STD 7 Basic Science: Chapter 09 Paths of Heat Flow - Questions and Answers | Teaching Manual
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Chapter 09: Paths of Heat Flow - Questions and Answers
Paths of Heat Flow
| City | Maximum temperature | Minimum temperature |
|---|---|---|
| Thiruvananthapuram | 26°C | 22°C |
| Kochi | 26°C | 23°C |
| Palakkad | 28°C | 23°C |
| Kozhikode | 26°C | 22°C |
1. The temperatures recorded during a day in some cities, telecast in the news, have been given here.
• Which cities recorded the lowest temperature?
Answer: Thiruvananthapuram and Kozhikode
• At what time might have the lowest temperature been recorded?
Answer: At night
You know that light and electricity are forms of energy. Similarly, heat is also a form of energy.
• Is it during the night or day that you feel hotter? Why?
Answer: During day time. we get direct sunlight during the daytime.
We get heat along with light from the sun. The sun is a source of heat just as it is a source of light.
• There are many situations in our daily life where we make use of heat. What are they?
Answer: For cooking, For ironing, For drying materials, For melting metals
• What are the sources of heat in the situations given below?
| Situation | Receives heat from |
|---|---|
| Cooking | Firewood, LPG, Electricity, Kerosene |
| Ironing | Electricity |
| Drying articles | The Sun |
| Melting metals | Fuels, electricity |
While heating
2. Introduce a steel spoon with water to the flame of a spirit lamp.
Answer: Water gets heated
• Does the spoon get heated?
Answer: Spoon also gets heated
3. Didn't the spoon receive heat from the flame of the spirit lamp?
• Where did water get heat from?
Answer: Water gets heat from the spoon.
Answer: The flow of heat from one place to another is termed heat transmission.
5. Try these activities. Fold a long piece of paper thick, burn it on a candle flame. Show an iron rod to a candle flame.
• In which situation did you experience heat? Why?
Answer: We experienced heat while showing an iron rod to the candle flame. Iron rod allowed heat to pass through.
6. Repeat the experiment using an aluminium rod, a piece of wood, a copper rod, a rib of coconut leaf etc.
• Which are the materials that allowed heat to pass through?
Answer: aluminium rod and copper rod
Heat transmission - different modes
7. Fix pins on a hacksaw blade at equal distances using wax. Heat one end of the blade using a candle flame as shown in the figure.
What is your observation?
• Did the pins fall down simultaneously?
Answer: All the pins didn't fall down simultaneously
• Which pin fell down first?
Answer: The pin that was near the flame of the candle.
• Which pin fell down last?
Answer: The pin that was farthest from the candle flame.
• Why do the pins fall in regular order?
Answer: The heat is transmitted along the hacksaw blade in an order.
8. Conduction
Answer: The mode of heat transfer from one end of a material to the other without the actual movement of the molecules.
9. Good conductors
Answer: The substances that conduct heat very well are termed good conductors.
10. Poor conductors
Answer: The substances which do not conduct heat are termed poor conductors.
11. Heat some materials known to you and classify them into good conductors and poor conductors.
| Good Conductors | Poor conductors |
|---|---|
| Copper Aluminium Tin Zinc | Wood Cloth Plastic Charcoal Rubber Glass |
12. The handle of the pressure cooker and nonstick vessels are made of substances like bakelite. Can you explain the reason?
Answer: Bakelite is a poor conductor of heat. It does not conduct heat. The handle does not become hot during cooking. So it is easy to handle the vessel.
Transmission of heat in metals
13. Take three metal rods made of copper, iron and aluminium of the same length and thickness, and arrange them on wooden blocks as shown in the figure. Instead of rods, you can use sheets of copper, iron and aluminium of the same size available in the school lab as well.
• What did you observe?
Answer: We will observe that the nails stuck to the metal strips begin to drop one by one.
• Did all the pins fall down at the same time?
Answer: No, all the pins did not fall at the same time.
• From which metal did the pinfall down first? From which metals did the pins fall secondly and thirdly?
Answer:
The pinfall down first - From the copper rod
The pinfall down secondly - From the Aluminium rod
The pinfall down the third - From the Iron rod
All good conductors do not transmit heat at the same rate.
• Write down the metals iron, copper and aluminium in the order of their rate of conducting heat.
Answer: Copper, Aluminium, Iron
Transmission of heat in liquids
14. You have understood about heat transfer in solids. Let us see how heat is transferred in liquids.
Fill three-fourths of a round-bottomed flask with water. Take a few crystals of potassium permanganate in an aluminium foil and pack it firmly. Make a hole in it with a pin and keep it immersed in water. Heat the flask using a spirit lamp.
