9. Heat
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.
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Exercises
(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 conductor.
(f) The earth receives heat from the sun by radiation.
All these substances absorb heat. But they absorb it at different rates.
- Steel spoon and iron tava are good conductors. They absorb heat quickly and become hot fast.
- Wooden board, wooden spoon, plastic plate, glass vessel and glass are bad conductors. They absorb heat slowly.
- Soil, water and wax also absorb heat. Wax melts on absorbing heat.
(a) How does a fever get lowered by putting a cold compress on the forehead of a patient?
During fever, the body temperature is high. Heat flows from a hot object to a cold object. The cold wet cloth takes heat from the forehead. The water in the cloth also evaporates and takes away heat. So the body temperature comes down.
(b) Why are the houses in Rajasthan painted white?
Rajasthan is very hot. White colour reflects most of the heat. So the walls do not get very hot. Hence the houses remain cool inside.
(c) What are the modes of heat transfer?
Heat is transferred in three ways:
- Conduction: Heat passes from the hot part to the cold part of a solid. Example: the handle of a spoon in hot milk becomes hot.
- Convection: Heat is carried by the movement of particles of a liquid or gas. Example: heating of water in a vessel.
- Radiation: Heat is transferred without any medium. Example: heat from the sun reaches the earth.
(d) Explain which mode of heat transfer causes sea breezes and land breezes.
Sea breezes and land breezes are caused by convection.
Convection currents in air near the coast by day and by night.
- Sea breeze (day): Land gets heated faster than sea water. The air above land becomes warm and rises. Cool air from the sea flows towards the land.
- Land breeze (night): Land cools faster than sea water. The air above the sea is warmer and rises. Cool air from the land flows towards the sea.
(e) Why is the outer coat of the penguins of Antarctica black?
Antarctica is extremely cold. Black colour absorbs the maximum heat. So the black coat absorbs heat from sunlight. This keeps the penguin’s body warm.
(f) Why are heaters fitted near the floor and air conditioners, near the ceiling of a room?
Heater near the floor: Hot air is light and rises up. Cool air comes down and gets heated. These convection currents warm the whole room.
Air conditioner near the ceiling: Cool air is heavy and moves down. Warm air rises up and gets cooled. So the whole room becomes cool.
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(a) An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not.
Glass is a bad conductor of heat. When boiling water is poured, the inner surface becomes hot and expands. The outer surface stays cool and does not expand. This unequal expansion cracks the bottle. Borosil glass expands very little on heating. So it does not crack.
(b) The telephone wires which sag in summer become straight in winter.
Metals expand on heating and contract on cooling. In summer, the wires expand and become longer. So they sag. In winter, the wires contract and become shorter. So they become straight.
(c) Dew drops form on the grass, in winter.
On winter nights, the grass becomes very cold. The air touching the grass also cools down. The water vapour in this air condenses. It forms tiny water drops on the grass. These are dew drops.
(d) In winter, why does an iron pillar feel colder than a wooden pole?
Iron is a good conductor of heat. When we touch it, heat from our hand flows into it quickly. So it feels very cold. Wood is a bad conductor. It takes heat from our hand slowly. So it does not feel so cold.
- Conduction: A tava becomes hot on the stove. The handle of a spoon in hot tea becomes hot. Ironing clothes.
- Convection: Heating of water or milk in a vessel. Smoke rising from a chimney. Sea and land breezes.
- Radiation: Heat of the sun reaching the earth. Warming hands near a fire. Drying clothes in sunlight.