# NCERT Class 11 Physics Solutions: Chapter 11 – Thermal Properties of Matter-Part 10

Get top class preparation for IMO-Level-2 right from your home: fully solved questions with step-by-step explanation- practice your way to success.

Question 11.21:

Explain why:

(a) A body with large reflectivity is a poor emitter

(b) A brass tumbler feels much colder than a wooden tray on a chilly day

(c) An optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives too low a value for the temperature of a red hot iron piece in the open, but gives a correct value for the temperature when the same piece is in the furnace

(d) The earth without its atmosphere would be inhospitably cold

(e) Heating systems based on circulation of steam are more efficient in warming a building than those based on circulation of hot water

Explanation:

A body with a large reflectivity is a poor absorber of light radiations. A poor absorber will in turn be a poor emitter of radiations. Hence, a body with a large reflectivity is a poor emitter.

Explanation:

Brass is a good conductor of heat. When one touches a brass tumbler, heat is conducted from the body to the brass tumbler easily. Hence, the temperature of the body reduces to a lower value and one feels cooler.

Wood is a poor conductor of heat. When one touches a wooden tray, very little heat is conducted from the body to the wooden tray. Hence, there is only a negligible drop in the temperature of the body and one does not feel cool.

Thus, a brass tumbler feels colder than a wooden tray on a chilly day.

Explanation:

An optical pyrometer calibrated for an ideal black body radiation gives too low a value for temperature of a red hot iron piece kept in the open. Black body radiation equation is given by:

Where,

Temperature of optical pyrometer

Temperature of open space

Constant

Hence, an increase in the temperature of open space reduces the radiation energy.

When the same piece of iron is placed in a furnace, the radiation energy,

Explanation:

Without its atmosphere, earth would be inhospitably cold. In the absence of atmospheric gases, no extra heat will be trapped. All the heat would be radiated back from earth’s surface.

Explanation:

A heating system based on the circulation of steam is more efficient in warming a building than that based on the circulation of hot water. This is because steam contains surplus heat in the form of latent heat

Question 11.22:

A body cools from in Calculate the time it takes to cool from . The temperature of the surroundings is

According to Newton’s law of cooling, we have:

Where,

Temperature of the body

Temperature of the surroundings

is a constant

Temperature of the body falls from in time,

Integrating equation we get:

The temperature of the body falls from 60°C to 30°C in time

Hence, we get:

Equating equations we get:

Therefore, the time taken to cool the body from .