9 - Heat
Q
1 Fill in the blanks with the proper word from the brackets :
(radiation,
white, conduction, blue, convection, bad conductor, absorb, good conductor,
black, reflection)
(1) Maximum heat is absorbed by a
..................... coloured object.
(2)
.......... of heat does not require a medium.
(3)
Conduction of heat takes place through a ................ substance.
(4)
The shining surface in a thermos flask decreases the outgoing heat by
(5)
Cooking utensils are made from metals due to their property of
(6)
The earth receives heat from the sun by.............
Ans.:
(1) black (2) Radiation (3) good conductor (4) reflection (5) conduction (6)
radiation
Q
2. Answer the following question in one sentence.
1]
How does a fever get lowered with cold compress on forehead of a patient?
Ans A cold compress
absorbs some heat from the patient’s forehead as water slowly evaporates from
the cloth and lower the body temperature.
2)
Q.
3. Answer the following questions in short:
(1)
Why are the houses in Rajasthan painted white?
In
Rajasthan, atmospheric air temperature is very high in summer. White colour reflects
most of the sun's incident radiant heat. Moreover, absorption of heat by white
colour also is very little, as a result houses do not get heated up. Therefore,
houses in Rajasthan are painted white.
(2) Explain which mode of heat transfer causes sea
breezes and land breezes?
Ans.
During day time the land gets heated up by sun's radiant energy more quickly
than water in the sea. As a result hot atmospheric air over the land rises up
due to lower density and transfer of heat by convection currents. The
relatively cold air with higher pressure (higher density) from the sea blows
towards land which is called sea breeze. At night land mass cools quickly while
sea remains warmer. Again warmer air above the sea rises up due to convection
and low pressure, as a result the winds blow from land to sea.
(3)
Why is the outer coat of penguins of Antarctica black?
Ans.
Black colour absorbs radiant heat energy from sun readily. In Antarctica there
is freezing cold temperature. Warm blooded animals struggle to keep their body
warm. Penguins outer black coat colour absorbs radiant heat from sun and helps
them to keep their body temperature warm.
(4)
Why heaters fitted near the floor and air conditioners, near the ceiling in
a room?
Ans.
The air warmed by the heater near the floor rises up easily due to its lower
density and the cold air near the ceiling comes down and replaces it due to
higher density. The density difference of air at upper and lower height sets in
convection currents by transfer of heat. Similarly, air conditioners fitted
near the ceiling blow cold air that settles down due to its higher density
while warm air near the floor rises up due to convection currents.
(5)
Which one of these will absorb the heat?
Steel
spoon, wooden board, glass, vessel, iron griddle (tava), glass, wooden
Spoon,
plastic plate, soil, water, wax
Ans.
All the above will take up heat depending on if it is a good or bad conductor
of heat. Metal objects such as steel spoon and iron griddle are good conductor
of heat and they will be hot. Water is a moderately good conductor and hence it
will take up heat slowly and become hot. Wooden board, wooden spoon and plastic plate are bad conductors of heat and
Q.
6. Give scientific reasons :
(1)
An ordinary glass bottle cracks when boiling water is poured into it, but a borosil
glass bottle does not.
Ans.
When boiling water is poured in ordinary glass bottle, the surface on which hot
water falls expands quickly. The heat is not conducted rapidly from this area.
As compared to this part the adjacent part does not heat up and does not get
expanded. Hence due to unequal expansion, there is a crack in the bottle. In
borosil bottle, the heat conductor uniform and is very quick. The expansion due
to heat is also uniform. Therefore ben bottles do not crack.
(2) The telephone wires which sag in summer
become straight in winter
Ans.
In summer the temperature of the surrounding air is high. Therefore, the wires
of telephone expand and start sagging. In winter, the surrounding temperature
is less and hence the wires undergo contraction and become straight.
(3) Dew drops are formed on the grass, in winter.
Ans.
There is a water vapour in the air. The water holding capacity of the air
depends upon the temperature. In winter, when the temperature falls down, the
water holding capacity also comes down. If temperature recedes much more, then
the water vapour condences and the dew
drops are formed.
(4)
In winter, why does an iron pillar feel colder than a wooden pole?
Ans.
Iron is a good conductor of heat. Wood is a bad conductor of heat. In winter
body temperature is more than the surrounding temperature. At such time, if we
touch iron pillar, there is a conduction of heat from our body to the iron
pillar. Therefore we feel that iron pillar is cold. But when we touch a wooden
pole, the conduction of heat does not take place and hence we do not feel that
the wooden pole is cold.
