# DAV Class 7 Science Chapter 5 Question Answer – Heat

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## DAV Class 7 Science Ch 5 Question Answer – Heat

### DAV Class 7 Science Ch 5 Solutions – Heat

Something To Know

A. Fill in the blanks.

Question 1.
Heat is a form of ________
Energy

Question 2.
The range of clinical thermometer is from ________°C to ________ °C.
35°C to 42°C.

Question 3.
Water is a ________ conductor of heat.
Poor

Question 4.
Land and sea breezes help to maintain the ________ of air.
Temperature

Question 5.
Transfer of heat from the sun to earth usually takes place by ________

B. Write True or False for the following statements.

1. Heat cannot be produced by doing work.
2. No gaps are left between the rails of railway tracks.
3. Poor conductors of heat are also good insulators of heat.
4. Ventilators, when provided in rooms, are located near their roofs.
5. A material medium is required for transfer of heat by the process of radiation.
1. Heat cannot be produced by doing work. (False)
2. No gaps are left between the rails of railway tracks. (False)
3. Poor conductors of heat are also good insulators of heat. (True)
4. Ventilators, when provided in rooms, are located near their roofs. (True)
5. A material medium is required for transfer of heat by the process of radiation. (False)

C. Tick (✓) the correct option.

1. A copper ball at 30°C is put in a container, containing water at 30°C. In this case
(a) Heat will get transferred from water to the copper ball.
(b) Heat will get transferred from the copper ball to water.
(c) Heat would flow first from copper ball to water and then from water to copper ball.
(d) There would be no transfer of heat between the copper ball and water.

2. The reading of the laboratory thermometer, in the figure shown here, is
(a) 10.2°C
(b) 10.8°C
(c) – 10°C
(d) 18°C

3. It is not convenient to use the laboratory thermometer to measure our body temperature. This is so because
(a) Its range is small.
(b) As the thermometer is taken out from the mouth, the level of mercury in it immediately starts falling.
(c) Our body does not transfer heat to its bulb.
(d) It takes a very long time to acquire the body temperature.

4. When we hold our hands near one side of a flame, they get warmed due to
(a) Conduction as well as convection
(b) Conduction
(d) Convection

5. The freezer compartment in a refrigerator is usually put near its top. This provides good cooling throughout the refrigerator by making use of
(a) The conductivity of air.
(b) The radiation of heat by the food items kept inside the refrigerator.
(c) The setting up of ‘convection currents’ in the air inside the refrigerator.
(d) Transfer of heat by conduction and convection.
1. (d) There would be no transfer of heat between the copper ball and water.
2. (c) 18°C
3. (b) As the thermometer is taken out from the mouth, the level of mercury in it immediately starts falling.
5. (c) The setting up of ‘convection currents’ in the air inside the refrigerator

D. Answer the following questions in brief.

Question 1.
In which direction does the transfer of heat normally take place?
The transfer of heat normally takes place from the region of high temperature to that of low temperature.

Question 2.
What is the cause of heat generation in the following situations:
(a) We apply brakes on our fast moving car.
Friction between the rim and brake shoes and that between the tyre and the road.

(b) People often jump up and down to feel warmer in cold weather.
Vigorous physical activity; like jumping up and down; raises the body temperature. Due to this, people often jump up and down to feel warmer in cold weather.

Question 3.
State two types of physical changes that may take place when a substance is heated.
The following physical changes take place when a substance is heated:

• Change of state: A solid changes to liquid and a liquid changes to gas.
• Thermal expansion: Most of the materials expand on heating. This phenomenon is called thermal expansion.

Question 4.
In what way(s) does a clinical thermometer differ from an ordinary thermometer?

 Clinical thermometer Ordinary thermometer Range is between 35 to 42°C. Range is between 0 to 100°C. Kink is present to prevent the fall in mercuiy level. Kink is absent. No need to hurry to take the reading. Reading needs to be taken quickly.

Question 5.
State the mode/different modes, of heat transfer, in the following situations:
(a) A paper cup, full of hot soup, lying on a table.

1. From soup to cup – by conduction.
2. From soup to air – by convection.

(b) Cooking vegetables in a pan.

