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N.B:  
The land gets warmer quickly than the sea, because sea water has a higher heat  
capacity than the land (sea water needs more time to raise its temperature)  
(vii)  
Motor – car cooling system  
Car engines are cooled by convection currents in the water pipes. Water is a very good  
substance to carry the unwanted heat away from the engine to the radiator.  
(viii)  
Hot Air balloons  
Air molecules at the bottom of the balloon get heated by a heat source and rise. As the  
warm air rises, cold air is pushed downward and it is also heated. When the hot air is  
trapped inside the balloon, it rises.  
Radiation of heat  
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If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known  
as radiation. The side of your body nearest the fire warms while your other side remains  
unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this  
transfer of heat.  
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Therefore; Radiation is the transfer of heat energy through space by electromagnetic radiation.  
Transfer of heat energy from the sun reaches us in the form of radiation. Radiation is emitted by  
all bodies above 0 K  
.
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Some objects absorb radiation and some other objects reflect them  
.
This can be shown using the demonstration set up shown in Figure below. In this figure the  
inside surface of one plate is shiny and of the other is dull black. Coins are stuck on the outside  
of each plate with candle wax. If the heater is midway between the plates they each receive the  
same amount of radiation.  
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Therefore, after few minutes the wax on the black plate melts and the coin falls off while the  
shiny plate stays cool and the wax on it is un-melted and the coin does not falls off.  
NB;  
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Radiant heat travels with the speed of light and can be reflected , absorbed and transmitted  
Heat loss by radiation is minimized by covering a body (surrounding) with shiny surfaces  
(polished surface).  
Radiant Detector  
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There are two instruments which can be used to detect radiations. These are Thermopile and  
Liquid in thermometer  
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Thermopile T is an instrument used to converts radiant heat energy into electrical energy  
Absorbers, Emitters and Reflectors  
Absorber  
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Absorber is a material that delivers/gain all radiant energy.  
A surface that absorbs all radiant energy is called black body.  
Good radiators of heat are also good absorbers  
Example, black cooking vessel, black clothes dry faster than others colored clothes etc  
Emitter  
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Emitter is a material that delivers out all radiant energy.  
A surface that emits all radiant energy is called black body  
Reflector  
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Reflector is a material (surface) that bounces back all radiant energy.  
Example, white or bright – colored surfaces are poor emitters and absorbers of heat, solar  
cookers etc  
Uses of good and bad radiators or absorbers  
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White clothes. In hot countries white clothes are slightly cooler than other clothes  
White buildings. These are cooler than buildings with dark colours on the outside. Storage  
tanks containing oil or petrol are usually painted with aluminium paint. The polished, silvery  
surface is a bad absorber.  
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Vacuum flask. This is used to keep hot liquids hot and cold substances cold  
Thermos Flask (Vacuum Flask)  
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Thermos flask is a device used to hold hot or cold liquid for long period of time.  
It consists of a double-walled glass (polished by coated with a thin layer of aluminium), container  
with vacuum between the walls. It has a stopper made of insulating materials or cork (See the fig.  
below)  
How thermos flask prevents heat Loss?  
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Vacuum flasks are designed to keep liquids hot by minimizing heat losses in three ways that  
is, conduction, convection and radiation.  
Heat losses by conduction is minimized by  
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The stopper which is a bad conductor (made of wood, rubber or cork)  
Glass flask (Poor conductor of heat)  
The supporting pad (separator). It is also a poor conductor of heat  
Heat losses by convection can be minimized by  
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Vacuum between the walls of the flask  
The stopper which is a bad conductor (made of wood, rubber or cork)  
Heat loss by radiation is harder to stop as radiant heat can pass through a vacuum. To  
minimize heat loss through radiation, the walls of the glass are silvered so as to reflect  
the radiant heat back into the hot liquid.  
Metal Foil  
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Metal foil Is a polished material used to cover items to minimize heat loss by radiation. It is  
used to prevents heat loss by reflecting radiant energy  
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For example, aluminium foils are used in wrapping hot food.  
Applications of radiation  
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Electric kettles, iron boxes and cooking pans have polished surfaces to reduce heat loss  
through radiation  
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Houses in hot regions are painted white to minimize heat absorption through radiation  
Green houses act as heat trap by preventing longer wavelength radiations from passing  
through glass  
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Cloudy nights are warmer than clear nights because clouds reflect radiation back to the earth  
We wear white or light-coloured clothes in summer because they are poor absorbers and  
good reflectors of heat. This way they keep us cool. On the contrary, we prefer to wear dark-  
coloured clothes in winters because they absorb most of the heat of sun and keep our  
body warm  
.
