CHP # 9
Q.1
TRANSFER OF HEAT
Explain conduction of heat. Describe its three applications.
(Ans) Conduction of heat
The process in which heat energy is transferred from particle to particle by
collision is called conduction of heat.
 Conduction takes place in solids.
 Conduction required a material medium.
 Substances that carry conduction are called conductors e.g Silver, Iron,
Copper,etc.
Mechanism of heat conduction
The mechanism of heat conduction can be explained by the behaviour of
atoms within the materials.
There are two ways by which the heat is transferred.
(i) Vibrations of the atoms in the metals.
(ii) Motion of free electrons present in the metals.
Example
The solid iron is made of closely packed iron atoms. The iron rod has also
a large number of free electrons. The atoms in hotter part of the rod
vibrate more violently than in the colder part. They collide with the
neighbouring atoms and molecules. The K-E of these molecules increases
and as a result, the heat is transfer from one end to the other.
Practical applications of conduction are as under:
1. Cooking pots and pans are made of metals that are good conductors.
They conduct heat quickly to the food inside the pots and pans. As a result
the food is prepared.
2. We use a wire gauze to place over a flame to conduct heat outwards from
the flame. A glass beaker can be heated saftly on the wire gauze without
any breaking.
3. Icebox has double wall made of iron or tin. The space between the two
walls is filled with cork that is a poor conductor of heat. The cork prevents
the flow of heat, as a result the icebox is cold for long time.
Q.2
Differentiate between good and bad conductors.
(Ans) Good conductors (bad insulators)
Those substances that conduct heat through itself easily are called good
conductors or bad insulators.
Examples
Silver, Iron, Copper, Aluminium, Gold,etc.
Bad conductors (good insulators)
Those substances that do not conduct heat through itself easily are called
bad conductors or good insulators.
Examples
Wood, Rubber, Plastic, Paper,etc.
Q.3
Explain thermal conductivity. On what factors it depends.
(Ans) Thermal conductivity
The ability of a substance to conduct heat energy is called thermal
conductivity. It is denoted by “k”. Its SI unit is “ WK 1 m 1 ”.
It can also be defined as “the amount of heat which flows through one
square meter of area of the substance in one second when a temperature
difference of one Kelvin is maintained across a thickness of one meter.”
Mathematically
k 
Q.4
QL
TAt
Explain convection of heat. Describe its three applications.
(Ans) Convection
The transferred of heat from one place to another by the actual motion of
the heated particles is called convection.
 Convection takes place in gases and liquids.
 Convection requires a material medium.
Mechanism of heat convection
The mechanism of heat convection can be explained by the behaviour of
medium between hot and cold objects. Convection occurs in all fluids,
whether liquids or gases.
Example
When water is heated, the heated portion of water at the bottom of the
beaker expands and becomes less dense. The denser water moves
upward to replace itcold water sinks down. This replacement produces a
current that are called convection current.
Practical applications of heat convection
1.
Heating water
Convection is a very effective process of heating water. When heat is
given to water, the water at the bottom absorbe the heat. It becomes less
dense and rise up the cold water from the top sinks to replace it. As a
result current is produced in water and the whole water is heated.
2.
Coastal breeze
During the day sun shines equally on the land and sea. The land heats up
nore quickly than the sea. The hot air rises over the land and cold air
blows from the sea to replace it. Thus there is a sea breeze during the day.
3.
Q.5
At night the process reverses. The land cools more quickly than the sea.
The hot air from the sea rises and the cold air from the land blows to
replace it. This movement of air is called land breeze.
Riding on thermals
Thermals are upward current of warm air. Actually they are convection
currents arise due to the replacement of hot and cool air in the
atmosphere.
Birds are able to fly for hours on thermals without flapping their wings.
Similarly gliders aeroplanes are able to rise by riding on the thermals.
Explain radiation of heat. Describe its three applications.
(Ans) Radiation
The method of heat transfer in which heat is transferred from a hotter
place to a colder place with or without having a material medium is called
radiation.
Mechanism of heat radiation
Radiation can transfer energy through a vaccum as well as through a
material medium like glass. The mechanism of radiation is not molecular
motion. It is an electromagnetic phenomenon.
Example
Heat from the sun reaches us after passing through millions of kilometers
in vaccum through conduction.
Practical applications of radiation
1.
Thermos flask
It is a vessel used to prevent heat transfer due to conduction, convection
and radiation.
Construction
It consists of a double walled glass vessel silvered on the inside. The
purpose of the silvering is to reflect all the heat radiation. The space
between the walls has vaccum that prevent convection of heat. The glass
is a poor conductor that minimizes conduction of heat.
2.
3.
Advantage
The heat loss through the flask is so small that a hot liquid placed in the
flask remains hot for a long time and a cold thing placed in the flask will
remain cold for a long time.
White clothes
White clothes are poor absorber of heat radiation and are goog reflector at
the same time.Therefore white clothes are preferred to wear in summer,
Because it reflect all the radiations of the sun falling on it.
In this way our body remains cool and we feel comfort by wearing
white clothes in summer.
The interior of a car parked in the hot sun warms easily because of suns
radiation. Heat is transmitted to the interior of the car in the form of high
energetic, high frequency and shorter wavelength from the sun.
On reflecting from the interior of the car they lose some energy
becomes less energetic, low frequency and longer wavelength infrared
radiations. These radiations cannot escape easily, that is why the interior
of a car parked in hot sun warms easily.
Q.6
Discuss “global warming”. How is it a threat to human life?
(Ans) Global warming
The earth surface is receiving thermal radiation from the sun. The
temperature of the sun surface is 600K where the earth’s surface
temperature is 300K. The thermal radiations emitted from sun are at very
high temperature is in the region of ultraviolet, visible and high frequency
infrared radiations having short wavelength.
The earth surface radiates infrared radiations of smaller energy, low
frequency and higher wavelength. These radiations are absorbed by the
water and carbon dioxide in the atmosphere. The resulting effect is that
the temperature of the earth getting higher, this is called global warming.
Global warming creat thermal pollution that is a great threat to human life.
Q.7 Discuss the greenhouse effect. How this effect keeps the
greenhouse warm?
(Ans) Greenhouse effect
Heat is transmitted into a greenhouse in the form of high frequency and
short wavelength radiation from the sun.
Glass has the property of being able to transmit sunlight and absorbe
infrared radiation. In the interior of greenhouse the radiations are
absorbed by plants and soil. These radiations are re-emitted in the form of
low frequency and longer wavelength. These radiations being less
energetic and cannot escape from the greenhouse.
Conceptual Questions
1.
Why wire gauze is often placed over a burning flame for heating an
object.
Ans. The wire gauze is often placed over a flame to conduct heat outward from
flame. The glass beaker can be heated safely on the wire gauze that
protects it from the concentrated heat of the flame.
2.
Ans.