1. From pan to vegetable – by conduction.
2. From one part of vegetable to another part – by conduction and convection,
3. From flame to utensil – by conduction, convection and radiation.

(c) Melting of a chocolate bar, in the school bag, on a hot day.

1. From air to school bag – convection and radiation.
2. from school bag to chocolate bar – conduction and convection.

(d) Cooking food in a microwave oven.

Question 6.
What is meant by sea breeze? When does it occur?
The wind moving from sea surface towards land is called sea breeze. Sea breeze happens during daytime.

E. Answer the following questions.

Question 1.
In the arrangement shown in the figure, pins A, B, C and D are fixed to a circular metal loop with the help of wax.

The circular metal loop is heated at the point A with the help of a candle flame. In which order would the pins fall if AB < AD? Justify your answer.
Pins would fall in the following order: A → B → D → C
A is nearest to the flame; followed by B, D and C in that order. Hence, A is the first to get heated; followed by B, D and C in that order.

Question 2.
Give reasons for the following:
(a) Iron rims are heated red hot before ‘fixing’ them on cart wheels.
Iron rims to be fitted on cart wheels always have smaller diameter than the diameter of wheel so that they can tightly fit. When iron rim is heated red hot, it expands because of heat. After that, it can be easily inserted over the wheel. Then cold water is poured over iron rim so that it cools down. Once the iron rim cools down, it contracts and becomes tightly fit on the wheel.

(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
The slight bend, or kink, in the capillary tube is because of a purpose. The kink prevents the mercury level from falling on its own. Thus, it becomes easier to take the reading.

(c) A new quilt is warmer than an old one.
A new quilt has cotton evenly fluffed out. This means there is plenty of air space inside the quilt. This helps in preventing body heat from escaping and thus the person feels the warmth during cold season. In old quilt, cotton becomes lumpy at most of the places and then there is less number of air pockets.

(d) A brass tumbler feels much cooler than a wooden tray on a chilly day.
Brass is a metal while wood is a non-metal. Metals are good conductors of heat, while non-metals are bad conductors of heat. When we touch a brass tumbler on a chilly day, the heat from our body is quickly transferred to the brass tumbler. This is not the case with wooden tray. Due to this, a brass tumbler feels much cooler than a wooden tray on a chilly day.

(e) The bottoms of cooking utensils are often kept black.
Black colour absorbs most of the radiation falling on it. Due to this, the bottom of cooking utensil is often kept black so that it can absorb most of the radiation. This helps the cooking utensil to utilize the maximum amount of heat.

Question 3.
Akshit visited Rishikesh for river rafting during summer holidays. At a campsite, there were two tents, one made with a black fabric and the other with a white fabric. Which one should Akshit prefer? Give reasons for the choice. Should Akshit prefer the same tent during winters?
White colour reflects most of the radiation falling on it, while black colour absorbs most of the heat. Due to this, white coloured fabric remains comfortable during summers, while black coloured fabric remains comfortable during winters. Hence, Akshit should prefer white tent during summers and black tent during winters.

Question 4.
Explain briefly how winds are caused.
Winds are caused due to uneven heating of the earth’s surface. Areas near the equator get the maximum amount of heat. The warm air from the equator rises up and creates low pressure near the surface. This area of low pressure is filled in by cooler winds from surrounding latitudes. This cycle is repeated and thus winds are formed.

Question 5.
Supriya, while doing an experiment in the Science Laboratory, kept a laboratory thermometer ‘P’ 10 cm away on one side of the flame of a candle. Her friend Riya kept a similar thermometer ‘Q’ 10 cm above the flame of the candle as shown in the figure.

In which of the thermometers, ‘P’ or ‘Q’, the rise in temperature will be faster? Give reasons for your answer.
We know that warm air rises up. Hence, thermometer ‘Q’ will get more amount of heat compared to thermometer ‘P’. Thus, the rise in temperature would be faster in thermometer ‘Q’.

