Absorptive power of perfectly black body is
  • Zero
  • Infinity
  • One
  • Constant
A black body at hot temperature at 227°C radiates heat at a rate of 5 cal/cm²s. at a temperature of 727°C the rate of heat radiated per unit area will be
  • 50 cal/cm²s
  • 250 cal/cm²s
  • 80 cal/cm²s
  • 100 cal/cm²s
A body cools from 50°C to 46°C in 5 minutes and to 40°C in the next 10 minutes. The surrounding temperature is
  • 30°C
  • 28°C
  • 36°C
  • 32°C
For which of the following process, the thermal conduction is maximum?
  • Combustion
  • Radiation
  • Convection
  • Conduction
A bucket full of hot water is kept in a room and it cools from 75°C to 70°C in t1 minutes from 70°C to 65°C in t2 minutes and from 65°C to 60°C in t3 minutes; then
  • t1 - t2 = t3
  • t1 < t2 < t3
  • t1 > t2 > t3
  • t1 < t2 > t3
The good absorber of heat are
  • Non-emitter
  • Poor-emitter
  • Good-emitter
  • Highly polished
Two stars A and B radiate maximum energy at 3600°A and 3600°A respectively. Then the ratio of absolute temperatures of A and B is
  • 256 : 81
  • 81 : 256
  • 3 : 4
  • 4 : 3
A person with dark skin as compared to a person with white skin will experience
  • Less heat and less cold
  • More heat and more cold
  • More heat and less cold
  • Less heat and more cold.
A cup of tea cools from 80°C to 60°C in one minute. The ambient temperature is 30°C. in cooling from 60°C to 50°C, it will take.
  • 50 sec
  • 90 sec
  • 60 sec
  • 30 sec
The best ideal black body is
  • Lamp of charcoal heated to high temperature
  • Metal coated with a black dye
  • Glass surface coated with coalter
  • Hollow enclosure blackened inside and having a small hole
The earth intercepts approximately one billionth of the power radiated by the sun. if the surface temperature of the sun were to drop by a factor of 2, the average radiant energy incident on earth per second would reduce by factor of
  • 2
  • 4
  • 8
  • 16
The process of heat transfer in which heat is transferred with actual migration of medium particles is known as (AFMC-94)
  • Conduction
  • Convection
  • Radiation
  • Reflection
Unit of Stefans constant is given by
  • W/ m K²
  • W/ m² K²
  • W²/ m² K4
  • W/ mK
A metal piece heated to T1°K. The temperature of the surrounding is T2°K. the heat in the surrounding due to radiation is proportional to
  • (T14 - T24)
  • (T1 - T2)4
  • (T14 + T24)
  • (T13 - T23)
The earth receives at its surface radiation from the sun at the rate of 1400 W/m². the distance of the centre of the sun from the surface of the earth is 1.5 1011 m and the radius of the sun is 7.0 108 m. treating sun as a black body, it follows from the above data that its surface temperature is,
  • 5801 K
  • 106 K
  • 50.1 K
  • 2801 C
A surface at temperature T0°K receives power P by radiation from a small sphere at temperature T >> T0 and at a distance d. if both T and d are doubled, the power received by surface will becomes approximately
  • P
  • 2p
  • 4p
  • 16p
The temperature of a piece of metal is raised from 27°C to 51.2°C. the rate at which metal radiates energy increases nearly
  • 1.36 times
  • 2.36 times
  • 3.36 times
  • 4.36 times
Emissivity of perfectly black body is
  • 1
  • 2
  • 5
  • 0
Co-efficient of reflection, coefficient of absorption and coefficient transmission are related as
  • a + r + 1 = 1
  • a + r + t1 1
  • a + r = -t
  • a1 r + 1
The wavelength of radiation emitted by a body depends upon
  • The nature of the surface
  • The area of the surface
  • the temperature of the surface
  • All the above
0 h : 0 m : 1 s

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