Heat

Objective Questions

Question 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.

(b) Heat flows from a body at a high temperature to a body at a low temperature when they are kept in contact.

(c) All solids expand by the same amount when heated to the same rise in temperature.

(d) Telephone wires are 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.

(g) A mercury thermometer makes use of the property of expansion of liquids on heating.

(h) Kerosene contracts on heating.

(i) Water is a bad conductor of heat.

(j) Medium is necessary for the transfer of heat by radiation.

(k) Land and sea breezes are convection currents of cold and warm air.

(l) Liquids are heated by conduction and radiation.

(m) Black surfaces are poor absorbers of heat radiations.

Answer

(a) True

(b) True

(c) False
Correct Statement — All solids expand by the different amount when heated to the same rise in temperature.

(d) False
Correct Statement — Telephone wires are kept loose between two poles in summer.

(e) True

(f) True

(g) True

(h) False
Correct Statement — Water contracts on heating from 0°C to 4°C.

(i) True

(j) False
Correct Statement — Medium is not necessary for the transfer of heat by radiation.

(k) True

(l) False
Correct Statement — Liquids are heated by conduction and convection.

(m) False
Correct Statement — Black surfaces are good absorbers of heat.

Question 2

Fill in the blanks:

(a) Heat is a form of ............... .

(b) ............... determines the degree of hotness or coldness of a body.

(c) On heating a body, its temperature ............... .

(d) We use a ............... for measuring the temperature of a body.

(e) The S.I. unit of temperature is ............... .

(f) In a thermometer, the commonly used liquid is ............... .

(g) The temperature of a normal human body is ............... °C.

(h) A person is said to have fever if his body temperature is more than ............... °F.

(i) A hot metallic piece is placed in tap water contained in a bucket. Heat will flow from ............... to ............... .

(j) The temperature of boiling water is ............... .

(k) Liquids expand ............... than solids.

(l) Gases expand ............... than liquids.

(m) Heat transfer in solids is by ............... .

(n) Heat transfer in liquids and gases is by ............... .

(o) Metals are ............... of heat.

(p) Still air is an ............... of heat.

(q) Black and dull surfaces are ............... of heat.

Answer

(a) energy

(b) temperature

(c) rises

(d) thermometer

(e) kelvin

(f) mercury

(g) 37

(h) 98.6

(i) metallic piece, water

(j) 100°C

(k) more

(l) more

(m) conduction

(n) convection

(o) conductors

(p) insulator

(q) good absorbers

Question 3

Match the following:

Answer

Question 4a

If we add a lump of ice to a tumbler containing water:

1. heat flows from water to ice
2. heat flows from ice to water
3. heat flows from water to ice if water is more
4. heat flows from ice to water if ice is more

Answer

heat flows from water to ice

Reason — Heat flows from a hot body to cold body when they are kept in contact.

Question 4b

The temperature of pure melting ice is:

1. 0°C
2. 100°C
3. 95°C
4. 98.6°F

Answer

0°C

Reason — The temperature of pure melting ice is 0°C.

Question 4c

A thermometer uses:

1. water
2. mercury
3. air
4. none of the above

Answer

mercury

Reason — Mercury is used in a thermometer.

Question 4d

Which statement is correct:

1. Iron rims are cooled before they are placed on cart wheels.
2. A glass stopper gets tight on warming the neck of the bottle.
3. Telephone wires sag in winter, but becomes tight in summer.
4. A little space is left between two rails on a railway track.

Answer

A little space is left between two rails on a railway track

Reason — A little space is left between two rails on a railway track to allow expansion of rails during summer otherwise rails will bend sideways.

Question 4e

Heat in a liquid is transferred by:

1. conduction
2. convection
3. radiation
4. conduction and radiation

Answer

convection

Reason — Heat in a liquid is mainly transferred by convection.

Question 4f

In the process of convection, heat travels:

1. sideways
2. downwards
3. upwards
4. in all directions

Answer

upwards

Reason — In convection the transfer of heat is always vertically upwards.

Question 4g

The vacuum kept in between the walls of a thermos flask reduces the heat transfer by:

1. conduction only
2. convection only
3. radiation only
4. conduction and convection

Answer

conduction and convection

Reason — The vacuum kept in between the walls of a thermos flask reduces the heat transfer by conduction and convection.

Short/Long Answer Questions

Question 1

What is heat? State its S.I. unit.

Answer

Heat is a form of energy which flows from a hot body to a cold body when they are kept in contact. This process of heat transfer continues till the temperature of both the bodies becomes equal. The S.I. unit of heat is joule(J).

Question 2

What is meant by the term temperature?