Question 6.
In the two ‘set-ups’ X and Y, shown on the next page, the wires AB and PQR are made of the same material and have equal ‘thickness’. The length of the wire AB, (in the set-up “X’) is equal to the diameter (=PR) of the semi-circle, formed by the wire PQR, (in the set-up ‘Y’). Pins, PI and P2, are attached, to wires AB and PQR respectively, with the help of wax. Which of the two pins, PI or P2, will fall off later? Give reason for your answer.

Diameter is always less than length of the arc of a semi-circle. Hence, heat will travel sooner to pin ‘PI’ than to pin ‘P2’. This means pin ‘P2’ would fall off later.

Value Based Question

During the summer holidays, Hridyika went to visit her grandparents in their village. During her morning walks there, she observed that a girl in the village would play very good music on her home-made musical instrument. Hridyika was very much impressed by her skill and wanted to learn playing that instrument from her. She, however, felt hesitant to ask her to teach her. When she discussed her problem with her mother, she advised her to politely and keenly request that girl to teach her that skill. She told Hridyika that just as heat always flows from a hot object to a cold object, knowledge and skill always ‘flow’ to a pupil from her/his mentor.

Question 1.
State the values displayed by Hridyika’s mother.
Humility, persuasion skills

Question 2.
Name the three different modes of transfer of heat and give one example of each.
Three different modes of transfer of heat are as follows:
(a) Conduction: Heating of an iron rod.
(b) Convection: Heating of water.
(c) Radiation: Heating of earth.

Question 3.
Have a group talk in which students talk about how they learnt some skill from a mentor.
Do it yourself.

Something To Do

Question 1.
Boil some water in vessel. Cover it with a plate. Remove the plate after sometime. Allow it to cool. We will see droplets of water on the surface of the plate. Try to find out why, and how, these water droplets are formed.
When water is heated, some water turns into vapour. When vapour reaches the undersurface of plate, it condenses into water droplets.

Question 2.
Describe the various ways in which heat energy can enter/escape from our houses. Learn about the measures that are now being taken to reduce such heat transfers.
Heat from our house can escape by conduction, convection and radiation. Heat from the house can escape to the ground by conduction. House is usually made of mud or brick, which are insulators. This prevents loss of heat through conduction. During winter, we keep the doors and windows closed, so that less amount of heat can escape through convection through air. The outer walls can be painted in dark colours to reduce the loss of heat, especially in colder climates.

Question 3.
Take a cup of hot water/tea. Measure its temperature accurately with a thermometer. What will happen to the hot water/tea if it is left in a room for, say, one hour? Measure the temperature at fixed intervals of five minutes and record (your observations! in the given table. Using these readings, draw temperature-time graph.
Do it yourself; following is an example of temperature-time graph of water.

Question 4.
Take some (moderately) hot water in a tumbler. Dip one end of a steel spoon, a plastic spoon, a glass rod, a wooden stick and a copper rod in the hot water. After waiting for few minutes (say 2-3 minutes) (carefully) touch the other end of each item. Observe which ones become hot and which ones do not. Hence, categorise them as ‘Heat conductors’ or as ‘Heat insulators’.

 Heat conductors Heat insulators Steel spoon, Plastic spoon Copper rod Glass rod Wooden stick

### DAV Class 7 Science Chapter 5 Solutions – Heat

I. Fill in the blanks.

Question 1.
The degree of of ________ a body is measured in temperature.
Hotness

Question 2.
Heat energy is also known as ________ energy.
Thermal

Question 3.
________ is the most preferred material for thermometer.
Mercury

Question 4.
The human body temperature does not go below ________°C.
35°C.

Question 5.
The boiling point of water is ________°C.
100

Question 6.
________ colour absorbs most of the radiation.
Black

Question 7.
________ colour reflects most of the radiation.
White

II. Tick (✓) the correct option.

1. Which of these is not a unit of temperature?
(a) Celsius
(b) Fahrenheit
(c) Kelvin
(d) Joule
Joule

2. What is the normal temperature of human body?
(a) 100°F
(b) 98.4°F
(c) 75°F
(d) 35°F
98.4°F

3. Which of the following materials is an insulator?
(a) Iron
(b) Copper
(c) Wood
(d) Aluminium
Wood

4. Transfer of heat by convection takes place in which of these materials?
(a) Iron
(b) Gold
(c) Ice
(d) Air
Air

5. What is the freezing point of water?
(a) 0°F
(b) 32°F
(c) 45°F
(d) 80°F
Air

III. Answer the following questions.

Question 1.
How will you define heat?
Heat is a form of energy which is transferred from one body to another due to the existence of a temperature difference between them.