Observe the changes inside the flask. (TextBook Page: 119)
• To which side does the water contain- Ing dissolved potassium permanganate move, on heating?
Answer: Heated potassium permanganate solution moves upwards.
• Subsequently in which direction does this water move?
Answer: This water then moves towards the bottom through the sides.
15. On heating water gets heated rises upwards and cold water moves down to the place. Isn't clear that heat transfer takes place in water due to the movement of the heated water molecules?
Answer: Colder particles from the surroundings move towards the bottom through the sides. The heated particles carry heat with them and when in contact with the colder particles they absorb heat from the heated particles. Since the coloured particles are heated particles we see the movements in it. Hence the heat is transferred from the hot bottom to the cold top by the actual, continuous movements of the water molecules themselves. After some time the colour spreads everywhere. It shows that heated molecules have spread everywhere in the liquid. This process of transference of heat is seen only in fluids.
Convection is the method by which heat is transferred through the movement of the molecules in gases and liquids.
16. What is Convection?
Answer: Convection is the process by which heat transfer takes place by the actual movement of molecules.
Path of Smoke (Textbook Page: 120)
17. Materials required: shuttlecock container, incense stick and candle.
Open the shuttlecock container at both ends. Make a small pencil-thick hole at a height of 8 cm on one side.
Fix a lighted candle on a table and place the container in such a way that the candle is inside the container. The hole pierced in the container should be towards the bottom of the container.
Bring a glowing incense stick near the hole on the side.
Observe the direction of the flow of smoke.
• In which direction does smoke from the incense stick flow?
Answer: Smoke from incense stick enters the container through a small hole and comes out through the opening above the candle flame
• Place your palm at the top of the container. Don't you feel hot?
Answer: Yes. Warm air rises vertically. It is through convection that heat transmits above the lighted candle.
• What do you understand from this?
When the air inside the container gets heated and moves upwards, cool air flows to that part through the hole. Along with this, smoke from the incense stick also enters. As in the case of liquid, here to heat is transferred from one place to another through the actual movement of particles. It is mainly by convection that heat transfer takes place in liquids and gases.
Transmission of heat in solids - Conduction
Transmission of heat in liquids and gases - Convection
A medium is required for heat transfer by conduction and convection
• Place your hand over a glowing chimney lamp. Don't you feel heat? How is heat transferred in this instance?
Answer: It is through convection that heat transmits above the burner lamp which is lit.
• What change do you notice when you close the holes around the burner of a chimney lamp? Why is it so?
Answer: The light will be extinguished when the burner of the lamp is closed because oxygen required for burning stops when convection flow stops.
• What arrangements are made in the kerosene oil stove to make the movements of hot and cold air smooth?
Answer: There are holes under the tin which cover all the wicks wholly. The place of these holes is under the burning wick. When the stove burns, hot air rises up. Then cold air enters the stove through these holes.
• Why are the smoke pipes of factories fixed at a height.
Answer: This helps to reach the smoke very high and make the burning strong by making a powerful convection flow.
• Why are some small spaces left open on the uppermost part of walls in the rooms of concrete houses?
Answer: The hot air in the rooms goes upwards. This flows outwards through the gap and then. cold air flows inwards through doors and windows. This airflow occurs in the room
From the sun
18. Haven't you now understood that a medium is required for heat transfer by conduction and convection?
• At the same time, the heat from the sun reaches the earth through space even though there is no medium. How does this happen?
Answer: There is no medium in between the sun and the earth. So heat cannot be transferred either by conduction or by convection. But heat is transferred from the sun. This is the third way of transference of heat called radiation.
• Don't you experience heat when you stand near a lighted firewood oven?
If heat should reach us by conduction, there should be a good conductor between the oven and us. When the firewood burns, the heated air moves only up. This means it is neither by convection that the heat reaches us. Still, we experience heat. This shows that heat transfer is possible without the presence of a medium as well
19. What is Radiation?
Answer: Radiation is the method of heat transmission without the help of a medium. A smooth surface reflects radiant heat.
20. Method of heat transfer
| Conduction | Convection | Radiation |
|---|---|---|
| The mode of heat transfer from one end of a material to the other without the actual movement of the molecules | The mode of heat transfer by the actual movement of molecules in liquids and gases. | The mode of heat transfer from one place to another without the help of a medium. |
Without losing heat
• A steel glass containing hot tea is kept open on a table. Won't the tea become cold after some time? What are the ways in which the tea loses heat?
Answer: By conduction, Convection and Radiation.
