Frank Brothers Solutions for ICSE Class 9 Physics Chapter 5.4 Heat Transmission Of Heat

Question 1. Name the three modes of transfer of heat.
Answer: Conduction, convection and radiation are three modes of transfer of heat.
In simple words: Heat can move from one place to another in three ways: by touching (conduction), through moving air or water (convection), or as invisible waves (radiation).

📝 Teacher's Note: Use the analogy of passing a ball: conduction is passing it hand-to-hand, convection is someone carrying the ball, and radiation is throwing the ball through the air.

🎯 Exam Tip: Make sure to list all three modes to get full marks in a definition question.

Question 2. Define the process of conduction.
Answer: The process of conduction involves the transmission of heat from the hot end to the cold end from particle to particle of the medium.
In simple words: Conduction is when heat travels through a solid object, like a metal spoon in hot tea, by passing the energy from one tiny part to the next.

📝 Teacher's Note: Emphasize that in conduction, the particles themselves don't move from one end to the other; they just vibrate and pass the energy along.

🎯 Exam Tip: The keyword here is "particle to particle." Mentioning this shows a clear understanding of the molecular level of heat transfer.

Question 3. Is conduction possible in a vacuum?
Answer: No, conduction is not possible in vacuum.
In simple words: Conduction needs a "path" made of material (like metal or wood) to travel through. Since a vacuum is empty space, there are no particles to pass the heat along.

📝 Teacher's Note: Remind students that conduction requires a medium. This is why a thermos flask has a vacuum layer—to stop heat from escaping through conduction.

🎯 Exam Tip: If asked why, state clearly that conduction "requires a material medium."

Question 4. What is meant by thermal insulation?
Answer: Thermal insulation means restriction of transfer of heat by any means.
In simple words: Insulation is like a barrier that stops heat from moving into or out of a place, helping things stay hot or cold.

📝 Teacher's Note: Relate this to winter clothes or the insulation in the walls of a house. It keeps the warmth inside where we want it.

🎯 Exam Tip: Use the word "restriction" or "prevention" to define insulation effectively.

Question 5. Distinguish between good and bad conductors of heat.
Answer: Good conductor allow heat to pass through them easily while bad conductor of heat does not allow heat to pass them.
In simple words: A good conductor (like a metal rod) gets hot quickly because heat moves through it easily. A bad conductor (like wood or plastic) stays cool because heat can't travel through it well.

📝 Teacher's Note: Ask students why cooking pans are metal but the handles are usually plastic or wood. This perfectly illustrates the use of both types of conductors.

🎯 Exam Tip: Remember that "bad conductors" are also called "insulators."

Question 6. Are liquids good conductors of heat?
Answer: No, liquids are poor conductors of heat.
In simple words: While heat can move through liquids, it doesn't do it very well through conduction. Liquids usually move heat using convection instead.

📝 Teacher's Note: Mention mercury as an exception—it is a liquid metal and is a good conductor, which is why we use it in thermometers.

🎯 Exam Tip: If the question is general, "poor conductors" is the correct answer for liquids.

Question 7. Which materials are the best and worst conductors of heat?
Answer:

• Metals are the best conductor of heat.
• Gases are the worst conductor of heat.

In simple words: Metals are great at moving heat, while gases (like air) are very bad at it.

📝 Teacher's Note: This is why double-pane windows work—they trap a layer of air, which is a terrible conductor, to keep heat inside the house.

🎯 Exam Tip: Metals are generally "good" and non-metals/gases are "bad" or "poor" conductors.

Question 8. Which metal is the most conducting?
Answer: Among all the metals, Silver is most conducting.
In simple words: Out of every metal we know, silver lets heat travel through it the fastest and easiest.

📝 Teacher's Note: Although silver is the best, we use copper or aluminum for pots and wires because they are much cheaper!

🎯 Exam Tip: Silver is the top answer for both heat and electrical conductivity.

