bo le but the two being 1. How does heat get transferred?
united only around the top. The inner bo le 2. What is conduc on?
contains the outer bo le whose temperature is 3. Can all kinds of solid transfer heat?
to be kept constant. The loss of heat from the 4. What is radia on?
material by conduc on is greatly reduced by 5. Differen ate between conductor and
providing no path along which the heat can insulator.

Heat : A form of energy that determines the hotness of a body

Kinetic energy : Energy produced by a moving object
Temperature : The degree of hotness or coldness of a body
Celsius : Unit of temperature

Thermometer : A device used to measure temperature
Conduction : Transfer of heat from one body to another when they are kept in close contact

38 Science-7