Selina Concise Class 7 Physics Solution Chapter No. 5- ‘Heat’ For ICSE Board Students.
Selina Concise Physics Solutions for Class 7th Chapter 5 Heat with easy language, expert answers and doubt clearing sections. The book is especially designed keeping in mind the syllabus of ICSE board and the difficulty level faced by the students of ICSE boards class 7.
Selina Class 7 Physics Chapter 5 Solution:
1.) Write true or false for each statement :
a) On touching a lump of ice, we feel cold because some heat passes from our body to the ice.
Answer: True.
The temperature of our body is greater than lump of ice. Therefore heat transfer from low temperature to high temperature.
b) Heat flows from a body at a high temperature to a body at a low temperature when they are kept in contact.
Answer: True.
Heat always flow transfer from low temperature to high temperature.
c) All solids expand by the same amount when heated to the same rise in temperature.
Answer: False.
Every solid expand with different rate when heated.
d) Telephone wires are kept tight between two poles in summer.
Answer: False.
Telephone wires are do not kept tight between two poles in summer.
e) Equal volumes of different liquids expand by different amounts when they are heated to the same rise in temperature.
f) Solids expand the least and gases expand the most on being heated.
Answer: True.
The molecules vibrates faster in gases than solid. If we heat solid and gases then gases expand faster than solid.
g) A mercury thermometer makes use of the property of expansion of liquids on heating.
Answer: True.
Mercury expands on heating so it indicates temperature.
h) Kerosene contracts on heating.
Answer: False. Kerosene expands on heating.
i) Water is a bad conductor of heat.
Answer: True.
Water does not allow to flow heat.
j) Medium is necessary for the transfer of heat by radiation.
Answer: False.
The heat reach from sun to earth by radiation but medium is absent in interstellar space.
k) Land and sea breezes are convection currents of cold and warm air.
Answer: True.
l) Liquids are heated by conduction and radiation.
Answer: False.
m) Black surfaces are poor absorbers of heat radiations.
Answer: False.
Black surfaces absorb maximum heat radiations than other colour.
2.) Fill in the blanks :
Answer:
a) Heat is a form of Energy.
b) Temperature determines the degree of hotness or coldness of a body.
c) On heating a body, its temperature rises.
d) We use a thermometer for measuring the temperature of a body.
e) Thee S.I. unit of temperature is kelvin.
f) In a thermometer, the commonly used liquid is mercury.
g) The temperature of a normal human body is37⁰C.
h) A person is said to have fever if his body temperature is more than 98.6⁰ F
I) A hot metallic piece is placed in tap water contained in a bucket. Heat will flow from metallic
j) The temperature of boiling water is100 ⁰ C.
k) Liquids expand more than solids.
l) Gases expand more than liquids.
m) Heat transfer in solids is by conduction.
n) Heat transfer in liquids and gases is by convection.
o) Metals are conductors of heat.
p) Still air is an insulator heat.
q) Black and dull surfaces are good absorbers of heat
3.) Match the following :
Answer:
Column A Column B
(a) mercury (v) thermometer
(b) wood (i) insulator
(c) aluminum (iv) conductor
(d) contracts (ii) water from O⁰C to 4⁰ C
(e) black surface (iii) absorbs
4.) a) If we add a lump of ice to a tumbler containing water :
(i) heat flows from water to ice
(ii) heat flows from ice to water
(iii) heat flows from water to ice if water is more
(iv) heat flows from ice to water if ice is more.
Answer: (i) heat flows from water to ice
The temperature of water is greater than ice and we know that heat flows from higher temperature to lower.
b) The temperature of pure melting ice is :
(i) 0 ⁰C
(ii) 100⁰ C
(iii) 95⁰C
(iv) 98.60F
Answer: (i) 0 ⁰C
c) A thermometer uses :
(i) water (ii) mercury
(iii) air (iv) none of the above
Answer: (ii) mercury.
Mercury is a good conductor of heat.
d) Which statement is correct :
(i) Iron rims are cooled before they are placed on cart wheels
(ii) A glass stopper gets tight on warming the neck of the bottle
(iii) Telephone wires sag in winter, but become tight in summer
(iv) A little space is left between two rails on a railway track.
Answer: (iv) A little space is left between two rails on a railway track.
