Thermodynamics - GATE practice papers - Mechanical

1)   Which condition of Rankine cycle has highest efficiency when operating between same pressure limits? (Marks: 02)
- Published on 19 Oct 15

a. Superheated cycle
b. Regenerative cycle
c. Reheat cycle
d. Saturated cycle
Answer  Explanation 

ANSWER: Regenerative cycle

Explanation:
The efficiency of ideal regenerative cycle is exactly equal to the corresponding Carnot cycle. Hence it is maximum.


2)   What is the relation between the specific heat of gas at constant pressure (Cv) and the specific heat of gas at constant volume (Cp) at the absolute zero point? (Marks: 02)
- Published on 19 Oct 15

a. (Cp) > (Cv)
b. (Cp) < (Cv)
c. (Cp) = (Cv)
d. unpredictable
Answer  Explanation 

ANSWER: (Cp) = (Cv)

Explanation:
The entropy is zero at the absolute zero. Also the value of γ is 1 at the absolute zero. γ = 1 = Cp/ Cv
Therefore (Cp) = (Cv) at absolute zero


3)   Which among the following has an infinite heat capacity?(Marks: 01)
- Published on 19 Oct 15

a. heat engine
b. thermal energy reservoir
c. both a. and b
d. none of the above
Answer  Explanation 

ANSWER: thermal energy reservoir

Explanation:
Thermal energy reservoir is either heat source or heat sink which can absorb or reject unlimited amount of heat energy without considerable change in temperature.


4)   What is a quasi-static process? (Marks: 02)
- Published on 19 Oct 15

a. a process which is random
b. a process which is spontaneous
c. a process which is infinitely slow
d. a process which is stationary
Answer  Explanation 

ANSWER: a process which is infinitely slow

Explanation:
The system has to pass through an infinitely slow process in order to experience equilibrium at every stage in the process. This infinitely slow process is called as quasi-static process


5)   A cyclic heat engine operates between source temperature of 1000 °C and a sink temperature 50 °C. Calculate least rate of heat rejection when net work output is 2 kW. (Marks: 02)
- Published on 19 Oct 15

a. 2.6799 kW
b. 0.6799 kW
c. 0.7463 kW
d. none of the above
Answer  Explanation 

ANSWER: 0.6799 kW

Explanation:
The rate of heat rejection will be least for reversible engine,

ηrev = ηmax = 1 – (Tsink / Tsource )

= 1 – (323 / 1273)
= 0.7463

ηmax = Wnet / Q1

Q1 = Wnet / ηmax
= 2 / 0.7463
= 2.6799 kW

Therefore the least heat rejection rate, Q2 is

Q2 = Q1 – Wnet

= 2.6799 – 2
= 0.6799 kW


6)   What is the Perpetual Motion Machine of First Kind (PMM1)? (Marks: 02)
- Published on 19 Oct 15

a. a machine which produces mechanical work with consumption of some energy
b. a machine which produces mechanical work without consumption of some energy
c. a machine which generates heat energy with consumption of some external work
d. none of the above
Answer  Explanation 

ANSWER: a machine which produces mechanical work without consumption of some energy

Explanation:
PMM1 is a machine which violates first law of thermodynamics. It states that energy neither be created nor be destroyed. Thus PMM1 is a machine which produces mechanical work without consumption of some energy


7)   Which is the example of path function?(Marks: 01)
- Published on 19 Oct 15

a. Heat energy
b. Work energy
c. both a. and b.
d. none of the aboves
Answer  Explanation 

ANSWER: both a. and b.

Explanation:
Heat energy as well as work energy both depend on the path followed by the system. Therefore both are the path functions.


8)   In which condition does the internal energy of the system increase? (Marks: 02)
- Published on 19 Oct 15

a. when the heat transfer into the system is less than the work transfer out of the system
b. when the heat transfer into the system is more than the work transfer out of the system
c. when the heat transfer into the system is equal to the work transfer out of the system
d. none of the above
Answer  Explanation 

ANSWER: when the heat transfer into the system is more than the work transfer out of the system

Explanation:
When a system undergoes a change of state along with the heat transfer as well as work transfer, the net energy transfer is collected and stored within the system. When heat transfer within the system Q is more than the work transfer out of the system W, then the change in energy (Q – W) is stored in the system. This is the internal energy and it increases.


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