50. Which of the following has least value of conductivity
A.Glass
B.Air
C.Plastic
D.Rubber

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51. Depending on the radiating properties, body will be transparent when
A.? = 0, ? = 0 and ? = 1
B.? = 1, ? = 0 and ? = 0
C.? = 0, ? = 1 and ? = 0
D.? = 0, ?+? = 1

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52. The concept of overall coefficient of heat transfer is used in case of heat transfer by
A.Conduction
B.Convection
C.Radiation
D.Conduction and convection

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53. Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by
A.Convection
B.Free convection
C.Forced convection
D.Radiation

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54. Thermal conductivity of glass-wool varies from sample to sample because of variation in
A.Composition
B.Porosity
C.Structure
D.All of the above

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55. Thermal conductivity of a material may be defined as the
A.Quantity of heat flowing in one second through one cm cube of material when opposite faces are maintained at a temperature difference of 1°C
B.Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
C.Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
D.All of the above

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56. Which of the following has maximum value of thermal conductivity
A.Aluminium
B.Steel
C.Brass
D.Copper

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