381. Thermal conductivity of water at 20°C is of the order of_____________________?
A. 0.1
B. 0.23
C. 0.42
D. 0.51
E. 0.64

382. The time constant of a thermocouple is_______________?
A. the time taken to attain the final temperature to be measured
B. the time taken to attain 50% of the value of initial temperature difference
C. the time taken to attain 63.2% of the value of initial temperature difference
D. determined by the time taken to reach 100°C from 0°C
E. none of the above

383. In heat transfer, conductance equals conductivity (kcal/hr/sqm/°C/cm) divided by ________________?
A. hr (time)
B. sqm (area)
C. °C (temperature)
D. cm (thickness)
E. kcal (heat).

384. Which of the following is expected to have highest thermal conductivity_______________?
A. steam
B. solid ice
C. melting ice
D. water
E. boiling water

385. 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 ^re 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
E. none of the above

386. Heat is transferred by all three modes of transfer, viz, conduction, convection and radiation in _________________?
A. electric heater
B. steam condenser
C. melting of ice
D. refrigerator condenser coils
E. boiler

387. The ratio of heat flow Q1/Q2 from two walls of same thickness having their thermal conductivities as ATj – 2K2 will be ________________?
A. I
B. 0.5
C. 2
D. 0.25
E. 4.0

388. Thermal diffusivity of a substance is ____________________?
A. proportional of thermal conductivity
B. inversely proportional to k
C. proportional to (k)
D. inversely proportional to k2
E. none of the above

389. According to Prevost theory of heat exchange ___________________?
A. it is impossible to transfer heat from low temperature source to t high temperature source
B. heat transfer by radiation requires no medium
C. all bodies above absolute zero emit radiation
D. heat transfer in most of the cases takes place by combination of conduction, convection and radiation
E. rate of heat transfer depends on thermal conductivity and temperature difference

390. Thermal conductivity of glass-wool varies from sample to sample because of variation in ________________ ?
A. composition
B. density
C. porosity
D. structure
E. all of the above

NOTE
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