Test: Second Law of Thermodynamics

10 Questions MCQ Test Physics For JEE | Test: Second Law of Thermodynamics

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Refrigerators X and Y are removing 1000 J of heat from the freezer. Refrigerator X is working between -5° C and 25° C and refrigerator Y is working between -20° C and 20 °C. Find efficiency of refrigerator X and Y?


We know that the efficiency of refrigeration for a refrigerator is T2 / T1 + T2
Where T1 is source temperature and T2 is sink temperature
For refrigerator X we have T1 = 298K and T2 = 268K
Hence the efficiency of refrigeration = 268 / 298 - 268
= 268 / 30
= 8.93
For refrigerator Y we have T1 = 293K and T2 = 253K
Hence the efficiency of refrigeration = 253 / 293 - 253
= 253 / 40
= 6.35


Kelvin- Planck statement states that


It works on the principle that no machine is 100% efficient. For instance if taking an ideal machine into consideration, work can be completely converted to heat. Now if the conditions are reversed, all heat cannot be converted to equal amount of work until and unless backed up by an external force.


Which of the following is an example of heat pump?


A heat pump is an electrical device that heats a building by pumping heat in from the cold outside. In other words, it’s the same as a refrigerator, but its purpose is to warm the hot reservoir rather than to cool the cold reservoir (even though it does both).


For proper utilization of exergy, it is desirable to make first law efficiency ____ and the source and use temperatures should ____.


If first law efficiency is close to unity, the all the energy carried in by heat transfer is used and no heat is lost to the surroundings.


The second law of thermodynamics says


The second law of thermodynamics gives a fundamental limitation to the efficiency of a heat engine and the coefficient of performance of a refrigerator. It says that the efficiency of a heat engine can never be unity or 100%, this implies that the heat released to the cold reservoir can never be made zero.
For a refrigerator the second law says that the coefficient through performance can never be infinite, this implies that the external work can never be zero.


If the door of refrigerator is left open inside a closed room, what would happen to the temperature of the room?


If you leave the door open, heat is merely recycled from the room into the refrigerator, then back into the room. A net room temperature increase would result from the heat of the motor that would be constantly running to move energy around in a circle.


Refrigerator transfers heat from the cold cooling coils to warm surroundings, which law of thermodynamics favour this process


Refrigerator follows the principle of Clausius statement of second law of thermodynamics. It does not violate second law of thermodynamics because it takes energy to transfer heat from low temperature body to high temperature body. Electrical work is given to refrigerator to extract heat from low temperature body and to transfer it to higher temperature body. If any refrigerator is transferring heat from low temperature body to higher temperature body without any external energy then we can say that it violates second law of thermodynamics. But in actual it takes energy to do.


Which word is defined by this statement: A measure of this disorder, or randomness?


Entropy is defined as a thermodynamic property which serves as a measure of how close a system is to equilibrium, as well as a measure of the disorder in the system.


What amount of work is needed to freeze 1 gm of water at 0˚C in a refrigerator .The temperature of surroundings is 320 K?


The coefficient of performance of a refrigerator is given by

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