Answer

Temperature is a quantity which tells us the degree of hotness or coldness of a body. When a body is heated its temperature rises and when it is cooled its temperature falls.

Question 3

State the three units of temperature.

Answer

The three units of temperature are degree Celsius (°C), degree Fahrenheit (°F) and Kelvin (K).

Question 4

Name the instrument used to measure the temperature of a body.

Answer

Thermometer is used to measure the temperature of a body.

Question 5

What is the Celsius scale of temperature?

Answer

On the Celsius scale the ice point is 0°C and the steam point is 100°C. The interval between the ice point and steam point is divided into 100 equal parts.

Question 6

What is the Fahrenheit scale of temperature?

Answer

On the Fahrenheit scale the ice point is 32°F and the steam point is 212°F. The interval between the ice point and steam point is divided into 180 equal parts.

Question 7

What is the Kelvin scale of temperature?

Answer

On Kelvin scale ice point is marked as 273 K and steam point is marked as 373 K. The interval between the ice point and steam point is divided into 100 equal parts. 0 K is the lowest possible temperature on this scale when no molecular motion can occur.

Question 8

How are the Celsius and Fahrenheit scales inter-related?

Answer

°C = $\dfrac{\text{5}}{\text{9}}$ (°F - 32)

Question 9

How is the size of a degree defined on a Celsius scale?

Answer

The interval between the ice point and steam point is divided into 100 equal parts and each division is called a degree on a Celsius scale (1°C).

Question 10

How is the size of a degree defined on a Fahrenheit scale?

Answer

The interval between the ice point and steam point is divided into 180 equal parts and each division is called a degree on a Fahrenheit scale (1°F).

Question 11

State the temperature of:

(i) ice point

(ii) steam point, on the Celsius scale.

Answer

(i) On the Celsius scale the ice point is 0°C.

(ii) On the Celsius scale the steam point is 100°C.

Question 12

Write down the temperature of:

(i) lower fixed point

(ii) upper fixed point, on the Fahrenheit scale.

Answer

(i) On the Fahrenheit scale, lower fixed point is 32°F.

(ii) On the Fahrenheit scale, upper fixed point is 212°F.

Question 13

The figure 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

The heat will flow from hot milk glass tumbler to tub of cold water.

Question 14

Draw a neat labelled diagram of a laboratory thermometer.

Answer

Labelled diagram of a laboratory thermometer is shown below:

Question 15

Write down the body temperature of a healthy person?

Answer

The body temperature of a healthy person is 37°C or 98.6°F.

Question 16

What do you understand by thermal expansion of a substance?

Answer

The expansion of a substance when heated is called thermal expansion.

Question 17

Name two substances which expand on heating.

Answer

Iron and brass.

Question 18

Why do telephone wires sag in summer?

Answer

Telephone wires sag in summer because they expand due to rise in temperature during summer.

Question 19

Iron rims are heated before they are fixed on the wooden wheels. Explain the reason.

Answer

The iron rims are made slightly smaller in diameter than the wheel. So they are heated because iron expands and is slipped over the wheel easily, so that on cooling it contracts and makes a tight fit on the wheel.

Question 20

Why are gaps left between successive rails on a railway track?

Answer

A little gap is left between successive rails on a railway track to allow expansion of rails during summer otherwise rails will bend sideways.

Question 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

When hot water is poured on the neck of the bottle, the neck expands and the glass stopper stuck in it gets space to loosen and then can be easily removed.

Question 22

Why is a cement floor laid in small pieces with gaps in between?

Answer

In summer when temperature rises cement expand. So a cement floor is laid in small pieces with gaps in between to allow expansion in summer and contraction in winter, otherwise the cement floor will crack.

Question 23

One end of a steel girder in a bridge is not fixed, but is kept on rollers. Give the reason.

Answer

One end of a steel girder in a bridge is kept on rollers instead of fixing it in a pillar because in summer or in winter when there is rise or fall in temperature, the girder may expand or contract freely without affecting the pillar or bridge.

Question 24

Describe one experiment to show that liquids expand on heating.

Answer

The following experiment demonstrates that liquids expand on heating:

1. Take an empty bottle with a tight fitting cork with a hole drilled in its middle, a drinking straw, two bricks, a wire gauze and a burner.
2. Fill the bottle completely with water. Add few drops of ink to make it coloured.
3. Fix the cork in the mouth of the bottle. Pass the drinking straw through the cork. Put some molten wax around the hole to avoid leakage of water.
4. Pour some more water in the drinking straw so that water level in the straw can be seen. Mark the water level in the straw as shown in figure.
5. Place the bottle on the wire gauze kept over the two bricks. Heat the bottle using a burner as shown in figure.
6. Look at the level of water in the straw.

