Class 7 Science NCERT Notes – Chapter 7: Heat Transfer in Nature (PDF, MindMap, Q&A, Quizzes)

Chapter 7 (Science): Heat Transfer in Nature – CBSE Class 7 NCERT Science Detailed Study Notes.

1. Introduction to Heat and Climate

The primary source of heat and light for Earth is the Sun. The climate of a location is significantly influenced by its proximity to the equator and geographical features like coastlines. For example, Kerala, being closer to the equator and having a long coastline, experiences warmer and more humid weather compared to the colder climate of Gangtok in Sikkim.

2. Modes of Heat Transfer

Heat is transferred from one place to another in three distinct ways: conduction, convection, and radiation. Understanding these processes is key to explaining many natural phenomena and technological applications.

2.1 Conduction

Conduction is the process of heat transfer from the hotter part of an object to its colder part. In this process, particles pass heat to their neighbors without moving from their own positions. Conduction is the primary mode of heat transfer in solids.

Conductors and Insulators:

• Good Conductors: Materials that allow heat to pass through them easily. Metals, such as aluminium, iron, and steel, are good conductors. This property is why cooking utensils are made of metal.
• Poor Conductors (Insulators): Materials that do not allow heat to pass through them easily. Examples include wood, glass, clay, porcelain, mud, and cow dung. Air is also a poor conductor of heat.

Applications of Conduction Principles:

• Cooking: Metal pans and utensils are used to efficiently transfer heat to food.
• Beverages: Clay and porcelain cups are used for hot drinks like tea or coffee because they are poor conductors, helping the beverage stay hot longer.
• Winter Clothing: Woollen clothes keep us warm by trapping air in their pores. Since air is a poor conductor, it reduces the flow of heat from our bodies to the colder surroundings.
• Blankets: Two thin blankets are often warmer than one single thick blanket. The layer of air trapped between the two thin blankets acts as an insulator, preventing heat loss.
• Housing in Extreme Climates:
  • In the Himalayas, houses are built with walls made of two wooden layers filled with cow dung and mud. These materials are poor conductors and prevent heat loss, keeping the house warm.
  • Hollow bricks are used in construction in both hot and cold climates. The air trapped inside the hollow bricks is a poor conductor, keeping houses warm in winter and cool in summer.

2.2 Convection

Convection is the process of heat transfer that occurs through the actual movement of particles. This is the primary mode of heat transfer in liquids and gases. When a fluid (liquid or gas) is heated, it expands, becomes less dense (lighter), and rises. Cooler, denser fluid moves down to take its place, gets heated, and also rises, creating a continuous cycle known as a convection current.

Examples and Applications of Convection:

• Rising Smoke: Smoke, which is a mixture of hot gases and particles, is warmer and lighter than the surrounding air, so it rises. This is why smoke detectors are most effective when placed on the ceiling.
• Heating Water: When water in a beaker is heated from the bottom, the water at the bottom gets hot, expands, and rises. Cooler water from the sides and top moves down to replace it, creating a convection current that heats the entire volume of water.
• Sea Breeze: In coastal areas during the day, the land heats up faster than the sea. The warm air above the land rises, and cooler, heavier air from the sea moves in to take its place. This movement of cooler air from the sea to the land is called a sea breeze.
• Wind Reversal: The direction of wind in coastal areas reverses between day and night due to the differential heating and cooling rates of land and sea, demonstrating a daily convection cycle.

2.3 Radiation

Radiation is the process of heat transfer that does not require any medium. Heat can travel through empty space via radiation. All objects radiate heat to their surroundings and absorb heat from them.

Examples and Applications of Radiation:

• Sun’s Heat: The heat from the Sun reaches Earth through the vacuum of space by the process of radiation.
• Fireplace: A person sitting near a fireplace feels warm due to the direct transfer of heat by radiation.
• Cooling Objects: A hot utensil left away from a flame cools down by radiating its heat to the surroundings.
• Clothing Color:
  • Summer: Light-colored or white clothes are preferred because they reflect most of the heat that falls on them, keeping the wearer cool.
  • Winter: Dark-colored clothes are preferred because they absorb more heat, keeping the wearer warm.

