Solutions – Class 7-General Science-Chapter-9-Heat-Maharashtra Board

(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 conduction.

(f) The earth receives heat from the sun by radiation.

Steel spoon, iron griddle (tava), water and wax are the good conductors of heat. Hence, these substances would absorb heat.

Wooden board, glass vessel, glass, wooden spoon, plastic plate and soil are bad conductors. Hence, they would absorb very small amount of heat that can be neglected.

Heat is transferred from a hot body to a cold body.

• During fever, body temperature gets very high e., the body is hot compared to the surroundings.
• When a cold compress is put on the forehead of a patient suffering from high fever, the heat flow takes place from the head of the patient to the cold compress.
• Therefore, the cold compress absorbs heat from the patient's body and lowers the body temperature. 
• This lowers the fever of the

Climate of Rajasthan city is very hot and has houses with their walls painted white. Because of white colours are reflectors of heat, most of heat from the Sun is reflected back to the atmosphere. This keeps the houses cool from inside.

There are three modes of heat transfer:

• Conduction: Conduction is a process in which heat is transferred from the hotter end of an object to its cooler end.
• Convection: The process of transference of heat through a fluid (liquid or gas) is called convection.
• Radiation: Radiation is a method of heat transfer which does not require a medium.

• During day time, air above the land gets heated by the sun to a higher temperature than the air above the
• The hot air above the land has less density than the cool air above the So air above the land moves upwards and is replaced by comparatively cooler air moving from the sea forming convection currents. In this way, sea breeze is set up.
• Similarly, during night time air above the land is cooler than air above the sea. The air above the sea being hotter moves upwards and is replaced by comparatively cooler air moving from the land forming convection currents. In this way, land breeze is set

Convection causes sea breeze and land breeze. Heat transfer through fluids takes place by the process of convection.

Black colour is good absorbers of heat. In cold regions like Antarctica, the black outer coating of Penguins helps them to absorb as much heat from the surrounding and keep their bodies warm.

Room heaters are fitted near the floor because this allows the hot air near the heater to rise up and warm the entire room. 
The air conditioners are fitted near the ceiling, so that the cold air from the AC takes the place of hot air rising up from the bottom. In this way, the entire room gets cooled.

When boiling water is poured into a glass bottle, the glass surface that comes into the contact with hot water heats up and expands.

• Glass being bad conductor of heat does not conduct heat which keeps the remaining glass surface of bottle
• This unequal heating and expansion cause cracking of glass
• On the other hand, borosil glass is made up of constituents different than that of ordinary
• Borosil glasses have very low coefficients of thermal expansion which makes them resistant to thermal shock, more than any other common glass. 
• So, Borosil glass does not crack when hot liquid is poured in it, but normal glass cracks.

All substances expand on heating and contract on cooling

• In summers, the surrounding temperature is very high.
• Due to high surrounding temperature, the metallic telephone wires are heated up and thus expands in length and sag down.
• In winters, the surrounding temperature is very low.
• Due to this low temperature, the metallic telephone wires are cooled down and thus contracts in length. Due to this, they become straight

In winters, the temperature of the air near the grass decreases to dew point. Thus, the air becomes saturated with water vapour. As a result, water vapour condenses into tiny water droplets which appears on the surface of the grass.

• An iron pillar feels colder than a wooden pole in winter because the thermal conductivity of iron is very high as compared to wood.
• So, when you touch wooden pole, heat from your finger warms the wooden pole just at that place but isn't conducted away.
• But for iron pillar, the heat conducts away rapidly from your finger to the whole pillar and your finger then keeps trying to supply more heat until it is as cold as the iron.
• So, the iron pillar feels colder because it doesn't warm under your finger like the wooden pole.