HEAT TRANSFER IN NATURE : Class 7 : Science
SUMMARY –
Heat always flows from a hotter object to a colder one. In nature, heat transfer happens in three main ways:
1. Conduction
Heat transfer through direct contact of particles.
Mostly in solids (like metals).
Example: Heating of sand or rocks by the Sun, cooking utensils getting hot.
2. Convection
Heat transfer by movement of fluids (liquids and gases).
Hot fluids rise, cold fluids sink → convection currents.
Examples in nature:
Sea breeze (cool air from sea moves towards land at day).
Land breeze (cool air from land moves towards sea at night).
Hot air balloon rising.
Convection currents inside Earth cause volcanic eruptions.
3. Radiation
Heat transfer without any medium (no contact needed).
Heat travels in the form of electromagnetic waves.
Example: Heat from the Sun reaching Earth, heat felt near a fire.
SHORT QUESTION ANSWER
Define conduction.
Transfer of heat through direct contact of particles without movement of matter.
Give two examples of conduction in daily life.
👉 Heating a metal spoon in hot tea, ironing clothes.
Why is metal a good conductor of heat?
👉 Because metals have free electrons that transfer heat quickly.
Why is wood a bad conductor of heat?
👉 It has air trapped inside which prevents heat transfer.
Why are cooking utensils made of metal but have plastic/wooden handles?
👉 Metal conducts heat for cooking, handles are insulators for safe holding.
Define convection.
👉 Heat transfer in liquids and gases due to movement of particles.
Give two natural examples of convection.
👉 Sea breeze, boiling of water.
Why does hot air rise?
👉 Hot air is lighter (less dense) than cold air.
Explain sea breeze.
👉 During day, land heats up faster than sea → hot air rises → cool air from sea moves to land.
Explain land breeze.
👉 At night, land cools faster → hot air rises over sea → cool air from land moves to sea.
How does convection help birds to fly high?
👉 Birds use upward-moving hot air currents to glide without much effort.
Give an example of convection in the human body.
👉 Warm blood circulation helps distribute heat inside the body.
Why do chimneys in kitchens or factories release smoke upward?
👉 Because hot air and smoke rise up due to convection.
How does a hot air balloon rise?
👉 Air inside balloon is heated, becomes lighter, and rises.
Convection inside the Earth causes which natural phenomenon?
👉 Volcanic eruptions and earthquakes.
Define radiation.
👉 Transfer of heat in the form of electromagnetic waves without a medium.
Give one natural example of radiation.
👉 Sunlight reaching the Earth.
Why does heat from the Sun reach Earth even though space has no air?
👉 Because radiation does not need any medium.
Why do we wear white clothes in summer?
👉 White reflects heat radiation and keeps us cool.
Why do we wear dark clothes in winter?
👉 Dark colors absorb more heat radiation and keep us warm.
Why does a black car become hotter than a white car in the sun?
👉 Black absorbs more radiation than white.
Give an example of radiation in cooking.
👉 Heat felt from a burning fire or microwave cooking.
Why does a desert become very hot during day and cool at night?
👉 Sand absorbs radiation quickly but loses heat rapidly.
Why do we feel warm near a campfire even if the air around is cold?
👉 Heat is transferred by radiation.
Which surfaces are good absorbers of radiation?
👉 Dark and rough surfaces.
Name the three modes of heat transfer.
👉 Conduction, convection, radiation.
In which medium does conduction mainly occur?
👉 Solids.
In which medium does convection mainly occur?
👉 Liquids and gases.
Which mode of heat transfer does not need any medium?
👉 Radiation.
Which is the fastest mode of heat transfer?
👉 Radiation.
Which is the slowest mode of heat transfer?
👉 Conduction.
Why do cooking pans have shiny bottoms?
👉 To reduce heat loss by radiation.
Why do we put a lid on a cooking pot?
👉 To reduce heat loss by convection.