The Bukhari: A traditional room heater used in the Himalayas, known as a bukhari, demonstrates all three forms of heat transfer.

• Radiation: The heater radiates heat, warming the room and people in it.
• Convection: The air around the bukhari gets heated and circulates throughout the room via convection currents.
• Conduction: The flat top of the iron stove gets hot through conduction, allowing it to be used for cooking.

3. The Water Cycle

The water cycle is the continuous movement of water on, above, and below the surface of the Earth. It is a vital process that redistributes and replenishes water resources and is driven by the Sun’s heat.

Key Stages of the Water Cycle:

1. Evaporation & Transpiration: The Sun’s heat causes water from oceans, rivers, and lakes to turn into water vapor and rise into the atmosphere (evaporation). Plants also release water vapor from their leaves in a process called transpiration.
2. Condensation: As the warm, moist water vapor rises, it cools down and condenses to form tiny water droplets or ice crystals, which come together to form clouds.
3. Precipitation: When the clouds become saturated, the water falls back to Earth in the form of rain, snow, or hail.

4. Groundwater System

A significant portion of Earth’s freshwater is stored underground. This groundwater system is replenished by the water cycle.

4.1 Infiltration and Seepage

• Infiltration (or Seepage): The process by which surface water from rain or melting snow seeps through soil and rocks into the ground.
• Rate of Seepage: The rate at which water infiltrates depends on the type of material. Water seeps fastest through materials with large, interconnected spaces between particles, such as gravel. It moves slower through sand and slowest through clay, which has very small spaces between its particles.

4.2 Groundwater and Aquifers

• Groundwater: Water that has infiltrated the ground and is stored in the pore spaces of sediments and openings in rocks.
• Aquifer: An underground layer of sediments and rocks that holds and stores groundwater. This is the source of water for wells and bore wells.

4.3 Groundwater Management

• Depletion: Groundwater is a limited resource that is being depleted due to excessive extraction for a growing population, decreased vegetation cover, and an increase in concrete surfaces in urban areas which limits infiltration.
• Replenishment: To ensure a sustainable supply, groundwater must be replenished. The water cycle naturally recharges these sources. Human-led efforts like rainwater harvesting and the construction of recharge pits also help replenish groundwater.

5. Water Conservation Innovations

• Ice Stupas (Ladakh): In regions like Ladakh, where streams dry up in spring, people have created ice stupas. During winter, water from mountain streams is piped down and sprayed into the cold air. The water freezes layer by layer, forming a large, cone-shaped ice structure. This ice stupa melts slowly during the spring and summer, providing a crucial water source for farming and other needs.

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II. Quiz and Q&A Section

Short-Answer Questions (Answer in 2-3 sentences)

1. Why is Kerala’s winter climate described as warmer and more humid compared to Gangtok’s?
2. What is the primary mode of heat transfer in solids, and how does it work at the particle level?
3. Explain why cooking utensils are typically made of metal.
4. Define what a poor conductor of heat (or insulator) is and provide two examples from the text.
5. How do woollen clothes help in keeping us warm during winter?
6. Explain the principle that makes two thin blankets warmer than a single thick one.
7. What is convection, and in what states of matter does it primarily occur?
8. Describe the process that causes smoke from a fire or an incense stick to rise.
9. What is a sea breeze, and what causes it to occur during the day?
10. Does heat transfer by radiation require a medium? Provide an example of radiation.
11. Why is it more comfortable to wear light-colored clothes in the summer?
12. Why are dark-colored clothes preferred during winter?
13. What is a bukhari, and how does it demonstrate all three modes of heat transfer?
14. Name the three main processes involved in the water cycle that move water from the Earth’s surface to the atmosphere and back.
15. What is the difference between evaporation and transpiration?
16. Define infiltration in the context of the water cycle.
17. Arrange the following materials in order of fastest to slowest seepage rate for water: sand, clay, gravel. Explain your reasoning.
18. What is an aquifer?
19. How is groundwater typically extracted for human use?
20. List two factors contributing to the depletion of groundwater.
21. What is the purpose of an ice stupa in Ladakh?
22. How is an ice stupa created?
23. Why are smoke detectors usually installed on the ceiling of a room?
24. Explain why houses in coastal areas often have windows facing the sea.
25. Who was Varahamihira and what was his contribution mentioned in the text?

