# UPSC  Physics

21. 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

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22. emissivity of perfectly black body is

 1 2 5 0

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23. Absorptive power of perfectly black body is

 Zero Infinity One Constant

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24. newton’s law of cooling is also applicable to

 Convection losses Natural convection losses Forced convection losses None of the above

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25. Which of the following statement is wrong

 Rough surfaces are better radiators tan smooth surfaces Highly polished mirror surfaces are very good radiators Black surfaces are better absorbers then white ones Black surfaces are better radiators then white ones

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26. 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

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27. 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

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28. If the temperature of the sun is doubled. The rate of energy received on the earth will be increased by a factor of

 2 4 8 16

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29. A body in a room cools from 90°C to 80°C in 5 minutes. the time taken to cool from 70°C to 60°C is

 Less than 5 minute 5 minute More than 5 minutes Less or more than 5 minutes depending upon the nature of the body

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30. A sphere, a cube and a thin circular plate, all made of the same material and having the same mass are initially heated to a temperature of 3000°K, which of these will cool fastest

 Sphere Cube Plate None

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