Which heat transfer causes winds on Earth?
👉 Convection.
Which heat transfer keeps the Earth warm from the Sun?
👉 Radiation.
Why do birds fly in circles during the day?
👉 They use rising hot air convection currents.
Which type of heat transfer helps water in a pot boil?
👉 Convection.
Why do metal roofs of houses become very hot in summer?
👉 Because metals conduct heat.
Why are thermos flasks shiny from inside?
👉 To prevent heat loss by radiation.
Write one difference between conduction and convection.
👉 Conduction: No movement of particles; Convection: Transfer by movement of particles.
LONG QUESTION–ANSWERS
Explain conduction of heat with examples.
👉 Conduction is the transfer of heat through direct contact of particles in solids. When one end of a metal rod is heated, particles vibrate and transfer heat to neighboring particles. Examples: heating a spoon in hot tea, ironing clothes.
Why are metals good conductors and wood/plastic bad conductors?
👉 Metals contain free electrons that transfer heat quickly, so they are good conductors. Wood/plastic contain trapped air pockets, which slow heat flow, making them insulators.
Why are cooking vessels made of metal but handles made of plastic or wood?
👉 Metals conduct heat well for cooking, but handles made of wood/plastic (insulators) prevent burns and make them safe to use.
Describe an activity to show conduction in metals.
👉 Fix pins with wax on a metal rod. Heat one end. Pins fall one by one as heat travels through conduction from particle to particle.
Why do we wear woolen clothes in winter? Explain in terms of heat transfer.
👉 Wool traps air (a poor conductor). Air prevents heat from escaping our body, keeping us warm.
What is convection? Explain with an example.
👉 Convection is heat transfer in liquids and gases through the movement of particles. Example: boiling water, where hot water rises and cold water sinks.
Describe the process of sea breeze with diagram explanation.
👉 During the day, land heats up faster than water. Hot air rises over land and cool sea air moves to replace it, creating sea breeze.
Describe the process of land breeze.
👉 At night, land cools faster than sea. Hot air rises over sea, and cool air from land moves toward sea, creating land breeze.
How does convection help in heating water?
👉 When water is heated at the bottom of a pot, hot water rises and cold water sinks, forming convection currents that evenly distribute heat.
Explain how convection is responsible for winds on Earth.
👉 The Sun heats Earth unevenly, creating pressure differences. Warm air rises, cool air sinks, producing winds and weather patterns.
How do birds and gliders make use of convection currents?
👉 Birds and gliders ride on rising hot air currents to fly high with less effort.
How does a hot air balloon rise?
👉 Air inside balloon is heated, becomes lighter and rises due to convection, lifting the balloon.
Explain why smoke from chimneys always rises upward.
👉 Smoke is hot and lighter than surrounding air. Convection currents push smoke upward.
How do convection currents inside the Earth cause volcanoes?
👉 Heat from Earth’s core causes molten rock to rise; pressure builds up, leading to volcanic eruptions.
Why is ventilation provided near ceilings in rooms?
👉 Hot air rises up, so ventilators at the top allow hot air to escape and cool air enters from below.
What is radiation? Explain with example.
👉 Radiation is heat transfer without medium, in the form of electromagnetic waves. Example: Sun’s heat reaching Earth.
How does Earth receive heat from the Sun?
👉 Space has no medium, so conduction/convection cannot occur. Sun’s heat travels by radiation.
Why are black surfaces hotter than white in the Sun?
👉 Black absorbs more radiation, white reflects radiation.
Why do we wear light-colored clothes in summer and dark-colored in winter?
👉 Light colors reflect heat (cool effect in summer). Dark colors absorb heat (warm effect in winter).
Why do desert areas become very hot in day and cold at night?
👉 Sand absorbs radiation quickly but also loses heat rapidly.
Explain why polished surfaces are poor absorbers but good reflectors of heat.