Multiple-Choice Questions (MCQ)

1. Which process of heat transfer is responsible for the Sun’s heat reaching Earth? a) Conduction b) Convection c) Radiation d) Infiltration
2. Materials that allow heat to pass through them easily are called: a) Insulators b) Poor conductors c) Aquifers d) Good conductors
3. In which of the following does heat transfer occur mainly through conduction? a) A metal strip b) Air in a room c) Water in a beaker d) The vacuum of space
4. A saucepan consists of a metal part (A) for cooking and a handle (B). Which statement is correct? a) A is a poor conductor and B is a good conductor of heat. b) Both A and B are good conductors of heat. c) A is a good conductor and B is a poor conductor of heat. d) Both A and B are poor conductors of heat.
5. The movement of cooler air from the sea towards the land during the day is known as: a) Land breeze b) Sea breeze c) Conduction current d) Radiation flow
6. Which of the following materials is the best insulator of heat? a) Steel b) Air c) Iron d) Aluminium
7. Why do hollow bricks help keep houses cool in summer and warm in winter? a) The brick material reflects all heat. b) The air trapped inside the bricks is a poor conductor of heat. c) The bricks absorb all external heat. d) The bricks allow for rapid heat transfer.
8. In the experiment with pins attached to a metal strip by wax, which pin falls first when one end is heated? a) The pin furthest from the flame. b) The pin in the middle. c) The pin closest to the flame. d) All pins fall at the same time.
9. The process where water vapor cools and forms clouds is called: a) Evaporation b) Precipitation c) Condensation d) Transpiration
10. The continuous movement of water on, above, and below the Earth’s surface is called: a) The groundwater system b) The water cycle c) Infiltration d) Precipitation
11. Underground layers of rock and sediment that store water are known as: a) Recharge pits b) Wells c) Aquifers d) Ice stupas
12. Water would seep fastest through which of these materials? a) Clay b) Sand c) Gravel d) A block of wood
13. The process of surface water seeping through soil and rocks is called: a) Condensation b) Transpiration c) Precipitation d) Infiltration
14. Dark surfaces are good absorbers of heat. Therefore, we feel more comfortable with dark-colored clothes during: a) Summers b) Winters c) Rainy season d) All seasons
15. Heat transfer by convection involves: a) The actual movement of particles. b) Particles vibrating in fixed positions. c) Transfer through a vacuum. d) Reflection of heat waves.
16. Where would be the most suitable place to fit a smoke detector in a room? a) On the ceiling b) Near the floor c) In the middle of a wall d) Behind a curtain
17. Which of these is NOT a form of precipitation? a) Rain b) Snow c) Hail d) Water vapor
18. What is the main reason land heats up faster than water during the day? a) Land is darker than water. b) The text does not specify the reason, only the phenomenon. c) Water reflects all sunlight. d) Land has a higher capacity for heat absorption.
19. An ice stupa is an innovative water conservation method used in: a) Kerala b) Gangtok c) Ladakh d) Uttarkashi
20. Heat transfer in liquids and gases primarily occurs through: a) Conduction b) Convection c) Radiation d) Insulation

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III. Essay Questions and Answers

