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MCQs

  1. If the absolute temperature of a radiating body is suddenly halved, the radiating power will reduce approximately by
    a.   25%                
    b. 12.5%                     
    c. 6.25%                     
    d. 3.12%
  2. Which relation for emissive power and temperature is correct?
    a. [latex]E\propto T[/latex]            
    b. [latex]E\propto T^2[/latex]               
    c. [latex]E\propto T^4[/latex]                 
    d. [latex]E\propto T^{-4}[/latex]
  3. The most appropriate material for making a cooking pot is the one having
    a. Low specific heat and high conductivity                  
    b. Low specific heat and low conductivity
    c. High specific heat and low conductivity
    d. High specific heat and high conductivity
  4. Two stars radiate maximum energy at 320 nanometer and 400 nanometer respectively. The ratio of their Kelvin temperature is
    a. 5:4                  
    b. 4:5                          
    c. 2:5                          
    d. 4:25
  5. Instrument used to measure temperature by radiation method is
    a. Pyrometer                               
    b. Radio micrometer
    c. Thermometer                          
    d. Barometer
  6. The ratio of maximum wavelength of sun and moon is in the ratio 1:4000. What is the ratio of their temperature?
    a. 200:1                          
    b. 400:1                      
    c. 1:200                      
    d. 1:400
  7. Unit of the coefficient of thermal conductivity is
    a. [latex]watt\ Km^{-1}[/latex]                                    
    b. [latex]Joule\ sK^{-1}[/latex]    
    c. [latex]watt\ K^{-1}m^{-1}[/latex]                                
    d. [latex]Joule\ s^{-1}K[/latex]
  8. If temperature of black body is increased by 50% percentage increase in emitted radiation
    a. 50                               
    b. 100                         
    c. 500                         
    d. 400
  9. From 0oC the coefficient of thermal expansion of water is
    a. Positive but not zero                                      
    b. negative
    c. zero                                                                
    d. Infinite
  10. A black body is heated from 27oC to 127oC the ratio of their energies of radiation is
    a. 9:16                            
    b. 27:64                      
    c. 81:256                    
    d. 3:4
  11. A perfectly black body emits radiation at temperature [latex]T_1K[/latex]. If it is to radiate 16 times this power, its temperature [latex]T_2K[/latex] will be
    a. [latex]T_2 = 16T_1[/latex]            
    b. [latex]T_2 = 8T_1[/latex]               
    c. [latex]T_2 = 4T_1[/latex]                 
    d. [latex]T_2 = 2T_1[/latex]
  12. Two rods of same length and diameter having thermal conductivities 2 and 3 units respectively are joined in series. The equivalent thermal conductivity is            
    a. 1.0                              
    b. 2.0                          
    c. 3.0                          
    d. none
Q1

CONCEPTUAL PROBLEMS

  1. Cooking utensils are blackened at the bottom and polished on the upper surface. Explain, why?
  2. What is the physical meaning of emissivity? Write its unit.
  3. Why are the polar regions much cooler than the equatorial regions despite the fact that the polar regions are periodically tilted towards sun?
  4. Air is a bad conductor of heat. Why do you feel cool without cloth in your body?
  5. Although aluminium is good conductor of heat, how can aluminium foil with shining surface can be used to keep food hot for a long time?
  6. Birds often swell their feathers in winter. Why?
  7. Animals curl into a ball, when they feel very cold. Why?
  8. Why are two thin blankets warmer than a single blanket of double the thickness?
  9. Why are good absorbers always good emitters?
  10. What is a blackbody? How is it realized in practice?
  11. How are water pipes used in the room painted black?
  12. Metal knob of door is colder than wooden parts at the same temperature. Why?
Show answer
Correct answer: (C)
Q2

THEORY BASED PROBLEMS

  1. Discuss the methods of heat transmission. Define reflection, transmission and absorption coefficient of heat radiation and relate them.
  2. What is radiation and how does this mode of heat transfer differ from conduction and convection?
  3. Define thermal conductivity. Derive an expression for thermal conductivity of a good conductor in steady state by Searle’s method.
  4. What do you mean by perfectly black body. State and explain Stefan’s law of black body radiation.
Show answer
Correct answer: (C)

Numerical problems

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