PHYSICS [3 Mark Question Answer]

We prefer to wear white clothes in summer. The reason is that the white clothes reflects most of the sun’s heat and absorb very little of the sun’s heat, thus they keep our body cool.
We prefer to wear black and dark coloured clothes in winter. The reason is that the black or dark colour clothes absorb most of the sun’s heat and keep our body warm

When we sit in the sun, we feel warm. We cannot get heat from the sun by the process of conduction or convection because most of the space between the sun and the earth is a vacuum and both of these modes of heat transfer require medium. Hence, one must be getting heat from the sun by the mode of radiation.

Freezing chest in a refrigerator is fitted near the top, because it cools the remaining space of the refrigerator by convection current. Air near the top comes in contact with the freezing chest gets cooled, becomes denser and therefore descends while the hot air from the lower part rises and hence convection currents produced cool the whole space inside.

Chimneys are provided over the furnace in factories. This is because the hot gases coming out of the furnace are less dense than the air. They rise up through the chimney. The smoke, fumes etc. around the furnace rush in so as to take their place and they are sucked out. Thus, the chimney helps to remove the undesired fumes, smoke etc. from the premises.

Quilts are filled with fluffy cotton. Air is trapped in the fine pores of cotton. Cotton and air are the insulators of heat. They check heat from our body to escape and thus keep us warm.
The newly made quilts are warmer than the old ones because in the old quilts, there is no air trapped in the cotton.

During very cold weather, the water pipes are covered with cotton. The cotton has air trapped in its fine pores. The cotton and air are the insulators of heat. They do not pass heat from water inside the pipes to the outside atmosphere. Thus, cotton prevents the water in the pipes from freezing.

Woolen clothes have fine pores filled with air. Wool and air both are bad conductors of heat. Therefore in winter, we wear woolen clothes as they check the conduction of heat from the body to the surroundings and thus keeps the body warm.

In summer, the ice is kept wrapped in a gunny bag or it is covered with saw dust. The air filled in the fine pores of the gunny bag or saw dust, is the insulator of heat. The air does not allow heat from outside to pass through it to the ice. Thus, the ice is prevented from melting rapidly.

Cooking utensils are made of metals such as copper, aluminium, brass, steel etc., so that heat is easily conducted through the base to their contents. But they are provided with handles of bad conductors (such as ebonite or wood) to hold them easily as handles will not conduct heat from the utensil to our hand.

Good conductors. “The substances through which heat is easily conducted are called good conductors of heat.”
Example : Copper, iron.
Bad conductors. “The substances through which heat is not conducted easily are called bad conductors of heat or poor conductors of heat.”
Example : Wood, cloth.

There are three modes of transfer of heat (i) Conduction (ii) Convection (iii) Radiation.
(i) Conduction “is that mode of transfer of heat, when heat travels from hot end to cold end from particle to particle of the medium, without actual movement of particles.”
(ii) Convection. “Is a process of transfer of heat by actual move-ment of the medium particles.”
(iii) Radiation. “Is that mode of transfer of heat in which heat directly passes from one body to the other body without heating the medium.”

In the construction of a bridge, steel girders are used. One end of the girder is fixed into the concrete or brick pillars and its other end is not fixed, but it is placed on rollers. The reason is that if there is any rise (or fall) in temperature of atmosphere, the girder can freely expand (or contract) without affecting the pillars.

The telephone wires will sag in summer due to expansions and will break in winter due to contraction.
Therefore, while putting up the wires between the poles, care is taken that in summer they are kept slightly loose so that they may not break in winter due to contraction.
While in winter they are kept light so that they may not sag too much in summer due to expansion.