1. Describe the three modes of heat transfer: conduction, convection, and radiation. For each mode, provide a real-world example discussed in the text.
   • Answer: The three modes of heat transfer are conduction, convection, and radiation.
     • Conduction is the transfer of heat through a material from a hotter part to a colder part, where the particles transfer energy to their neighbors without moving from their positions. It is the primary method of heat transfer in solids. A key example is the heating of a metal cooking pan on a stove; the heat travels from the bottom of the pan to the food inside through conduction.
     • Convection is the transfer of heat through the actual movement of particles within a fluid (a liquid or gas). Warmed particles become less dense and rise, while cooler, denser particles sink, creating a convection current. An example is the formation of a sea breeze, where cooler air from the sea moves towards the land as the warmer air over the land rises.
     • Radiation is the transfer of heat in the form of electromagnetic waves, which does not require a medium and can travel through a vacuum. An example is the heat from the Sun reaching the Earth across the emptiness of space, or the warmth felt from a fireplace without touching it.
2. Explain the scientific principles behind the advice to wear light-colored clothes in summer and dark-colored clothes in winter.
   • Answer: The advice on clothing color is based on the principle of heat absorption and reflection through radiation. Dark-colored surfaces are excellent absorbers of heat radiation. In winter, wearing dark clothes helps the body absorb more heat from the sunlight and the surroundings, which helps in keeping warm. Conversely, light-colored or white surfaces are poor absorbers and good reflectors of heat. In summer, wearing light-colored clothes reflects most of the Sun’s heat that falls on them, reducing the amount of heat absorbed and helping the wearer feel more comfortable and cool.
3. Detail the entire water cycle as described in the source text, defining each key stage and explaining the Sun’s role in the process.
   • Answer: The water cycle is the continuous movement of water on Earth, driven primarily by heat from the Sun. It begins with evaporation, where the Sun’s radiation heats water in oceans, lakes, and rivers, turning it into water vapor that rises into the atmosphere. Plants also contribute to this process through transpiration, releasing water vapor from their leaves. As this warm water vapor rises, it cools and undergoes condensation, forming clouds of tiny water droplets. When these clouds become saturated, the water returns to Earth’s surface through precipitation in forms like rain, snow, or hail. This water then flows into water bodies or seeps into the ground, ready to begin the cycle anew, thus redistributing and replenishing Earth’s water.
4. How is groundwater formed and stored? Discuss the factors that affect the rate of water seepage and the challenges related to groundwater depletion.
   • Answer: Groundwater is formed through a process called infiltration or seepage, where surface water from rain and melting snow trickles down through soil and rocks. This water gets stored in underground layers of sediments and rocks called aquifers. The rate of seepage is affected by the composition of the ground material; water infiltrates fastest through materials with large, open, and interconnected spaces like gravel, and slowest through materials with small spaces like clay. Groundwater faces significant challenges, primarily depletion. This is caused by excessive extraction to meet the needs of an increasing population, as well as reduced infiltration due to decreased vegetation and the proliferation of concrete surfaces in urban areas which prevent water from seeping into the ground.
5. Explain why a house built with hollow bricks can be more comfortable in both hot summers and cold winters. Relate this to the properties of conductors and insulators.
   • Answer: A house built with hollow bricks provides better temperature regulation due to the insulating properties of air. Air is a poor conductor of heat, or an insulator. In hollow bricks, air gets trapped in the empty spaces. During hot summers, this trapped air reduces the amount of heat conducted from the hot exterior to the cooler interior of the house. During cold winters, the same trapped air acts as a barrier, slowing down the loss of heat from the warm interior to the cold outside. This use of air as an insulator helps keep the house cooler in summer and warmer in winter.
6. Using the experiment of heating a metal strip with pins attached by wax, explain the process of conduction in solids.
   • Answer: The experiment demonstrates conduction in a solid metal strip. When one end of the strip is heated by a candle, the heat energy causes the particles at that end to vibrate more intensely. These vibrating particles transfer their energy to their immediate neighbors, which in turn start vibrating and pass the energy further down the strip. This process continues from the hot end to the cold end. As the heat travels along the strip, it reaches each pin sequentially, melts the wax holding it, and causes the pin to fall. The observation that the pin closest to the flame (pin I) falls first, followed by II, III, and IV, proves that heat is being transferred progressively along the strip, which is the hallmark of conduction.
7. Describe the phenomenon of sea breeze. What conditions are necessary for it to occur, and what heat transfer process is responsible for it?
   • Answer: A sea breeze is the movement of cooler air from the sea to the land. It occurs in coastal areas during the day. The necessary condition is the differential heating of land and water; land heats up much faster than the sea under the Sun’s radiation. This causes the air above the land to become hot, expand, and rise. To fill the low-pressure area created by the rising warm air, the cooler, denser air from over the sea moves in towards the land. The heat transfer process responsible for this entire phenomenon is convection, which involves the bulk movement of air masses.
8. What is an ice stupa? Explain the scientific principles and the societal need behind this innovative water conservation technique.
   • Answer: An ice stupa is a man-made, cone-shaped glacier created to conserve water in high-altitude desert regions like Ladakh. The societal need arises from water scarcity during the spring, when streams dry up before the mountain glaciers have melted enough to supply water for farming. The technique involves channeling water from mountain streams through pipes during the winter and spraying it into the extremely cold air. The water freezes as it falls, gradually building up a large cone of ice. Because of its shape and volume, the ice stupa melts much more slowly in the spring and summer than a flat sheet of ice would, providing a steady and reliable source of water for the community’s needs.
9. Compare and contrast the three modes of heat transfer—conduction, convection, and radiation—in terms of their mechanisms and the requirement of a medium.
   • Answer:
     • Mechanism: Conduction transfers heat through direct particle-to-particle interaction without the particles changing position. Convection transfers heat through the bulk movement of the particles themselves in a fluid (liquid or gas). Radiation transfers heat via electromagnetic waves.
     • Medium Requirement: Conduction and convection both require a material medium for heat to be transferred. Conduction occurs mainly in solids, while convection occurs in liquids and gases. In contrast, radiation does not require a medium and can travel through a vacuum, which is how the Sun’s heat reaches Earth.
10. Explain how the water cycle is crucial for replenishing groundwater sources. What human activities are hindering this natural process?
    • Answer: The water cycle is essential for replenishing groundwater. Through the process of precipitation (rain, snow), water is returned to the Earth’s surface. A portion of this water then undergoes infiltration, seeping through the soil and rocks to recharge underground aquifers. This natural recharge ensures the long-term sustainability of groundwater supplies. However, human activities are hindering this process. Deforestation and the removal of vegetation cover reduce the soil’s ability to absorb water, leading to more runoff and less infiltration. Furthermore, urbanization involves creating large concrete surfaces (roads, buildings, parking lots) that are impermeable, preventing rainwater from seeping into the ground and recharging the aquifers below.

