Question Answers For All Chapters – General Science Class 7
Heat
Exercise
1. Fill in the blanks with the proper word from the brackets.
(a) Maximum heat is absorbed by a black coloured object.
(b) Radiation of heat does not require a medium.
(c) Conduction of heat takes place through a good conductor substance.
(d) The shining surface in a thermos flask decreases the outgoing heat by reflection.
(e) Cooking utensils are made from metals due to their property of good conduction.
(f) The earth receives heat from the sun by radiation.
2. What will absorb heat?
Objects that absorb heat well:
- Steel spoon
- Iron griddle (tava)
- Soil
- Water
Objects that do not absorb heat well (poor conductors):
- Wooden board
- Glass vessel
- Glass
- Wooden spoon
- Plastic plate
- Wax
3. Write answers to the following questions.
(a) How does a fever get lowered by putting a cold compress on the forehead of a patient?
→ The cold cloth absorbs heat from the forehead, cooling the body. This helps lower the body temperature.
(b) Why are the houses in Rajasthan painted white?
→ White color reflects most of the heat from the sun, keeping houses cooler in the hot climate.
(c) What are the modes of heat transfer?
→ The three modes of heat transfer are:
- Conduction – Transfer of heat through solids.
- Convection – Transfer of heat through liquids and gases.
- Radiation – Transfer of heat without a medium.
(d) Explain which mode of heat transfer causes sea breezes and land breezes.
→ Convection causes sea and land breezes.
- In the day, land heats up faster than water, making the air above it rise and cooler sea air move in (sea breeze).
- At night, land cools faster than water, so warm air over the sea rises and cold land air moves in (land breeze).
(e) Why is the outer coat of the penguins of Antarctica black?
→ Black color absorbs more heat from sunlight, helping penguins stay warm in the cold climate of Antarctica.
(f) Why are heaters fitted near the floor and air conditioners near the ceiling of a room?
→ Hot air rises and cold air sinks due to convection.
- Heaters are placed near the floor so heat rises and warms the room.
- ACs are placed near the ceiling so cool air moves down to cool the room.
4. Give scientific reasons.
(a) An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not.
→ Ordinary glass expands quickly and cracks due to sudden heat. Borosil glass expands slowly, so it does not crack.
(b) The telephone wires which sag in summer become straight in winter.
→ In summer, metal expands due to heat, making wires sag. In winter, metal contracts due to cold, making them tight.
(c) Dew drops form on the grass in winter.
→ At night, air cools down, and moisture in the air condenses into tiny water droplets on grass, forming dew.
(d) In winter, why does an iron pillar feel colder than a wooden pole?
→ Iron is a good conductor of heat, so it absorbs heat from our hand, making it feel cold. Wood is a bad conductor, so it does not feel as cold.
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