Give three ways in which insulating materials can be used to reduce
heat losses from a house.
In order to avoid heat losses from houses, the three ways are:
The roof of the building should be insulated by using fibreglass.
The walls should be made as double wall with cavity filled with insulating
materials.
The windows should be made of double wall contain air.
3.
Why are white clothes preferred wearing in summer? Explain briefly
Ans. See Q.5 (2)
4.
Why is the freezer compartment kept at the top of the refrigerator?
Explain
Ans. In refrigerator the cooled air in freezer compartment at the top, easily sinks
in the lower portion of refrigerator which contain food. The hot air from the
food rises up from below and thus the circulation of cold and hot air takes
place in the refrigerator.
5.
Why does thermal radiation pass easily into the greenhouse than out
of it?
Ans. The thermal radiation from the sun having short wavelength can pass
easily through the glass in the greenhouse. However, the radiation emitted
fr4om the earth is of long wavelength which cannot pass through glass
and hence make the greenhouse warm.
6.
How heat losses are reduced in a thermos flask?
Ans. See Q.5 (1) Thermos flask
7.
How the interior of car parked in the hot sun warm easily?
Ans. See Q.5 (3)
8.
A black car standing in the sun warms up more quickly than other.
Why?
Ans. The black coulor is good absorber of heat radiations. It can absorbe all the
sun light falling on it. Therefore a black car standing in the sun warms up
quickly than other cars.
9.
How air filled cavity walls keep a house warmer in winter than a solid
brick wall.
Ans. The solid brick wall has a capacity to conduct heat easily from inside to
the outside. While air filled cavity wall cannot conduct heat from inside to
the outside. Therefore air filled cavity walls keep a house warmer in winter
than a solid brick wall.
10.
Why a tile floor feels colder to bare feet than carpeted floor?
Ans. The tile floor is a good conductivity as compared to the carpeted floor.
Therefore the tile floor absorbe more heat from our feet as compared to
the carpeted floor hence the tile floor feels colder than carpet floor.
11.
How woolen sweaters keep us warm in winter?
Ans. The woolen sweaters contain a large number of pores filled with air. As air
is a bad conductor of heat, therefore heat from our body does not flow out
to the atmosphere. Thus the woolen sweaters keep our body warm in
winter.
12.
In certain places the birds are fly for hours without flapping their
wings. Explain
Ans. See Q.4 (3) Riding on thermals
Numerical Problems
1.
Window glass has thermal conductivity of 0  8Wm 1 K 1 . Calculate the
rate at which heat is conducted through a window of area 2  0m 2 and
thickness 4  0mm . The temperature inside an air-conditioned room
is 20 C . The out doors temperature is 35 C.
Given data
K  0  8Wm 1 K 1
A  2  0m 2
L  4  0mm  0  004m
T1  20 C  20  273  293K
T2  35 C  35  273  308K
T  T2  T1  308  293  15K
Q
?
t
We know that
QL
k 
TAt
Q
KAT