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IV. Answer Keys

Short-Answer Questions – Answer Key

1. Kerala is closer to the equator, making its climate generally hot. Its long coastline also contributes to more humid weather conditions compared to Sikkim, which is at a higher altitude and further from the equator.
2. The primary mode of heat transfer in solids is conduction. In this process, the particle that gets heated passes the heat to its neighbor through vibration, but the particles themselves do not move from their positions.
3. Cooking utensils are made of metal because metals are good conductors of heat. This property allows heat from the stove to be transferred efficiently and quickly through the utensil to the food being cooked.
4. A poor conductor, or insulator, is a material that does not allow heat to pass through it easily. Examples from the text include wood, glass, clay, and air.
5. Woollen clothes have fine pores that trap a layer of air. Since air is a poor conductor of heat, it acts as an insulator, reducing the flow of heat from our bodies to the colder surroundings, thus keeping us warm.
6. When using two thin blankets, a layer of air is trapped between them. Air is an insulator, so this trapped air reduces heat loss from the body more effectively than the single layer of a thick blanket.
7. Convection is the process of heat transfer through the actual movement of particles. It primarily occurs in liquids and gases (fluids).
8. The air and particles in smoke are heated by the fire, causing them to expand and become less dense (lighter) than the surrounding cooler air. This lighter, hot smoke then rises due to convection.
9. A sea breeze is the movement of cooler air from the sea to the land. It is caused by the land heating up faster than the water during the day, which makes the air above the land rise and creates a pressure difference.
10. No, heat transfer by radiation does not require a medium. An example is the heat from the Sun reaching Earth through the vacuum of space.
11. Light-colored clothes reflect most of the heat radiation that falls on them. This reduces the amount of heat absorbed by the person wearing them, making them feel more comfortable in the hot summer sun.
12. Dark-colored clothes absorb more heat radiation than light-colored clothes. In winter, this property helps a person absorb more warmth from the sun, making them feel more comfortable.
13. A bukhari is a traditional room heater in the Himalayas. It demonstrates radiation by warming the room directly, convection by heating the air which then circulates, and conduction through its hot metal top used for cooking.
14. The three main processes are evaporation (and transpiration), condensation, and precipitation.
15. Evaporation is the process of water from bodies like oceans and lakes turning into vapor. Transpiration is the specific process of water evaporating from trees and plants.
16. Infiltration is the process where surface water, such as rainwater, seeps down through soil and rocks to become groundwater.
17. The order is: Gravel (fastest), Sand (slower), and Clay (slowest). This is because the spaces between gravel particles are the widest, allowing water to pass through most easily.
18. An aquifer is an underground layer of sediments and rocks that stores groundwater in its pore spaces.
19. Groundwater is typically extracted by digging wells or drilling bore wells into aquifers.
20. Two factors are excessive groundwater extraction due to increasing population and limited water infiltration caused by decreased vegetation and increased concrete surfaces.
21. The purpose of an ice stupa is to store winter water in the form of ice. This ice then melts slowly during the dry spring and summer months, providing a reliable source of water for farming.
22. An ice stupa is made by piping water from mountain streams in winter and spraying it into the cold air. The freezing water builds up layer by layer into a large, cone-shaped structure.
23. Smoke is hot air and rises due to convection. Placing a smoke detector on the ceiling ensures it will detect the rising smoke from a fire as quickly as possible.
24. These windows allow the cool sea breeze, which blows from the sea to the land during the day, to enter the houses and provide relief from the heat.
25. Varahamihira was an astronomer and mathematician from the sixth century CE. In his work Brihatsamhita, he gave methods for predicting seasonal rainfall based on natural phenomena.