Conclusion — When water is heated more and more, the level of water in the drinking straw rises. This shows that water expands on heating.

Question 25

State one application of thermal expansion of liquids.

Answer

Thermal expansion of liquids is used in the working of mercury thermometer.

Question 26

Describe an experiment to show that air expands on heating.

Answer

The following experiment demonstrates that air expands on heating:

1. Take an empty bottle. The empty bottle contains air.
2. Attach a rubber balloon to its neck as shown in figure.
3. The balloon is deflated initially.
4. Place the bottle in a water bath containing boiling water. It heats the air present in bottle.
5. After sometime it is noticed that the balloon gets inflated as shown in figure.

Conclusion — This shows that on heating, air present inside bottle expands and fills the balloon. So the balloon gets inflated. This shows that air expands on heating.

Question 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

The empty glass bottle is filled with air. When the bottle is heated, the air inside the bottle expands and it escapes out through the tube into water which is seen in the form of bubbles.

Question 28

State which expands more, when heated to the same temperature: solid, liquid or gas?

Answer

Gas will expand more as compared to solid and liquid when heated to the same temperature.

Question 29

Name the three modes of transfer of heat.

Answer

The three modes of transfer of heat are:

1. Conduction
2. Convection
3. Radiation

Question 30

Name the mode of transfer of heat in the following:

(a) solid

(b) liquid

(c) gas

(d) vacuum.

Answer

(a) conduction

(b) conduction and convection

(c) conduction and convection

(d) radiation

Question 31

What are good and bad conductors of heat? Give two examples of each.

Answer

The substances which allow heat to pass through them easily are called good conductors of heat. Examples — silver, copper.

The substances which do not allow heat to pass through them easily are called bad conductors of heat or insulators. Examples — wood, plastic.

Question 32

Name a liquid which is a good conductor of heat.

Answer

Mercury

Question 33

Name a solid which is a good conductor of heat.

Answer

Copper

Question 34

Select good and bad conductors of heat from the following:

copper, mercury, wood, iron, air, saw-dust, cardboard, silver, plastic, wool.

Answer

Question 35

Why is an oven made of double walls with the space in between filled with cork?

Answer

An oven is made of double walls with the space in between filled with cork because cork is insulator of heat and prevent the heat of the oven to escape.

Question 36

Why do we use cooking utensils made up of copper?

Answer

We use cooking utensils made up of copper because it is a very good conductor of heat and gets heated up rapidly.

Question 37

Why is a tea kettle provided with an ebonite handle?

Answer

Ebonite is an insulator of heat. It does not pass heat from the tea kettle to our hand, so we can hold the tea kettle comfortably by the handle. Therefore, the tea kettle is provided with an ebonite handle.

Question 38

In summer, ice is kept wrapped in a gunny bag. Explain the reason.

Answer

The air filled in the fine pores of the gunny bag is insulator of heat. The air does not allow heat from outside to pass through it to the ice thus preventing the ice from melting rapidly. So in summer, ice is kept wrapped in a gunny bag.

Question 39

Explain why:

(a) we wear woollen clothes in winter?

(b) the water pipes are covered with cotton during very cold weather?

Answer

(a) Woollen clothes have fine pores which are filled with air and both wool and air are insulators of heat. They do not allow the heat to escape through them and keep us warm. So we wear woollen clothes in winter.

(b) Cotton has air trapped in its fine pores. Both cotton and air are insulators of heat which do not pass heat from the water inside the pipes to the outside atmosphere and prevents the water from freezing. So the water pipes are covered with cotton during very cold weather

Question 40

Why are quilts filled with fluffy cotton?

Answer

Cotton has air trapped in its fine pores. Both cotton and air are insulators of heat which prevent heat from our body to escape and thus keep us warm. So quilts are filled with fluffy cotton.

Question 41

State the direction of heat transfer by way of convection.

Answer

The transfer of heat is always vertically upwards in the process of convection.

Question 42

Why is a ventilator provided in a room?

Answer

Ventilator is provided in a room for proper ventilation because when we breathe out in a room it becomes warm and air inside the room becomes impure. The warm air is less dense and rises up and moves out through the ventilator. So cold, fresh air comes inside the room through windows to take its place. Thus, the continuous circulation of fresh air keeps the air in the room fresh.

Question 43

Why are chimneys provided over furnaces in factories?

Answer

Chimneys are provided over furnaces in factories because the hot gases are less dense and rises up and comes out through the chimneys. Smoke, fumes etc. around the furnace rush in to take their place and they are sucked out. Thus chimneys remove undesired smoke, fumes from the premises.