• Which mode of heat transfer is controlled when the glass is covered with paper?
Answer: Heat loss by convection.
Hot food becomes cold after some time.
• Food can be kept hot for a long time if we can minimise heat transfer by conduction, convection and radiation. What are the devices we use for this?
Answer: Flask, Casserole, Thermal cooker, hot box, etc.
21. What are the utensils used in houses for keeping cooked food hot?
Answer:
Kept in hot box
Kept in casserole
Kept in thermal cooker
In flask
22. A steel glass containing hot tea is kept on a table. Won't the tea become cold after some time? What are the ways in which the tea loses heat?
Answer: The tea loses heat by conduction, convection and radiation.
23. Which mode of heat transfer is controlled when the glass is covered with paper?
Answer: Heat transfer due to convection is controlled here.
24. The food and drinks stored in such devices remain hot for a long time. How is the heat loss due to conduction, convection and radiation controlled here?
25. Examine the arrangements in a casserole, thermal cooker etc. to retain heat. Record your findings in the science diary.
Answer: The main part of a thermos flask is a glass vessel with two walls. The region between the two walls is made empty. This vessel is fixed by using bad conductors like plastic and cork. It is better to examine an old thermos flask.
Heat transmission through conduction will be very less as the flask is made of bad conductors like glass, plastic and cork.
Heat transmission by convection is prevented as there is empty space between the walls. Outside of inner wall and inside of outer wall of the glass vessel are coated with silver. When the hot substance is placed inside the flask, the silver-coated inner side reflects heat to the inside part inwards itself. Thus the loss of heat radiation is prevented. If cold substance is placed in the flask the loss of heat through radiation is prevented as the silver-coated outer
side reflects heat outside.
In casseroles and hot boxes the transmission of heat by conduction, convection and radiation is prevented by the same principle.
Thermal expansion in solids
26. What are the changes that happen when a solid is heated? Let us do an experiment.
Cut off the middle portion of a cardboard sheet. Fix two aluminium plates using
cycle screws on the cardboard sheet as shown in the figure. Connect these plates to a battery and a bulb using wires. Heat the aluminium plate using a lighted candle.
• What do you observe?
Answer: The bulb glows
• Why does the bulb glow?
Answer: The circuit is completed and the bulb glows.
• Why do the sheets come into contact with each other on heating?
Answer: The aluminium sheets get heated up and expand. The circuit is completed and the bulb glows.
• Put out the flame and allow the sheets to cool. What do you observe now? Why does the bulb stop glowing?
Answer: Aluminium plate contracts on cooling. Due to this contraction, the circuit is opened and the bulb stops glowing.
27. Collect the equipment as shown in the figure. Pass the metal ball through the ring, and it passes exactly through the ring. Try to pass it once again through the ring after heating the metal ball.
Answer: After heating the metal ball it could not pass through the ring.
• What is the reason?
Answer: Because the ball expands during heating. If the ring also is heated the metal ball will pass through the ring again. This is because the ring has expanded while heating.
• Discuss how the thermal expansion of objects in the following situation is useful. Prepare notes.
• The method of fixing metal frame to the wheel of the bullock cart.
Answer: The metal frame expands while heating. So it slips easily onto the wheel. As the frame cools it contracts to the original size and fits tightly on the wheel.
• To loosen the cap of a fountain pen that is tightened, it is heated.
Answer: As the top of the pen is metal it expands more on heating. So we can loosen it easily.
• To take out a glass tumbler that got jammed into another, it is dipped in hot water.
Answer: When immersed in hot water glass expands. Because of this, it is easy to take out a glass tumbler that got jammed into another.
Find out other similar situations and record them.
• Telephone lines in summer days are not stretched tightly because the line gets tightened during winter days due to contraction.
• Little gaps are left between the sections of a railway track because the rails expand on sunny days. Otherwise, the rails will be deformed causing dangers.
• When we fit the iron ring to the handle of a cutting knife we heat it. The iron ring expands when heated and contracts when it cools. So the iron ring fits tightly to the handle.
• Concrete bridges are built leaving gaps between the joints of concrete slabs. The gaps in joints are needed to keep the bridge from buckling when the concrete expands on hot summer days.
28. Observe the situations given below.
• Long buildings were built as two.
Concrete expands during summer. If there is no space for this expansion, it will damage.
• Long bridges were built as spans.
The space is left between girders. So as to allow their expansion during summer.
• To take out a glass tumbler that got jammed into another, it is dipped in hot water
When immersed in hot water glass expands. Because of this, it is easy to take out a glass tumbler that got jammed into another.