Multiple-Choice Questions – Answer Key

1. c) Radiation
2. d) Good conductors
3. a) A metal strip
4. c) A is a good conductor and B is a poor conductor of heat.
5. b) Sea breeze
6. b) Air
7. b) The air trapped inside the bricks is a poor conductor of heat.
8. c) The pin closest to the flame.
9. c) Condensation
10. b) The water cycle
11. c) Aquifers
12. c) Gravel
13. d) Infiltration
14. b) Winters
15. a) The actual movement of particles.
16. a) On the ceiling
17. d) Water vapor
18. b) The text does not specify the reason, only the phenomenon.
19. c) Ladakh
20. b) Convection

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V. Glossary of Key Terms

• Aquifer: An underground layer of sediments and rocks that stores water in its pore spaces.
• Bukhari: A traditional room heater used in the Himalayan region, consisting of an iron stove where wood or charcoal is burnt.
• Condensation: The process where water vapor rises, cools down, and turns into liquid water droplets or ice crystals, forming clouds.
• Conduction: The process of heat transfer from the hotter part of an object to a colder part, where particles pass heat to their neighbors without moving from their positions. It is the main mode of heat transfer in solids.
• Convection: The process of heat transfer in liquids and gases that occurs through the actual movement of particles.
• Evaporation: The process where water in oceans, rivers, and lakes gets heated by the Sun and turns into water vapor.
• Good Conductor of Heat: A material that allows heat to pass through it easily (e.g., metals).
• Groundwater: Water that has seeped through the ground and is stored in the pore spaces of sediments and rocks.
• Ice Stupa: An innovative water conservation technique used in Ladakh where water is frozen during winter into a tall, cone-shaped structure that melts slowly in spring to provide water.
• Infiltration: The process of surface water seeping through soil and rocks into the ground. Also referred to as seepage.
• Insulator (Poor Conductor of Heat): A material that does not allow heat to pass through it easily (e.g., wood, glass, air).
• Precipitation: The process by which clouds release water back to Earth in the form of rain, snow, or hail.
• Radiation: The process of heat transfer that does not require a medium and can travel through a vacuum.
• Sea Breeze: The movement of cooler air from the sea to the land that occurs during the day, caused by the land heating up faster than the water.
• Transpiration: The process by which water evaporates from trees and plants.
• Water Cycle: The continuous movement of water on, above, and below the Earth’s surface, involving processes like evaporation, condensation, and precipitation.