The railway track expands in summer due to temperature. Thus this space is filled. If we do not put little space between two rails on railway track then the railway track does not lie in a straight line when expands it because of temperature in summer. To avoid this Bending of track in summer, we must put a little space between two rails on a railway track.
e) Heat in a liquid is transferred by :
(i) conduction
(ii) convection
(iii) radiation
(iv) conduction and radiation
Answer: (ii) convection
The actual movement of molecules takes place in liquid while transferring heat.
f) In the process of convection, heat travels :
(i) sideways
(ii) downwards
(iii) upwards
(iv) in all directions.
Answer: (iii) upwards
The density of hot air is less than cool air. Thus hot air moves from lower to high and transfer of heat takes place.
g) The vacuum kept in between the walls of a thermos flask reduces the heat transfer by :
(i) conduction only
(ii) convection only
(iii) radiation only
(iv) conduction and convection.
Answer: (iv)conduction and convection.
B.) 1) What is heat ? State its S.I. unit.
Answer: Heat is the form of energy which flows between two bodies having different temperature.
Heat has unit similar to energy. The SI unit of heat is Joule. Heat is also measured in calories, kilo calories etc.
2) What is meant by the term. ‘temperature’ ?
Answer: The physical quantity gives us the magnitude of coldness or hotness of the body is called temperature.
3) State the three units of temperature.
Answer: Units of temperature-
i) Kelvin (K)
ii) Degree Celsius (⁰C)
iii. Degree Fahrenheit (⁰F)
4) Name the instrument used to measure the temperature of a body.
Answer: We can measure the temperature of body using thermometer.
5) What is the Celsius scale of temperature ?
Answer: The Swedish astronomer Celsius finds out Celsius scale. On this scale, temperature of ice is fixed on 0⁰C and steam of water is fixed at 100⁰ C.
The space between 0⁰ and 100⁰ is numbered equally from 0 to 100.
6) What is the Fahrenheit scale of temperature ?
Answer: The scientist Fahrenheit finds out Fahrenheit scale. On this scale, temperature of ice is fixed on 32⁰ F and steam of water is fixed at 212⁰ F.
The space between 32⁰ F and 212⁰ F is divided into 180 equal parts. (212-32 = 180).
7) What is the Kelvin scale of ?
Answer: Kelvin is the SI unit of temperature. It is introduced by a scientist Kelvin.
The possible lowest temperature is 0⁰ K.
On this scale, temperature of ice is fixed on 273⁰ K and steam of water is fixed at 373⁰ K.
8) How are the Celsius and Fahrenheit scales inter related ?
Answer: We know that temperature of ice is 0⁰C or 32⁰ F and temperature of ice is 100⁰C or 212⁰F.
The relationship between Celsius and Fahrenheit is as,
⁰F = 9/5 ⁰C + 32.
9) How is the size of a degree defined on a Celsius scale ?
Answer: The lowest point on Celsius scale is at 0⁰ C and upper point is fixed at 100⁰ C. The values of temperature varies from 0⁰ C to 100⁰ C.
Thus the size of a degree on Celsius scale is 1/100.
10) How is the size of a degree defined on a Fahrenheit scale ?
Answer:
The lowest point on Fahrenheit scale is at 32⁰ F and upper point is fixed at 212⁰F. The values of temperature varies from 32⁰ F to 212⁰F.
Thus the size of a degree on Celsius scale is 1/180.
11) State the temperature of
(i) ice point and (ii) steam point, on the Celsius scale.
Answer:
i.) The temperature of ice point on Celsius scale is 0⁰ C.
ii.) The temperature of steam on Celsius scale is 100⁰C.
12) Write down the temperature of (i) lower fixed point, and (ii) upper fixed point, on the Fahrenheit scale.
Answer:
(i) The lowest temperature on Fahrenheit scale is freezing point of water or simply temperature of ice. Thus lower fixed point on Fahrenheit scale is 32⁰ F.
(ii) The upper fixed point on Fahrenheit scale is fixed at boiling point of water. The upper fixed point is 212⁰ F.
13) The Fig. 5.22 shows a glass tumbler containing hot milk which is placed in a tub of cold water. State the direction in which heat will flow.
Answer: We know that beat will from hot body to cold body. The upper side is hot as compared to lower side. Thus heat will flow vertically downwards.
14) Draw a neat labelled diagram of a laboratory thermometer.
Answer: The diagram of laboratory thermometer is as,
15) Write down the body temperature of a healthy person.