👉 Smooth polished surfaces reflect radiation, while rough dark surfaces absorb it.
How does radiation help in cooking?
👉 Heat radiated from flames or a microwave oven cooks food without direct contact.
Why do we feel warm near a fire even without touching it?
👉 Heat travels by radiation directly to our body.
Why are thermos flasks shiny inside?
👉 Shiny surface reflects radiation, reducing heat loss.
Which surfaces are good absorbers of heat? Which are poor absorbers?
👉 Dark, rough surfaces = good absorbers. Light, shiny surfaces = poor absorbers.
Compare conduction, convection, and radiation with examples.
👉 Conduction = solids (metal rod heating), Convection = liquids/gases (boiling water, winds), Radiation = vacuum (sunlight).
Which mode of heat transfer is fastest and which is slowest? Why?
👉 Fastest = radiation (no medium needed). Slowest = conduction (particle-to-particle transfer).
Explain the role of heat transfer in sea and land breeze.
👉 Land and water heat differently, causing convection currents that create sea and land breezes.
Explain with examples how heat transfer is important in daily life.
👉 Cooking (conduction), boiling water (convection), Sunlight warming Earth (radiation).
Why does a metal roof heat faster in summer?
👉 Metal conducts heat quickly and absorbs radiation strongly.
How does greenhouse effect involve radiation?
👉 Sunlight enters atmosphere, Earth radiates heat, but gases trap it, warming Earth.
Why does a pressure cooker cook faster?
👉 Convection of steam inside increases pressure and temperature, cooking food quickly.
Explain how heat transfer plays a role in weather and climate.
👉 Convection currents create winds, ocean currents, and rainfall patterns. Radiation from Sun drives the process.
Describe an experiment to show convection currents in water.
👉 Heat water with potassium permanganate at bottom of beaker. Colored water rises showing convection current.
Why do we need all three modes of heat transfer in nature?
👉 Conduction warms solids, convection distributes heat in fluids, radiation brings heat from Sun. Together they balance Earth’s temperature.
MCQS
Heat transfer in solids mainly takes place by –
a) Convection
b) Conduction
c) Radiation
d) Evaporation
👉 b) Conduction
A metal spoon becomes hot when kept in hot soup due to –
a) Convection
b) Radiation
c) Conduction
d) Evaporation
👉 c) Conduction
Which of the following is a good conductor of heat?
a) Wood
b) Plastic
c) Copper
d) Rubber
👉 c) Copper
Which of the following is a bad conductor of heat?
a) Iron
b) Aluminum
c) Glass
d) Steel
👉 c) Glass
Cooking vessels are made of metals because –
a) They are cheap
b) They are shiny
c) They are good conductors of heat
d) They are light
👉 c) They are good conductors of heat
Why do handles of cooking utensils have plastic/wood?
a) To look nice
b) To save cost
c) To prevent burns
d) To conduct more heat
👉 c) To prevent burns
In which medium is conduction the slowest?
a) Solids
b) Liquids
c) Gases
d) Vacuum
👉 c) Gases
Which one is an insulator?
a) Aluminum
b) Silver
c) Copper
d) Rubber
👉 d) Rubber
Which of these is an example of conduction?
a) Sea breeze
b) Heating water
c) Ironing clothes
d) Sunlight
👉 c) Ironing clothes
Which of the following materials is NOT a conductor?
a) Iron
b) Copper
c) Plastic
d) Steel
👉 c) Plastic
Convection
Heat transfer in liquids and gases mainly occurs by –
a) Conduction
b) Convection
c) Radiation
d) Reflection
👉 b) Convection
Sea breeze is caused due to –
a) Conduction
b) Convection
c) Radiation
d) None
👉 b) Convection
At night, cool air from land moves to sea. This is called –
a) Sea breeze
b) Land breeze
c) Wind storm
d) Cyclone
👉 b) Land breeze
Why does hot air rise upward?
