Question

If you cover the beaker’s surface with tin foil the rate of cooling
(A) Increases
(B) Decreases
(C) Remain same
(D) None

Answer 1

Hint: In case of a hot beaker Heat gets transferred by two methods, 1. Radiation and 2 Convection. Tin foil is a metal, which is a good reflector that reduces the radiation. And covering the beaker makes sure that the hot particles from the surface are not able to leave the surface and allow convection. This means that heat is being trapped.

Complete step by step answer
Tin foil is a very good reflector. This means that it has a high reflectance or reflecting power. Also, its absorbing power and transmitting power is low. So, good reflectors are bad absorbers. Hence, if a body has low absorbing power this means that it won’t absorb the heat from its surroundings. As a result, its rate of cooling due to radiation will decrease.
Covering the beaker stops hot air particles from running away from the beaker. This way the cooling process due to convection is affected.
 So, if we cover the beaker’s surface with a tin foil, the rate of cooling will decrease.

Therefore, option (b.) is the correct answer.

Note: When a body cools by radiation the rate of cooling depends on the following factors:
1. Nature of the radiating surface, i.e., greater the emissivity, faster will be the cooling.
2. Area of the radiating surface, i.e., greater the area of the radiating surface, faster will be the cooling.
3. Mass of the radiating body, i.e., greater the mass of the radiating body, slower will be the cooling.
4. Specific heat of the radiating body, i.e., greater the specific heat of the radiating body slower will be the cooling.
5. Temperature of the radiating body, i.e., greater the temperature of the body faster will be the cooling.
6. Temperature of the surroundings, i.e., greater the temperature of the surrounding slower will be the cooling.