Answer: The body temperature of a healthy normal person is 36⁰ C.
16) What do you understand by thermal expansion of a substance ?
Answer: Molecules of the substance vibrates about its mean position when we supply heat to that substance. Due to these vibrations the actual size of the substance increases and we say that substance expand when we supply heat to it.
17) Name two substances which expand on heating.
Answer: Two substances which expand on heating are-
Iron rod, ice bar, gold chain.
18) Why do telephone wires sag in summer ?
Answer: We know that solids expand when we supply heat. The temperature increases on summer so the linear expansion takes place in wire results it sag.
19) Iron rims are heated before they are fixed on the wooden wheels. Explain the reason.
Answer: We know that solids expand when heated and contract when cooled. We have to fix Iron rims on the wooden wheels tightly. Firstly we heat up it so it expands and then fix this rim on a wooden wheel. Lastly the rims cooled and fixed tightly on wheel because of contracting.
20) Why are gaps left between successive rails on a railway track ?
Answer: We know that solids expand when heated. If we put gaps between rails on racetrack then this gaps fills up in summer becomes thermal expansion of solid in summer.
21) A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle. Explain why ?
Answer: We know that, temperature changes the shape of the substance. When we pour hot water on the neck of bottle then it expand the area of neck of bottle because of thermal expansion. Therefore we can easily remove it.
22) Why is a cement floor laid in small pieces with gaps in between ?
Answer: We know that, solid expands due to heat. A cement floor laid in small pieces with gaps in between provides sufficient expansions due to heat.
23) One end of a steel girder in a bridge is not fixed, but is kept on rollers. Give the reason.
Answer: The free end of girder in a bridge can allows to move due to thermal expansion.
24) Describe one experiment to show that liquids expand on heating.
Answer:
Mercury thermometer is good example to show that liquids expand on heating. When the atmospheric temperature increases then the mercury in thermometer expands and gives the magnitude of temperature. There is linear relationship between temperature and expansion of liquids.
25) State one application of thermal expansion of liquids.
Answer: The most common application of thermal expansion of liquid is mercury thermometer.
As we know that, liquid expands because of heat. Thus mercury indicates the temperature of surrounding.
26) Describe an experiment to show that air expands on heating.
Answer- Air expands on heating. We can understand this by performing a simple experiment.
Aim- Air expands on heating.
Apparatus -rubber Balloon, hot water, beaker, empty bottle.
Procedure-
- Take a empty bottle.
- Attach a Balloon on the tip of bottle.
- Put it in hot water beaker.
- Wait for some time till Balloon inflated.
Observations and Conclusion-
Air expands and strike on the wall of Balloon therefore Balloon inflated.
27) An empty glass bottle is fitted with a narrow tube at its mouth. The open end of the tube is kept in a beaker containing water. When the bottle is heated, bubbles of air are seen escaping into the water. Explain the reason.
Answer: We know that, air expands on heating. This heated air try to escape into the bottle because air expands when heated.
28) State which expands more, when heated to the same temperature : solid, liquid or gas?
Answer: When we heated solid, liquids and gas to the same temperature then gas expands most than liquid or solid. The atoms of gases are very far from each other thus they separate by gaining temperature.
29) Name the three modes of transfer of heat.
Answer: Three modes of transfer of heat are
- Conduction
- Convection
- Radiation
30) Name the mode of transfer of heat in the following :
(a) solid. (b) liquid, (c) gas, (d) vacuum
Answer:
a) Solid- mode of transfer of heat in solid can be done by conduction and convection.
b) Liquid- mode of transfer of heat in the liquid can be done by conduction and convection.
c) Gas- Transfer of heat in gas can be done by radiation only.
d) Vacuum- Transfer of heat in vacuum can be done by radiation only.
31) What are good and bad conductors of heat ? Give two examples of each.
Answer:
Good conductors of Heat- The substance which allows heat transfer through it are said be good conductor of heat. Good conductor acts as a medium for conduction of heat. Mostly good conductors are metals only.
Examples- iron, Nickel, aluminum, etc.
Bad conductors- The substance which does not allow heat transfer through it are said be bad conductors. Good conductors blocks heat transfer. Mostly bad conductors are non metals.
Example- wood, paper, glass, etc
32) Name a liquid which is a good conductor of heat
Answer: Mercury is example of good conductor of heat in liquid.
33) Name a solid which is a good conductor of heat.