• To join two p.v.c. pipes having the same diameter, one of them is heated.
If one P.V.C. pipe is heated it will expand and join to another easily.
Solids expand on heating.
• Analyse the situations. Explain this situation on the basis of thermal expansion.
Solids expand on heating. On losing heat, they contract.
Thermal expansion in liquids
29. Take a big injection bottle filled with coloured water. Make a small hole in its rubber lid. Remove the metallic portion of an empty pen - refill and fix it into the hole.
Keep this bottle partially immersed in a vessel containing hot water.
• Why does the coloured water come out of the injection bottle?
Answer: Coloured water expands on heating and it comes out from the injection tube.
Remove the bottle from hot water and let it cool for some time.
• Is there any change in the liquid level In the refill?
Answer: The liquid level falls.
• Record the inferences made from your observations in your science diary. To The Science Diary
Inference
Like solids liquid also expands on heating. and contract on cooling.
Liquids expand on heating.
They contract on cooling.
• How does this principle used in thermometers?
Answer: Due to heat the mercury level in the thermometer rises up.
Measuring temperature
The laboratory thermometer and clinical thermometer work on the basis of the ability of liquids to expand and contract. Let us familiarize ourselves with these two types of thermometers.
The bulb at the end of the thermometer contains mercury. There is a narrow vacuum tube adjacent to the bulb. Measurements are marked on the tube.
Place the clinical thermometer at the nook of your elbow and check your body temperature. Won't you record the temperature?
Examine the expansion and contraction of mercury in a laboratory thermometer by dipping it first in hot water and then in cold water.
• Write down the working principle of the thermometer in your science diary.
Answer: A thermometer works on the basis of the principle that the objects expand on heating. The working of a mercury thermometer depends on liquid expansion
30. Thermometers
• Unlike the laboratory thermometer, there is a narrow portion above the mercury bulb in a clinical thermometer.
• Clinical thermometer is designed only to measure body temperature.
• Laboratory thermometer is used to measure temperatures below 200° C.
31. Examine a thermometer and note down its special features.
Answer:
• Mercury is filled in a bulb.
• Some mercury climbed on to narrow tube (will be equal to atmospheric temperature).
• There are markings on the outer side of the thermometer.
32. Which thermometer is used to find the body temperature?
Answer: Clinical thermometer
33. What is the use of a laboratory thermometer?
Answer: To find a temperature below 200°C
34. Clinical thermometer is not used Instead of the ordinary thermometer. Discuss why it is not used?
Answer: Usually clinical thermometer shows temperature on a Fahrenheit scale. In clinical thermometer, the marking is usually from 95°F to 108°F (35°C to 42°C). The maximum temperature that can be measured using a clinical thermometer is 108°F (42°C). The temperature of boiling water is 100°C (212°F). This temperature is not marked on the clinical thermometer. So the clinical thermometer is not used to measure this.
35. Find out the difference between ordinary thermometers and clinical thermometers and record them in the Science Book.
Answer: In ordinary thermometers, the markings are from -10°C to 110°C. The markings on the clinical thermometer are from 35°C to 42°C. It has a small kink (bend) in the glass tube.
Thermal expansion in gases
36. Fix a plastic tube in the lid of an injection bottle. Immerse the free end of the tube in water taken in a vessel. Keep the bottle partially immersed in hot water.
• What did you observe?
Answer: Bubbles come out from the other end of the tube.
• Why do bubbles come out through water?
Answer: When the bottle immerse in hot water, the air inside it expands and comes through the water as bubbles.
(When this bottle immerse in ice water. The air inside the bottle contracts on cooling and the water from the vessel moves inside the bottle through the tube)
Gases expand on heating and contract on cooling.
37. What is thermal expansion?
Answer: Substances expand on heating and contract on cooling. It is called thermal expansion.
Thermal expansion in daily life
38. Some situations in daily life are given below. Explain them on the basis of thermal expansion.
• Air holes are made near the ceiling in rooms.
Answer: When the air becomes hot in the room, it escapes through the air hole at the top and cold air enters the room through Windows and doors.
• Long bridges are constructed as different spans.
Answer: The bridge will expand by getting heat and bends if there is only one span. If it is constructed as different spans, the bridge will not bend on hot days.
• Hot water is poured in order to separate two glass plates that are stuck together.
Answer: The outer glass expands when hot water is poured into it. So the glass inside it can easily be taken out.
Wind
39. Observe the picture. Does the wind blow to the land or to the sea?
• Wind blows from sea to land during the day. What is the reason for this?