Question 2.
What is the difference between heat and temperature?
Heat is the energy of a body due to the motion of its particles. Temperature is just an indicator of this energy.

Question 3.
Explain the structure of a common thermometer.
The most common thermometer is composed of mercury in a glass tube. It consists of a narrow capillary tube which is made of glass. The tube is closed at the upper end and has a bulb at its lower end. The bulb is filled with mercury. The mercury can be seen as a shining thread inside the capillary tube. The mercury; in the bulb; expands with increase in temperature. This results in the shining thread of mercury appearing longer; which gives the reading of temperature. Proper scales are made on the thermometer.

Question 4.
What do you understand by conduction of heat?
When transfer of heat takes place from one molecule LO the next molecule, it is called conduction of heat. Transfer of heat in solids takes place through conduction.

Question 5.
Write about three significances of conduction of heat in our daily life.
Significance of conduction of heat in our daily life:

• Cooking utensils are usually made up of metals. Metals are good conductors of heat and hence are ideal for making cooking utensils.
• Handles of cooking utensils are usually made of wood or plastic. This ensures that one can safely hold these utensils from their handles.
• Houses are made from bricks or mud. These are insulators and hence shield the house against temperature variations.

Question 6.
What do you understand by radiation of heat?
When transfer of heat takes place without the involvement of any media, it is called radiation. Transfer of heat from the sun to the atmosphere of the earth takes place because of radiation. A white body reflects most of the radiation. A black body absorbs most of the radiation.

Question 7.
What is the working principle of a mercury thermometer?
Mercury is a metal which readily expands or contracts with change in temperature. The mercury inside the thermometer quickly expands with an increase in temperature. This is observed as the shining thread appearing longer; which gives the reading of temperature.

Question 8.
What is the normal direction of heat transfer?
Heat is usually transferred from higher temperature to lower temperature.

Question 9.
In what way(s) does a clinical thermometer differ from an ordinary thermometer?

 Clinical thermometer Laboratory thermometer The range is from 35°C to 42°C The range is from – 10°C to 110°C. There is a kink near the bulb. Kink is absent. Used for measuring body temperature. Used in laboratories and factories.

Question 10.
Why is a new quilt warmer than an old one?
In a new quilt, the cotton is fluffy and thus air spaces are more inside the quilt. This helps in trapping more air and thus in trapping heat. In an old quilt, the cotton often gets squeezed and thus there is less air space. Since older quilt can trap less volume of air so it can trap a less amount of heat. Due to this, a new quilt is warmer than an old one.

Question 11.
A brass tumbler feels much cooler than a wooden tray on a chilly day. Why?
When we touch a cold thing, heat transfer takes place from our skin to the
cold object. Copper is a good conductor of heat and hence more heat is quickly transferred from our skin to the copper tumbler. Wood being a bad conductor; less heat is transferred from our skin to the wooden tray. Due to this, a brass tumbler feels much cooler than a wooden tray on a chilly day.

Question 12.
Take a long rod of a metal like iron, copper or aluminium. Heat it from one side while holding the other end. We will see that after some time the other end also becomes too hot to hold. Try to reason out why this happens. (Caution: Do not hold the iron rod for ‘too long’ or ‘too firmly’. This can cause burns on the hand and fingers.)
Heat from one end of metallic rod reaches its other end by conduction. That is why, when a rod is kept near a flame for some time; its other end also becomes hot.

IV. Read the following statements and choose the correct answer.

1. Out of the following, the only one, that is not an effect of heat, is
(a) Thermal expansion
(b) Release of nuclear energy
(c) Change of state
(d) Many chemical reactions
1. (b) Release of nuclear energy

V. Projects

Question 1.
Take a clinical thermometer and measure the body temperature of your 10 classmates. Find the average temperature of your 10 classmates.