Answer: copper is a good conductor of heat.
34) Select good and bad conductors of heat from following :
copper, mercury, wood, iron, air, saw-dust, cardboard, silver, plastic, wool.
Answer:
Good conductors: copper, mercury, silver, iron.
Bad conductors: wood, air, saw-dust, cardboard, plastic, wool.
35) Why is an oven made of double walls with the space in between filled with cork ?
Answer: The main function of oven is that the heat can not go outside the oven. And we know that cork is a bad conductor of heat so heat can not goes through the cork. Thus outside layer of oven does not heat up and avoid heat loss.
36) Why do we use cooking utensils made up of copper ?
Answer: Copper is a good conductor of heat. It can easily transfer heat through it and helps to cook food.
37) Why is a tea kettle provided with an ebonite handle ?
Answer: We know that, ebonite is a bad conductor of heat. It does not allow heat so it prevent us from burning.
38) In summer, ice is kept wrapped in a gunny bag. Explain the reason.
Answer: Gunny bag is a bad conductor of heat. It does not pass heat from surrounding to ice. So we use gunny bag to store ice. Thus ice does not melt.
39) Explain why
(a) we wear woolen clothes in winter ?
(b) the water pipes are covered with cotton during very cold weather ?
Answer:
(a) Woolen cloth has small holes and it acts as a bad conductors of heat. It does not allow to flow heat from surrounding to our skin. So we protect from cold.
(b) Cotton is a bad conductor of heat because of small gaps in it. Therefore it does not allow heat to transfer from surrounding to water in pipes.
40) Why are quilts filled with fluffy cotton ?
Answer: Quits has a tiny holes in it. That tiny holes acts as bad conductors of heat. Therefore it does not allow to pass heat from our body to floor. In this way Quits prevents us from cold.
41) State the direction of heat transfer by way of convection.
Answer: The direction of heat transfer by the way of convection is from ground to vertically upwards.
42) Why is a ventilator provided in a room ?
Answer: When we heat up gases, they expands and the density of that gas decreases. The expansion of air takes place in room because of breathing. This air goes vertically upwards. The fresh, more dense air comes in room through windows and low dense air goes out through ventilator.
Ventilator maintains the temperature of room normal. Thus ventilator provided in a room.
43) Why are chimneys provided over furnaces in factories ?
Answer: The smoke provides in factories has low density also it contains harmful gases. The smoke in factories goes vertically upwards through chimneys. It does not spread in factory.
- What are land and sea breezes ? Explain their formation.
Answer:
- Land breezes- we know that, land cools fast than the sea at night. The air becomes more hotter at the surface of sea than land. The density of this hot air is less. This low dense air on surface of sea goes vertically upwards. More dense air on surface of land flows towards sea surface to fill the gap. This motion of air from land surface to sea surface at night is called as Land breezes.
- Sea breezes- similarly land becomes more warm than sea surface at daytime. Therefore the density of air on land surface becomes less dense than air of sea surface. This this low dense air goes upwards. To fill this gap, fresh air flows towards land surface. This flow of air from sea surface to land surface is called sea breezes.
45) Why is the freezing chest in a refrigerator fitted near its top ?
Answer: The density of freezing air is more. It cools other part by convection. The remaining hot which lies at bottom also cools by convection.
Freezing chest in a refrigerator fitted near its top.
46) Explain briefly the process of heat transfer by radiation.
Answer:
Heat transfer by radiation-
- The heat transfer by radiation is the fastest way to transfer heat.
- Heat can be transfer from hot body to cold body which put far from each other.
- This thermal radiations can be travels in vacuum.
- No medium is required for propagation of heat radiations.
- When we sit on sunlight, we feel warm because the temperature of sun is large than us. The heat transfer takes place from sun by radiation.
47) Give one example of heat transfer by radiation.
Answer:
Heat transfer by radiation- When we sit on sunlight, we feel warm because the temperature of sun is large than us. The heat transfer takes place from sun by radiation.
48) Why do we prefer to wear white or light coloured clothes in summer and black or dark coloured clothes in winter ?
Answer:
- White colour reflects all the wavelength (colours) in lights. Thus white coloured cloths protect us from sunlight in summer.
- Black or dark colors absorbs all type of wavelengths (colours). We need maximum sunlight in winter. Thus black colours absorbs heat from sunlight.
49) The bottom of a cooking utensil is painted black. Give the reason.