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Radiators of heat in cars, machines and air conditioners are painted black so as to have  
cooling effect by radiating most heat.  
Base of cooking utensils is made black. Such a black surface absorbs more heat from the  
surroundings. This is because conduction, convection and radiation of heat are minimum.  
In cold areas, a greenhouse is used for the better growth of plants. Radiation from the sun  
passes through the glass or plastic and warms up the soil and plants. Plants and soil absorb  
and emit radiation and increase the temperature in the greenhouse. Plants grow well in  
the increased temperature of the greenhouse.  
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Teapots, the shiny surfaces are bad emitters of radiation, shiny teapots can keep tea  
warmer than a black teapot for a longer time.  
The cooling fins at the back of our refrigerator need to radiate their heat quickly to the  
surroundings. Its surface is made rough and painted black.  
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When we sit beside a fire, the heat of fire reaches us by radiation.  
Room (electric) heaters have bright polished surfaces which act as good reflectors of  
heat. Such surfaces absorb very little heat and reflect towards us most of the heat radiations.  
These surfaces remain cool even after continuous use of heaters. The highly polished  
surfaces of spacecraft reflect most of the heat radiated from the sun.  
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Cloudy nights are warmer than clear nights sky  
Radiation from the sun reaches the earth and heats the earth's surface. The clouds act as a  
good reflector of heat energy. At night, the heat radiated from the earth is reflected by the  
clouds. This in turn makes the night warmer. On clear nights, there are no clouds in the sky to  
trap heat. So clear night become cool.  
Self Assessment – 7  
1. Explain the following:  
(a) A wooden knob and a metal latch on a door are both at room temperatures. Why  
the latch is colder to touch.  
(b) Water is heated generally from below.  
(c) Land becomes warmer than water during the day.  
(d) Why in cold countries windows have two glass panes with a thin layer of air between  
them?  
(e) In winter the human body covered with a blanket keeps warm.  
(f) It is better to use thin blankets to keep the body warm rather than using a single  
blanket of thickness equal to their combined thickness.  
(g) In winter the birds fluff up their feathers.  
(h) Old quits are less warm than the new ones.  
(i) On a hot sunny day, it is advisable to wear light coloured clothes.  
(j) We feel warmer in a fur coat. Why?  
(k) If air is bad conductor of heat, why do we not feel warm without clothes?  
(l) Why snow is a better heat insulator than ice?  
(m) When glass and copper rods are heated up to some temperature and then if the hot  
areas are touched, why copper piece seems warmer than the glass rod?  
(n) Ventilators are provided in rooms just below the roof. Why?  
(o) Why coolant coil fitted on the ceiling of the refrigerator?  
(p) Why is it hotter at the same distance over the top of a fire than in front of it?  
(q) A body with a large reflectivity is a poor emitter. Why?  
(r) Why are steam pipes wrapped with insulating materials?  
(s) Animals curl into a ball when they feel very cold. Why?  
(t) An ink dot on a cup of porcelain appears dark. When the same cup is heated to a high  
temperature, the dot becomes brighter than rest of the cup?  
(u) Why in summer days does the fan give hot air first in a closed room when it is  
switched on?  
(v) Why are houses often built with high ceilings?  
(w) A fan rotating in a room may cause you to feel much cooler but a thermometer in the  
room will show the same temperature as before the fan was switched on.  
2. Explain the following:  
(a) Why is radiator of motor car painted black?  
(b) Water should not be used to put off electrical fires.  
(c) Heat from the sun heats up earth, but the air in the atmosphere does not get heated up.  
(d) The top of a roof made of asbestos is painted white  
(e) A tile floor feels colder than a wooden floor when you walk on it, even when both are at  
same temperature. Why?  
(f) Stainless steel pans are usually provided with copper bottoms. Why?  
(g) When a pan is removed from the fire, it slowly cools down. Why does it cool down?  
(h) A brass tumbler feels much cooler than wooden tray on a chilly day  
(i) Why should air coolers be kept higher up in the room, and not down like the heaters?  
(j) why in hot water sugar is dissolved  
(k) Why room heaters should be placed at the floor of the room?  
(l) It is hotter at the same distance over the top of a fire than it is on the side of it. Why?  
(m) The bottoms of the cooking utensils are painted black. Give the reason.  
(n) Why is the freezing chest in a refrigerator fitted near its top  
(o) Why is an oven made of double walls with the space in between filled with cork?  
(p) Why do we use cooking utensils made up of copper?  
(q) Why is a tea kettle provided with an ebonite handle?  
(r) In summer, ice is kept wrapped in a gunny bag. Explain the reason.  
(s) the water pipes are covered with cotton during very cold water  
(t) Why are chimneys provided over furnace in factories?  