Question 9. Which substance is the most non-conducting?
Answer: Air is most non-conducting.
In simple words: Air is one of the best "stoppers" of heat, which is why fluffy blankets and jackets with air trapped inside keep us so warm.

📝 Teacher's Note: Explain that trapped air is an excellent insulator. If air could move, it would transfer heat by convection, but when trapped, it stops heat flow.

🎯 Exam Tip: Air is often the "worst conductor" mentioned in textbooks.

Question 10. Is air a good conductor of heat?
Answer: No, air is not a good conductor of heat.
In simple words: Air is a very poor conductor, which is why it is used in things like thermoses and woolly clothes to keep heat from moving.

📝 Teacher's Note: This is a key reason why we wear layers of clothes. The air trapped between layers provides insulation.

🎯 Exam Tip: Always associate air with "insulation" or "poor conduction."

Question 11. Are ebonite and asbestos poor conductors of heat?
Answer: Yes, this is a correct statement. Ebonite and asbestos are worst conductor of heat.
In simple words: These materials are extremely good at stopping heat, which is why asbestos was often used in fireproofing.

📝 Teacher's Note: Ebonite is a very hard rubber. Both are industrial examples of high-quality insulators.

🎯 Exam Tip: These are classic examples of insulators used in labs and industry.

Question 12. Why are cooking utensils made of metals?
Answer: Cooking utensils are made up of metals because metals are good conductors of heat and they easily allow the heat to pass through them.
In simple words: We want our food to cook fast, so we use metal pots that let the heat from the stove reach the food quickly.

📝 Teacher's Note: Connect the physical property (conductivity) to the practical use (cooking). If pots were made of wood, the food would never get hot!

🎯 Exam Tip: The reason is specifically "good thermal conductivity."

Question 13. Why are bricks made hollow for building houses?
Answer: Brick is bad conductor of heat, hence bricks greatly reduce the conduction of heat from outside to the room. In order to ensure better insulation, bricks are made hollow so that air filled between these gaps provides insulation to heat.
In simple words: Hollow bricks have air inside them. Since air is a very bad conductor, it stops the summer heat from entering the house and keeps the winter warmth from leaving.

📝 Teacher's Note: This is a brilliant example of using the "poor conductivity of air" to our advantage in architecture.

🎯 Exam Tip: Mention both the "brick material" and the "trapped air" as insulators.

Question 14. Is mercury a good conductor of heat?
Answer: Mercury is good conductor of heat.
In simple words: Mercury is a liquid metal, and like other metals, it lets heat travel through it easily. This is why it reacts so quickly in a thermometer.

📝 Teacher's Note: This makes mercury special among liquids, as most liquids (like water) are poor conductors.

🎯 Exam Tip: Use this to explain why mercury is used in clinical thermometers—it responds fast to body heat.

Question 15. Why do windows in cold countries have two glass panes?
Answer: In cold countries, windows have two glass panes and a thin layer of air in between these two glass panes because air is a poor conductor of heat; it obstructs the conduction of heat from the room to the outside.
In simple words: The layer of air "trapped" between the two panes of glass acts like a wall that stops the warm air in the house from escaping into the cold outside.

📝 Teacher's Note: This is called "double glazing." It is a standard energy-saving feature in modern homes.

🎯 Exam Tip: Focus on the "layer of air" being a "poor conductor."

Question 16. Arrange air, water, mercury, and copper in increasing order of heat conductivity.
Answer: Order of increasing heat conductivity:
Air < water < mercury < copper.
In simple words: Air is the worst at moving heat, followed by water, then mercury, and copper is the best of these four.

📝 Teacher's Note: Help students recognize the states of matter here: Gas (worst) < Liquid < Liquid Metal < Solid Metal (best).

🎯 Exam Tip: Be careful with the "<" symbols; they point towards the smaller/lesser value.