Answer: We know that, black coloured surface absorbs maximum heat than other coloured surfaces. Cooking utensil absorbs maximum heat due to painted black. Thus the bottom of a cooking utensil is painted black.
50) Draw a labelled diagram of a thermos flask. Explain how the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.
Answer
The diagram Of thermos flask is as shown in figure
The neat labelled diagram of thermos flask is as shown in figure below.
Thermos flask is mainly used to keep the hot water hot and cold water cold for long duration of time and which is not possible in case of other utensils.
We know that, when we put hot tea in a cup it will be get cold after sometime as heat is transferred from high temperature to low temperature. Here, heat is transferred from cup of tea to the surrounding. So we can say that, it is the open system.
But, thermos flask is an isolated system, in which no heat transfer takes place and hence temperature of the system is maintained as it is.
As we already know that, heat transfer takes place through conduction, convection and radiation.
Conduction is the process of heat transfer in which heat energy is transferred from high temperature region to low temperature region without actual transfer of molecules of the substance.
Convection is the process of transfer of heat from high temperature region to low temperature region with the actual migration of particles of the medium.
And radiation is also the process of transfer of heat which does not require any material medium like solar radiation comes to earth through space.
In case of thermos flask, the vacuum present in between the walls of the bottle helps in preventing the heat by the process of conduction and convection.
While the heat transfer through the process of radiation is reduced due to the shining surface on the outer side of the inner wall and inner side of the outer wall.
Also due to the presence of cork stopper the heat transfer through convection is reduced.
In this way, in thermos flask the heat transferred from inside to outside or outside to inside is in the very small amount. Due to which the temperature of the substance or liquid present inside the thermos flask is maintained.
Thus, the cold substance remains cold and hot substance remains hot for a long time of duration.
C.) Numericals:
1) The temperature of a body rises by 1⁰ C. What is the corresponding rise on the (a) Fahrenheit scale (b) Kelvin scale?
Answer: Given,
Rise in temperature = 1⁰ C.
- We know that, the interval between ice point and boiling point of water is divided into 180 equal parts. Thus each scale having magnitude of 1.8 .
Therefore the temperature rise in Fahrenheit scale is 1.8⁰ F.
- We know that, the Kelvin scale varies same with Celsius scale. Thus the rise in temperature also 1⁰ K in Kelvin scale.
2) The temperature rises by 18⁰ F. What is the rise on the Celsius scale ?
Answer:
We know that, the interval between ice point and boiling point of water is divided into 180 equal parts. Thus each scale having magnitude of 1.8 .
Therefore
1⁰ C = 1.8⁰ F
1⁰ F = 1/1.8 ⁰C.
Given, rise in temperature = 18⁰ F.
18⁰ F = (1/1.8 ×18)⁰ C.
18⁰ F = 10⁰ C.
Thus the rise in temperature is 10⁰ C.
3) Convert 5⁰ F to the Celsius scale.
Answer: Given, Temperature = 5⁰ F
we know the relationship between ⁰F and ⁰C.
⁰C/5 = (F – 32)/ 9
Put given value of temperature in above equation.
⁰C/5 = (5 – 32)/ 9
⁰C/5 = (-27)/ 9
C = -3 × 5
C = -15⁰ C.
Therefore,
5⁰ F = 15⁰ F.
4) Convert 40⁰C to the (a) Fahrenheit scale (b) Kelvin Scale.
Answer: Given,
Temperature = 40⁰ C.
a) We know that the relationship between Fahrenheit and Celsius.
⁰F = 9/5 ⁰ C + 32
Put the value of temperature in above equation.
⁰F = 9/5 ×40 + 32
⁰F = 72 + 32
⁰ F = 104
Therefore,
40⁰ C = 104⁰ F.
b) We know that the relationship between Kelvin and Celsius.
t⁰ C = (273 + t) K
Put t = 40.
40⁰ C = (273 + 40) K
40⁰ C = 313⁰ K.
5) Convert 40⁰ F to the Celsius scale.
Answer:
Given, Temperature = -40⁰ F
we know the relationship between ⁰F and ⁰C.
⁰C/5 = (F – 32)/ 9
Put given value of temperature in above equation.
⁰C/5 = (-40 – 32)/ 9
⁰C/5 = (-72)/ 9
C = -8× 5
C = -40⁰ C.
Therefore,
-40⁰ F =-40⁰ C