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 the circulation of steam are more efficient in warming a building than those based on the circulation of hot water
(a) A body with a large reflectivity is a poor absorber of heat radiation. A poor absorber will be a poor emitter of radiations. Therefore, a body with a large reflectivity is a poor emitter.
(b) Brass is a good conductor of heat and wood is a poor conductor of heat. When we touch a brass tumbler, heat is conducted from our hand to the tumbler and there is a drop in the body temperature Therefore, we feel cold. When a wooden tray is touched on a chilly day, very less heat is conducted from the hand to the wooden tray and body temperature is not decreased much. So, we do not feel cold.
(c) The radiation energy from a red hot iron piece place in a furnace is given by the relation
When the iron piece is placed in the open, the radiation energy is given by the relation
Here, E is the energy radiation
T is the temperature of optical pyrometer
σ is a constant
T0 is the temperature of the open space.
The increase in the temperature of the open space reduces the radiation energy.
(d) If there is no atmosphere, extra heat will not be trapped. All the heat from the sun will be radiated back from the surface of the earth. So without the atmosphere, the earth would be inhospitably cold.
(e) Steam at 100°C is much hotter than water at 100°C. This is because steam will have a lot of heat in the form of latent heat (540 cal/g). Therefore, heating systems based on the circulation of steam are more efficient in warming a building than those based on the circulation of hot water.Answered by Pragya Singh | 1 year ago
Answer the following questions based on the P-T phase diagram of carbon dioxide:
(a) At what temperature and pressure can the solid, liquid and vapour phases of \( CO_2\) co-exist in equilibrium?
(b) What is the effect of the decrease of pressure on the fusion and boiling point of \( CO_2\)?
(c) What are the critical temperature and pressure for \( CO_2\)? What is its significance?
(d) Is \( CO_2\) solid, liquid or gas at
(a) –70°C under 1 atm,
(b) –60°C under 10 atm,
(c) 15°C under 56 atm?
A brass boiler has a base area of 0.15 m2 and thickness 1.0 cm. It boils water at the rate of 6.0 kg/min when placed on a gas stove. Estimate the temperature of the part of the flame in contact with the boiler. Thermal conductivity of brass = 109 J s–1 m–1 K–1; Heat of vaporisation of water = 2256 × 103 J kg–1.
A ‘thermacole’ icebox is a cheap and an efficient method for storing small quantities of cooked food in summer in particular. A cubical icebox of side 30 cm has a thickness of 5.0 cm. If 4.0 kg of ice is put in the box, estimate the amount of ice remaining after 6 h. The outside temperature is 45°C, and the co-efficient of thermal conductivity of thermacole is 0.01 J s–1 m–1 K–1. [Heat of fusion of water = 335 × 103 J kg–1]