t
L
Q
0  8  2  0  15

 6000watts
t
0  004
2.
One end of a metallic rod of cross sectional area 90mm2 and thermal
conductivity 0.32wm-1K-1 is kept at high temperature. When steady
condition is attained, the temperature gradient from one end to the
other end is 4.6x102Km-1. Calculate the rate of flow of heat along the
rod.
Given data
K  0  32Wm 1 K 1
A  90mm 2  90  106 m
T
 4  6  102 Km 1
L
Q
?
t
We know that
QL
TAt
Q KAT

t
L
Q
 0  32  103  90  106  4  6  102
t
Q
 13  248watts
t
k
3.
The external wall of a brick house has an area of 16m2 and thickness
0.3m. The temperatures inside and outside the house are respectively
200 C and 00 C. Calculate the rate of heat loss through the wall?
Given data
K  0  5Wm 1 K 1
A  16m 2
L  0  3m
T1  20 C  20  273  293K
T2  0  C  273K
T  T2  T1  273  293  20K
Q
?
t
We know that
QL
k 
TAt
Q
KAT

t
L
Q
0  5  16  20

 533watts
t
03
Negative sign shows that the heat is loss.
4.
Two vessels of different metals are similar in shape and size. They are
fully filled with ice at 00 C.By the heat from out side all the ice in one
vessel melts in 25 minutes and that in the other vessel in 20 minutes.
Compare their thermal conductivitis.
Given data
K1
?
K2
A A
L  L
T1  T2
t1  20 min
t 2  25 min
We know that
k1 
QL
TAt1
k2 
QL
TAt 2
QL
QL

TAt1 TAt 2
k1 t1 20
 
 08
k 2 t 2 25
5.
A house loses a lot of heat through windows. Calculate the rate of
heat flow through a glass window of area 3.0m2 and thickness 3.3mm.
The temperatures at the inner and outer surfaces are respectively
150C and
-50C.
Given data
K  0  8Wm 1 K 1
A  3  0m 2
L  3  2mm  0  0032m
T1  15 C  15  273  288K
T1  35 C  5  273  268K
T  T1  T2  288  268  20K
Q
?
t
We know that
QL
k 
TAt
Q
KAT

t
L
Q
0  8  3  20

 15000watts
t
0  0032