Question 44

What are land and sea breezes? Explain their formation.

Answer

During summer in places near the sea a breeze blows from land to sea during night which is called land breeze and during the day time a breeze blows from sea to land known as sea breeze. They are formed by the process of convection.

Sea breeze — During day time, land becomes more warmer than sea. So air above the land is warmer, less dense and rises up. Cold air from the sea blows towards the land to take its place. This is called sea breeze.

Land breeze — During night, land cools much faster than sea. So sea water is warmer than land and air above sea is also warmer. It rises up being less dense and cold wind from land blows towards sea to take its place. This is called land breeze.

Question 45

Why is the freezing chest in a refrigerator fitted near its top?

Answer

The freezing chest in a refrigerator is fitted near its top because air in the top when contact with the freezing chest gets cooled and become denser and descends down. The hot air from lower part rises up thus convection currents are formed.The cold air coming down from the cooling chest cools the entire space along with the things present in the refrigerator.

Question 46

Explain briefly the process of heat transfer by radiation.

Answer

Radiation is the process of heat transfer in which heat directly passes from one body at high temperature to another body at low temperature without affecting the medium. In vacuum heat transfer occurs by radiation process. We get heat from the sun by radiation.

Question 47

Give one example of heat transfer by radiation.

Answer

We feel warm when we sit near fire. Heat from fire reaches us by radiation process.

Question 48

Why do we prefer to wear white or light coloured clothes in summer and black or dark coloured clothes in winter?

Answer

We prefer to wear white or light coloured clothes in summer because white clothes reflect most of sun's heat and absorb very little of it, thus keep our body cool. But we wear black or dark coloured clothes in winter because they absorb most of the heat of sun and keep us warm.

Question 49

The bottom of a cooking utensil is painted black. Give the reason.

Answer

Black surface absorbs more heat and the contents inside cooking utensil gets cooked rapidly, so the bottom of a cooking utensil is painted black.

Question 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

Labelled diagram of a thermos flask is shown below:

The vacuum present between the walls of the bottle prevents transfer of heat by conduction and convection. The shining surface on the outer side of the inner wall and inner side of the outer wall reduces the transfer of heat by radiation. The cork stopper prevents the transfer of heat by convection. In this way the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.

Numericals

Question 1

The temperature of a body rises by 1°C. What is the corresponding rise on the:

(a) Fahrenheit scale

(b) Kelvin scale?

Answer

(a) 100 divisions on the Celsius scale = 180 divisions on Fahrenheit scale

1 division on Celsius scale = $\dfrac{\text{180}}{\text{100}}$ = 1.8 divisions on Fahrenheit scale

So the corresponding rise on the Fahrenheit scale = 1.8°F.

(b) 100 divisions on the Celsius scale = 100 divisions on Kelvin scale

1 division on Celsius scale = $\dfrac{\text{100}}{\text{100}}$ = 1 division on Kelvin scale

So the corresponding rise on the Kelvin scale = 1 K.

Question 2

The temperature rises by 18°F. What is the rise on the Celsius scale?

Answer

180 divisions on the Fahrenheit scale = 100 divisions on Celsius scale

1 division on Fahrenheit scale = $\dfrac{\text{100}}{\text{180}}$ divisions on Celsius scale

18 divisions on Fahrenheit scale = $\dfrac{\text{100}}{\text{180}}$ x 18 =10 divisions on Celsius scale

So the corresponding rise on the Celsius scale = 10°C.

Question 3

Convert 5°F to the Celsius scale.

Answer

°C = $\dfrac{\text{5}}{\text{9}}$ (F - 32)

= $\dfrac{\text{5}}{\text{9}}$ (5 - 32)

= $\dfrac{\text{5}}{\text{9}}$ x (-27)

= - 15

So 5°F = -15°C.

Question 4

Convert 40°C to the:

(a) Fahrenheit scale

(b) Kelvin scale

Answer

(a) °F = $\dfrac{\text{9}}{\text{5}}$ C + 32

= $\dfrac{\text{9}}{\text{5}}$ x 40 + 32

= 9 x 8 + 32

= 72 + 32

= 104

So 40°C = 104°F.

(b) t°C = (273 + t) K

= 273 + 40

= 313 K

So 40°C = 313 K.

Question 5

Convert -40°F to the Celsius scale.

Answer

°C = $\dfrac{\text{5}}{\text{9}}$ (F - 32)

= $\dfrac{\text{5}}{\text{9}}$ (-40 - 32)

= $\dfrac{\text{5}}{\text{9}}$ x (-72)

= - 40

So -40°F = -40°C.