Question 17. Explain several facts about convection and conduction.
Answer:

• Water is heated from below because water is heated by process of convection and in process of convection heat always flows in upward direction from downward direction.
• In process of convection heat is transferred from one point to another by actual movement of particles of medium and as particles of solid cannot move so process of convection is not possible in case of solids.
• Conduction is not possible in Vacuum because process of conduction require a material medium for transfer of heat.
• Inside the refrigerator, cooling takes place by convection. The freezing chamber at the top produces cool and heavy air which descends and hot air from below goes up setting convection current. Thus, cooling in a refrigerator is kept near the top.
• As earth surface gets heated by sun rays and due to process of convection the air near earth surface open to sunlight get heated while in shade sun rays does not fall on earth surface so air near that surface would not heat up.

In simple words: Convection is when hot air or water moves physically to carry heat. In a fridge, the cold air falls from the top to keep everything cool. Conduction is different—it needs solid matter to travel through and won't work in empty space.

📝 Teacher's Note: The refrigerator example is very practical. Explain that "hot air rises and cold air sinks"—this is the heart of convection.

🎯 Exam Tip: For convection, remember the phrase "actual movement of particles." For conduction, remember "requires a material medium."

Question 18. Describe the circulation of air in a convection box with a candle.
Answer:
(i) Air is entering into the box from tube Q and as air reaches near the candle its temperature increase and air become lighter and leaves out from tube P.
(ii) If opening Q is closed then circulation of air stops and there is no way for air to enter. Air inside box will leave out from tube P and after some time candle stops because for burning it requires air which is not circulating in the box now.
In simple words: The candle heats the air, which rises up out of one chimney (P). This pulls fresh air in through the other chimney (Q). If you block the intake (Q), the candle will run out of air and go out.

📝 Teacher's Note: This demonstrates a convection current. You can show the air movement by holding a piece of smoking incense near tube Q.

🎯 Exam Tip: Explain that "hot air is lighter" so it rises, creating a vacuum that "sucks" in cool air.

Question 19. How does smoke move in a convection box?
Answer: Smoke is entering into the box from tube P and as smoke reaches near the candle its temperature increase and smoke become lighter and leaves out from tube Q.
In simple words: Smoke follows the movement of air. It gets pulled in one chimney and, once heated by the candle, it rises and floats out of the other chimney.

📝 Teacher's Note: This is a visual proof of air movement. Smoke is heavier than air but it still rises when heated because the air around it is moving up.

🎯 Exam Tip: Smoke moves due to the "convection currents" set up by the heat source.

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Question 20. How do we receive heat from the sun?
Answer: We receive heat from sun by process of Radiation.
In simple words: The sun's heat travels to Earth through millions of miles of empty space. Since there is no air or matter in space, only radiation can carry the heat to us.

📝 Teacher's Note: Point out that conduction and convection cannot work in space. Radiation is the only mode that doesn't need a material "medium."

🎯 Exam Tip: Radiation is the *only* mode of heat transfer that can happen in a vacuum.

Question 21. How can the radiating power of a surface be increased?
Answer: Radiating power of a surface can be increased by blackening it.
In simple words: Painting an object black makes it much better at giving off (radiating) heat energy.

📝 Teacher's Note: This is why the back of a refrigerator is often painted black—to help it get rid of heat more efficiently.

🎯 Exam Tip: Black surfaces are both "good absorbers" and "good emitters" of heat.

Question 22. What is the speed and path of heat radiation?
Answer: Heat radiation travels with speed of light and always follows straight path.
In simple words: Heat radiation is like light—it moves incredibly fast and travels in a perfectly straight line unless something gets in its way.

📝 Teacher's Note: Because it moves in a straight line, we can stop radiation just by standing in the shade or using a screen.

🎯 Exam Tip: Mention "speed of light" and "straight path" for a complete answer.

Question 23. Explain how a thermos flask prevents heat loss.
Answer: The flask consists of a double walled glass container with vacuum between the walls A and B. The inner side of A and the outer side of B are silvered. It has a narrow mouth which is closed by a non-conducting rubber stopper. If a hot liquid is placed inside the flask, it cannot lose heat by conduction or convection, as there is no air between the walls. The small amount of heat lost from the liquid by radiation is reflected back into the liquid by silvering of the inner wall A and any radiation striking the outer wall B, is reflected back and prevented from reaching the liquid.
In simple words: A thermos uses an empty space (vacuum) to stop heat from touching or moving through air, and shiny mirrors (silvering) to bounce heat radiation back where it belongs.