(u) A house with thick walls is likely to be cooler during hot seasons.  
(v) Eskimos make double walled ice houses. Why?  
(w) Why no heat sensation is felt when a burning match stick is held in the fingers?  
(x) At what temperature would a block of wood and a block of metal feel equally cold or equally  
hot, when touched?  
(y) A cloudy night is hotter than a clear sky night. Why?  
(z) Why do fire workers wear bright clothes?  
3. Explain what is meant by the conduction of heat .Use the kinetic theory to explain how heat is  
transferred along a metal road  
4. Explain briefly the reason why steam pipes are covered with felt or asbestos  
5. A piece of iron and a piece of wood are both cooled to a temperature of 0 0 C .When touched  
with a finger ,the iron feels colder than the wood .Why ?  
6. Why are cloudy nights generally warmer than clear ones  
?
7. Why are thermometers that are used in weather stations shielded from the sunshine? What  
does a thermometer measure if it is shielded from the sunshine and also if it is not?  
ANS;  
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Sunshine emits electromagnetic radiation that increases the heat energy surrounding the  
thermometer. As a result, the mercury in the thermometer will show a higher reading than  
the normal temperature of the environment when it is exposed to sunshine. So, if you are  
shielding it from the sunshine, the thermometer will show the exact temperature that is  
required here. Therefore, if a thermometer is shielded from sunshine, it will measure the  
exact temperature of the environment.  
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However, if it is not shielded from sunshine, the thermometer will show a higher reading  
than the actual temperature of the environment.  
8. On average, would Earth be warmer or cooler without the atmosphere? Explain your answer.  
Ans;  
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Atmosphere plays a crucial role in maintaining Earth's temperature. It contains  
greenhouse gases that are transparent to solar radiation. These gases allow the sun's  
radiation to reach the Earth's surface. When the Earth's surface is heated, it emits infrared  
radiation. The greenhouse gases in the atmosphere trap this radiation and re-emit it back to  
the Earth. This helps to maintain Earth's temperature, and is responsible for a phenomenon  
known as global warming.  
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In the absence of an atmosphere, the radiation from the sun hitting the Earth would heat  
up the ground, and then escape into space. Since there would be no greenhouse gases to  
trap this radiation, heat would quickly dissipate into space. As a result, the temperature of  
the Earth would decrease and it would become cooler.  
9. Discuss the uses of good and bad conductors of heat energy in everyday life  
10. What do you understand by convection? Describe how you would demonstrate the formation of  
convection currents in a liquid  
11. Explain the following:  
(a) Cork is packed between the double walls of refrigerating chambers  
(b) Most cooking pots are made of Aluminium  
(c)Ice blocks are wrapped in paper when they are stored  
(d) Water tanks in the tropics are painted white  
(e) White clothes are worn in the Arctic in preference to dark ones  
12. A thermometer having a blackened bulb records a higher than an ordinary thermometer when  
they are both held at an equal distance from a fire Explain this  
13. Explain briefly how a fire can assist in the ventilation of a room  
14. (a) Explain how heat transfer by radiation takes place.  
(b)Why does heat transfer by radiation not require a medium?  
(c) A good cooking vessel should be black on the outside and not shiny white. Explain.  
(d) Give one way through which heat losses by radiation can be prevented.  
15. List three areas where heat transfer through each of the following methods is applied:  
(a) conduction  
(b)convection  
(c)radiation  
16. (a) explain how heat transfer by conduction takes place  
(b) Why are gases poor conductors of heat  
(c) Why are cooking vessels made of aluminium and not iron?  
(d) Why are stadium seats made of plastic and not steal?  
(e) Give two ways through which heat losses by convection can be  
prevented?  
17. Why is it not possible for heat transfer by convection to take place in solids?  
18. Explain the importance of making ventilation on the top of the walls in a room  
19. Explain briefly how heat travels in metals  
20. The figure below shows a solar panel used for heating water  
(a) What is the name of this type of electromagnetic radiation from the sun? What is the  
significance of this radiation?  
(b) What color should the surface covered with a transparent glass sheet?  
(c) Why is the surface covered with a transparent glass sheet?  
(d) If no pump was available, how could the water be made to circulate naturally?  
(e) Why is the solar panel tilted at an angle to the horizontal?  
(f) What is the material Y behind the panel? Why is it used?  
21. The diagram below shows two air filled bulbs connected by U – tube partly filled with a  
low density (or light) oil.  
(a) Describe how the levels of oil in the limbs X and Y change after the electric bulb is  
lighted.  
(b) Account for your observation.  
22. How is heat loss by conduction ,convection and radiation reduced in a vacuum flask  
23. Explain, how land and sea breeze occurs