NCERT Class 11 Physics Chapter 12 Thermodynamics CBSE Board Sample Problems

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Question 17:

Consider a Carnot’s cycle operating between producing of mechanical work per cycle. Find the heat transferred to the engine by the reservoirs.

Answer:

Question 18:

A person of mass wants to lose by going up and down a high stairs. Assume he burns twice as much fat while going up than coming down. If of fat is burnt on expending kilo calories, how many times must he go up and down to reduce his weight by ?

Answer:

Question 19:

Consider a cycle tyre being filled with air by a pump. Let be the volume of the tyre (fixed) and at each stroke of the pump ) of air is transferred to the tube adiabatically. What is the work done when the pressure in the tube is increased from to ?

Answer:

Question 20:

In a refrigerator one removes heat from a lower temperature and deposits to the surroundings at a higher temperature. In this process, mechanical work has to be done, which is provided by an electric motor. If the motor is of power, and heat is transferred from , find the heat taken out of the refrigerator per second assuming its efficiency is of a perfect engine.

Answer:

Efficiency of refrigerator

If Q is the heat/s transferred at higher temperature then

or ,

and heat removed from lower temperture = 19 kJ.

Question 21:

If the co-efficient of performance of a refrigerator is 5 and operates at the room temperature, find the temperature inside the refrigerator.

Answer:

Question 22:

The initial state of a certain gas is . It undergoes expansion till its volume becomes . Consider the following two cases:

(a) The expansion takes place at constant temperature.

(b) The expansion takes place at constant pressure.

Plot the P-V diagram for each case. In which of the two cases, is the work done by the gas more?

Answer:

The P-V diagram for each case is shown in the figure. In case (i); therfore process is isothermal. Work done area under the PV curve so work done is more when the gas expands at constant pressure.

P-V diagram

P-V Diagram

LA

Question 23:

Consider a P-V diagram in which the path followed by one mole of perfect gas in a cylindrical container is shown in Fig. 12.9.

P-V diagram

P-V Diagram

(a) Find the work done when the gas is taken from state 1 to state 2.

(b) What is the ratio of temperature ?

(c) Given the internal energy for one mole of gas at temperature T is , find the heat supplied to the gas when it is taken from state 1 to 2, with .

Answer:

(a) Work done by the gas

(b) Since

Thus,

(c) Then, the change in internal energy

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