📝 Teacher's Note: This is a masterpiece of thermal engineering. It addresses all three modes of heat transfer: Conduction and Convection (stopped by the vacuum) and Radiation (stopped by the silvering).

🎯 Exam Tip: Match each part of the flask to the mode of heat it stops: Vacuum → Conduction/Convection; Silvering → Radiation.

Question 24. What is a greenhouse and why does it stay warm?
Answer: Green house is a glass house constructed to keep delicate and rare plants that require warmth and protection from weather. It is constructed by panels of glass. The temperature in a green house rises because heat enters the house through the glass but cannot escape out, because the glass reflects the heat back to the inside of the house. This makes glass house warmer than the outside environment.
In simple words: A greenhouse works like a trap for sunlight. The sun's energy gets in through the glass, but the heat can't easily get back out, making it a cozy place for plants.

📝 Teacher's Note: This is why a car gets so hot when parked in the sun with the windows up. The glass lets in light but traps the heat inside.

🎯 Exam Tip: The glass acts as a "one-way door" for solar energy.

Question 25. Describe an experiment to show that black surfaces are better absorbers of heat than polished ones.
Answer: Take two sheets of tin plates, one blackened and the other is polished brightly. Fix some drawing pins on the outer side of both sheets using wax. Fix these plates vertically at a very short distance apart. Place a Bunsen burner midway between them. When you light the burner you will observe that the pins from the black painted sheet start dropping while the pins on the polished surface remain in the same position for much longer time. Thus, we can conclude that black surface is better absorber of heat than the polished one.
In simple words: If you put a black plate and a shiny plate near a flame, the black one gets hot so fast that the wax on it melts and its pins fall off. The shiny plate reflects the heat and stays cool longer.

📝 Teacher's Note: This perfectly explains why we wear light-colored clothes in summer—they reflect the sun's heat instead of absorbing it like black clothes would.

🎯 Exam Tip: The melting wax is the indicator that one surface is absorbing more heat than the other.

Question 26. How can we minimize heat loss due to convection, conduction, and radiation?
Answer:

• As convection require medium for transfer of heat so heat loss due to convection can be minimized if the substance is kept away from any material medium.
• Similarly for conduction, as conduction also require medium for transfer of heat so heat loss due to conduction can be minimized if the substance is kept away from any material medium.
• Heat loss due to radiation can be minimized by polishing the surface as polished surfaces radiate less heat.

In simple words: To stop heat from moving by touching or through air, keep it in a vacuum. To stop heat from escaping as waves, make the surface shiny.

📝 Teacher's Note: This is the theory behind the vacuum flask discussed earlier. Vacuum stops the first two, and polishing (silvering) stops the third.

🎯 Exam Tip: Use the word "vacuum" or "away from material medium" for conduction and convection.

Question 27. How does a suspended ball in a vacuum lose heat?
Answer:

• As no material medium surrounds the ball so there is no possibility of losing heat by conduction or convection. The ball will lose heat by Radiation.
• Ball will lose minimum amount of heat if it is polished.
• As loss of heat take place by Radiation and process of radiation does not require any material medium, so there is no role of air in loss of heat.

In simple words: In a vacuum, heat can't touch or move through air, so it must travel as radiation waves. If the ball is shiny, it holds its heat better.

📝 Teacher's Note: This is why satellites in space are often covered in shiny gold or silver foil—to control how much heat they lose or gain by radiation.

🎯 Exam Tip: Radiation is the "lone survivor" of heat transfer modes when there is no air.