Answer: During day time the land gets heated up more quickly than the sea. The air above the land gets heated and expands. The air with lower density rises up. The comparatively cooler air above the sea blows towards the land.
40. What is sea breeze?
Answer: During day time, the land gets heated fast by the heat of the sun, while sea water gets heated slowly. The air above the land gets heated and rises up. The less heated air from the sea rushes to the land at this time. This is a sea breeze.
• Which is hotter during day time - the air above sea or the air above land?
Answer: The air above land
• Where does the heated air move to?
Answer: The heated air expands and moves upwards.
• State the direction of the wind by observing the direction in which the flag waves.
Answer: The direction of the wind is from sea to land.
41. Take two flat vessels of the same size. Take the sand in one vessel and water in the other, to the same measurement. Record the temperature. Expose them To the heat of the sun. Record the temperature of both sand and water. What difference do you notice? Which of these got heated soon?
Answer: Though both get heated at the same time, it can be observed that the increase in temperature is great for sand. From this, we can conclude that sand gets heated more easily.
42. Place both sand and water under shade or in a room so as to allow it to cool down. Measure the temperature of both and notice the difference. Answer the following questions related to the experiment.
• Which gets heated up sooner, sand or water?
Answer: Sand
• Which gets cooled down sooner?
Answer: Sand
43. Choose the correct statement from those given below, in relation to the activity you have done.
• Land gets heated sooner during the daytime.
• Sea gets heated sooner during daytime.
• During daytime the air above the land gets heated and rises up.
• During the night land cools down sooner.
• During the night the air above land cools down quickly.
• During night the air above the sea cools down only slowly.
• During the night the hot air above the sea rises up
Answer:
• Land gets heated sooner during the daytime.
• During daytime the air above the land gets heated and rises up.
• During the night land cools down sooner.
• During the night the air above land cools down quickly.
• During night the air above the sea cools down only slowly.
• During the night the hot air above the sea rises up.
44. What is a Land breeze?
Answer: Land gets cooled faster at night. Sea gets cooled slowly. Hence the air above the sea will expand more than the air above land and rise up. Then the air above the land will blow to the sea. This is a land breeze.
• Which is hotter at night - the air above land or the air above the sea?
Answer: The air above the sea is hotter.
• In which direction does the wind blow then?
Answer: The wind flows from land to sea.
• Does the wind blow only from the land to the sea and from the sea to the land? Is the direction of the wind the same always?
Direction of wind
45. Why does the wind blow from the North and South to the equatorial region?
Answer: The sun's rays fall vertically on the equatorial region. The air above this region gets heated and moves upwards. So the air from the less heated areas of North and South move towards this region.
46. How does the wind become useful to us?
Answer: The wind helps to regulate the temperature of a region. They attract rain clouds and avail us of the monsoon seasons.
47. What are the damages caused by wind? Try to write them down.
Answer:
• A powerful wind can cause hazards to our property and surrounding areas.
• Trees and human-made structures are damaged or destroyed
• Causes soil erosion
48. Wind and rain
• It is dangerous to stand under a tall tree when there is heavy rain and strong wind. Why?
Answer: Lightning has a tendency to strike the tallest objects in the area.
• Why are special weather forecast warnings given to fishermen?
Answer: Special weather forecast warnings are given to fishermen so that if any natural calamities like tsunamis or tides occur in the sea they can save their life.
49. What are the precautionary steps that one has to follow when there are strong wind, rain and lightning?
Answer:
• Do not stand under tall and isolated trees.
• If you are travelling, stop and take shelter in a safe place.
• Take precautions foreseeing the chance of electric cables breaking and falling down.
• Do not stay inside weak buildings
• If you happen to be in a lake, get to the shore immediately and take shelter in a safe place.
Let us Assess
1. At which place is the land breeze and sea breeze felt strongly?
A. High Land
B. Mid Land
C. Coastal areas
D. Coastal areas and highland
Answer: C. Coastal areas
2. At noon, water in the deeper areas of a pond is cooler than that at the top. Why?
A. The pond is shallow.
B. The heat that reaches the bottom is transmitted to soil.
C. Sun rays do not pass through water.
D. Water does not transmit heat from top to bottom.
Answer: D. Water does not transmit heat from top to bottom.
3. Hold your palm above and on the sides of a candle flame. Where do you experience more heat? Why?
Answer: Above the candle. Due to convection and radiation.
4. Observe the figures
Answer: The cup moves up. The air in the cup gets heated, expands and goes out. The air below the cup makes it rise.
5. Observe the figure.
Answer: 1. Convection, 2. Conduction, 3. Radiation
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