a) It is heavier
b) It is lighter
c) It reflects sunlight
d) It is cooler
👉 b) It is lighter
Which of the following is NOT due to convection?
a) Sea breeze
b) Land breeze
c) Boiling water
d) Heating a spoon
👉 d) Heating a spoon
In which case does convection NOT occur?
a) Water
b) Air
c) Vacuum
d) Steam
👉 c) Vacuum
Why do smoke and hot gases rise in chimneys?
a) Due to radiation
b) Due to conduction
c) Due to convection
d) Due to reflection
👉 c) Due to convection
Which process helps hot air balloons rise?
a) Radiation
b) Conduction
c) Convection
d) Evaporation
👉 c) Convection
Birds fly high using –
a) Radiation currents
b) Conduction currents
c) Convection currents
d) None
👉 c) Convection currents
Which natural phenomenon is caused by convection currents inside the Earth?
a) Rainbow
b) Volcanoes
c) Eclipse
d) Rainfall
👉 b) Volcanoes
Radiation
Which heat transfer does not need any medium?
a) Conduction
b) Convection
c) Radiation
d) Evaporation
👉 c) Radiation
Earth receives heat from the Sun by –
a) Conduction
b) Convection
c) Radiation
d) Reflection
👉 c) Radiation
Which surfaces are good absorbers of radiation?
a) White and shiny
b) Dark and rough
c) Transparent
d) Silver
👉 b) Dark and rough
Which type of clothing is best in summer?
a) Black
b) White
c) Red
d) Blue
👉 b) White
Which type of clothing is best in winter?
a) White
b) Black
c) Yellow
d) Green
👉 b) Black
Why do polished surfaces feel cooler in sunlight?
a) They absorb heat
b) They reflect heat
c) They produce heat
d) They store heat
👉 b) They reflect heat
Which of the following is an example of radiation?
a) Boiling water
b) Heating a rod
c) Sunlight
d) Sea breeze
👉 c) Sunlight
Why do deserts become very hot in day?
a) Due to conduction
b) Due to convection
c) Due to radiation
d) Due to rain
👉 c) Due to radiation
Why are thermos flasks shiny inside?
a) To look pretty
b) To reflect radiation
c) To store food
d) To prevent convection
👉 b) To reflect radiation
Why do we feel warm near a campfire?
a) Due to convection
b) Due to conduction
c) Due to radiation
d) Due to condensation
👉 c) Due to radiation
General / Mixed
Which is the slowest mode of heat transfer?
a) Conduction
b) Convection
c) Radiation
d) Reflection
👉 a) Conduction
Which is the fastest mode of heat transfer?
a) Conduction
b) Convection
c) Radiation
d) None
👉 c) Radiation
Which heat transfer occurs in vacuum?
a) Conduction
b) Convection
c) Radiation
d) Evaporation
👉 c) Radiation
Which heat transfer occurs in boiling water?
a) Conduction
b) Convection
c) Radiation
d) All
👉 b) Convection
Which is responsible for weather and winds on Earth?
a) Radiation
b) Conduction
c) Convection
d) Reflection
👉 c) Convection
Which heat transfer takes place when we touch a hot pan?
a) Convection
b) Radiation
c) Conduction
d) None
👉 c) Conduction
Which transfer of heat is responsible for greenhouse effect?
a) Conduction
b) Convection
c) Radiation
d) None
👉 c) Radiation
Why do woolen clothes keep us warm?
a) Wool is conductor
b) Wool traps air, which is insulator
c) Wool reflects sunlight
d) Wool absorbs water
👉 b) Wool traps air, which is insulator
Which process is used in microwave ovens?
a) Conduction
b) Convection
c) Radiation
d) Reflection
👉 c) Radiation
In a pressure cooker, food is cooked faster due to –
a) Conduction only
b) Convection of steam under pressure
c) Radiation only
d) Evaporation
👉 b) Convection of steam under pressure