Question 28. Define CFC, UV, and WHO.
Answer:

• CFC means Chlorofluorocarbons which are produced mainly by refrigeration industries, aerosol cans, coolants, propellants and plastic foams. They cause global warming.
• UV means ultraviolet range of radiations. These are the radiations which are emitted by sun and they enter our atmosphere.
• WHO is World Health Organization which put a check on diseases and global warming etc.

In simple words: CFCs are chemicals from fridges that hurt the environment. UV is a type of sunlight that can be harmful. WHO is the global group that looks after everyone's health.

📝 Teacher's Note: Connect CFCs to the "Ozone layer" as well. They are famous for being greenhouse gases and for destroying Earth's protective shield.

🎯 Exam Tip: Memorize the full forms of these abbreviations as they frequently appear in environmental science sections.

Question 29. What is the greenhouse effect?
Answer: Green house is a glass house constructed to keep delicate and rare plants that require warmth and protection from weather. It is constructed by panels of glass. The temperature in a green house rises because heat enters the house through the glass but cannot escape out, because the glass reflects the heat back to the inside of the house. This makes glass house warmer than the outside environment. This phenomenon is called green house effect.
In simple words: The greenhouse effect is when the Earth's atmosphere acts like a glass roof, trapping heat from the sun and keeping the planet warm.

📝 Teacher's Note: Explain that while some greenhouse effect is good (it keeps Earth from being a block of ice), too much of it causes global warming.

🎯 Exam Tip: The term "trapping of solar radiation" is the core concept to remember.

Question 30. Name three greenhouse gases.
Answer: Carbon dioxide, Carbon monoxide, methane are green house gases.
In simple words: These are the gases in the air that are responsible for trapping heat and making the world warmer.

📝 Teacher's Note: Carbon dioxide is the most famous one, but methane is actually much more powerful at trapping heat!

🎯 Exam Tip: Carbon dioxide (\( \text{CO}_2 \)) and Methane (\( \text{CH}_4 \)) are the two most important ones to list.

Question 31. Why should we not use fossil fuels extensively?
Answer: As fossil fuels are non-renewable sources of energy they are not available again and again if are fully exhausted, so they should not be used extensively.
In simple words: We only have a limited amount of coal and oil. Once they are all used up, they are gone forever, so we must be careful with them.

📝 Teacher's Note: Mention that burning fossil fuels also releases the greenhouse gases we just discussed, which is another reason to use less of them.

🎯 Exam Tip: Use the term "non-renewable" to describe why fossil fuels are limited.

Question 32. What causes global warming?
Answer: Global warming occurs due to the presence of carbon di- oxide, CFCs, methane in the atmosphere. Carbon dioxide acts as a transparent gas to incoming shortwave radiations which the earth re-radiates into space. It therefore traps the outgoing longwave radiations thus warming lower atmosphere of the earth thereby causing global warming.
In simple words: Gases like \( \text{CO}_2 \) let sunlight in but won't let the heat leave. This "trapped heat" is what makes the whole Earth's temperature go up.

📝 Teacher's Note: Explain the difference between "shortwave" (light) and "longwave" (heat). Greenhouse gases are like a one-way filter for these rays.

🎯 Exam Tip: Focus on the "trapping of outgoing longwave radiation" for a high-scoring answer.

Question 33. What are the harmful effects of global warming?
Answer: Harmful effects of global warming are:

• The atmospheric temperature of earth would increase thereby making it difficult for a living being to survive.
• It would melt down the polar caps thus increasing the size of the ocean and leading to floods, tsunami, etc.
• The increase in temperature would affect climate and rainfall thus affecting flora and fauna.
• Human beings would be vulnerable to diseases as microbes would get warmth to grow.

In simple words: Global warming makes the world too hot, melts ice at the poles (causing floods), changes the weather, and makes it easier for germs to spread.

📝 Teacher's Note: Discuss the "melting of polar caps"—this is a major concern because it destroys the habitats of animals like polar bears and raises sea levels globally.

🎯 Exam Tip: List at least three distinct points, such as "melting ice caps," "climate change," and "health risks," to get full marks.