Q. 21 A4.8( 4 Votes )
a body with large reflectivity is a poor emitter
A body with a large reflectivity is a poor absorber of light radiations since it doesn’t absorb energy. A poor absorber will in turn be a poor emitter of radiations. Hence, a body with a large reflectivity is a poor emitter.
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Calculate the temperature which has same numeral value on Celsius and Fahrenheit scale.Physics - Exemplar
According to Stefan’s law of radiation, a black body radiates energy σT 4 from its unit surface area every second where T is the surface temperature of the black body and σ = 5.67 × 10-8 W/ m2K4 is known as Stefan’s constant. A nuclear weapon may be thought of as a ball of radius 0.5 m. When detonated, it reaches temperature of 106K and can be treated as a black body.
(a) Estimate the power it radiates.
(b) If surrounding has water at 30 C°, how much water can 10% of the energy produced evaporate in 1 s?
[Sw = 4186.0 J/ kgK and Lv = 22.6 X 105 J / kg]
(c) If all this energy U is in the form of radiation, corresponding momentum is p = U/c. How much momentum per unit time does it impart on unit area at a distance of 1 km?
Physics - Exemplar
100 g of water is supercooled to –10° C. At this point, due to some disturbance mechanized or otherwise some of it suddenly freezes to ice. What will be the temperature of the resultant mixture and how much mass would freeze?
[Sw = 1 cal/g/° C and Lw fusion = 80 cal/ g]
Physics - Exemplar
One day in the morning, Ramesh filled up 1/3 bucket of hot water from geyser, to take bath. Remaining 2/3 was to be filled by cold water (at room temperature) to bring mixture to a comfortable temperature. Suddenly Ramesh had to attend to something which would take some times, say 5-10 minutes before he could take bath. Now he had two options: (i) fill the remaining bucket completely by cold water and then attend to the work, (ii) first attend to the work and fill the remaining bucket just before taking bath. Which option do you think would have kept water warmer? Explain.Physics - Exemplar
During summers in India, one of the common practices to keep cool is to make ice balls of crushed ice, dip it in flavored sugar syrup and sip it. For this a stick is inserted into crushed ice and is squeezed in the palm to make it into the ball. Equivalently in winter, in those areas where it snows, people make snow balls and throw around. Explain the formation of ball out of crushed ice or snow in the light of P–T diagram of water.Physics - Exemplar
A copper block of mass 2.5 kg is heated in a furnace to a temperature of 500 °C and then placed on a large ice block. What is the maximum amount of ice that can melt? (Specific heat of copper = 0.39 J g–1 K–1; heat of fusion of water
= 335 J g–1).NCERT - Physics Part-II
heating systems based on circulation of steam are more efficient in warming a building than those based on circulation of hot waterNCERT - Physics Part-II
the earth without its atmosphere would be inhospitably coldNCERT - Physics Part-II
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 furnaceNCERT - Physics Part-II
In an experiment on the specific heat of a metal, a 0.20 kg block of the metal at 150 °C is dropped in a copper calorimeter (of water equivalent 0.025 kg) containing 150 cm3 of water at 27 °C. The final temperature is 40 °C. Compute the specific heat of the metal. If heat losses to the surroundings are not negligible, is your answer greater or smaller than the actual value for specific heat of the metal?NCERT - Physics Part-II