Thermal Physics : MCQ
28B 29A 30C 31B 32A 33C 34D 35D 36A 37D 38B
Comments :
28. The air gun pellet absorb 0.5 of its own kinetic energy . Ke converted to heat energy. Hence equating the energy conversion , mass can be cancelled. .
29. Change in state requires the separation of the molecules from one state to another state.
30. The potential energy is greater if the forces are stronger. At 20 degrees the forces are larger than at 100degrees.Hence the latent heat is correspondingly greater.
31. The water is spilled during the expt , the mass evaporated would be an overestimate.Hence the latent heat would be an underestimate. Hence decrease in value of latent heat.The water drip back , the mass evaporated would be an underestimate and hence the value of the latent heat would be an overestimate.
32. The power used is the same . Since the 2 processes have the same power and equating mL = mc@ where m is mass per unit time and @ is temp change per unit time.
33. The same power would mean that the mL = mc@ where m is mass per unit time and @ is temperature change per unit time. Same as 32.
34. A. The substance is all liquid only at 7 min.(wrong)
B. After 10 min , the susbstance is still in liquid state.(wrong)
C The rate of temp rise is greater in liquid hence the shc is smaller.(wrong)
D Power = mL /t where t= 3 min. Hence L = 2000x 3=6000 J/min(correct)
35. If latent heat is large the time taken to change state would be large, power remain constant.Using the formula : Power =mL/t. L/t = constant. Hence the horizontal line is longer at 373K due to latent heat of vaporisation having a large value.
36. Write the 2 equations , one for rise in temp and the other for latent heat but the rate of heat supplied is the same . Equating them with having the same power. Solve.
37.Heat loss by water = heat gained by ice to melt. The amount of ice melted depends on the amount of heat released by the water in changing to 0 degrees.
38. Heat absorbed when the temp rise plus heat absorbed when the solid liquefy.The total heat required (mc@ + mL) divide by R gives the time taken.
