First Law of Thermodynamics - Mechanical Engineering (MCQ) questions and answers

1)   A quasi-static process has main characteristics as it is

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

ANSWER: an infinitely slow process

Explanation:
Consider a system of gas exists in a cylinder. The piston consists of many very small pieces of weights. Initially system is in an equilibrium state. When the gas system is isolated, the weights on piston are removed one by one slowly, at any instant of upward travel of the piston. So every state passes through by the system will be in an equilibrium state. Thus the system passes through the locus of all equilibrium points. This infinitely slow system is a quasi-static process. If the same small weights are now placed slowly one by one on top of the piston then the process will reverse in the same manner.


2)   A machine which can supply mechanical work continuously without consumption of any energy is called as

a. Perpetual Motion Machine of the First kind (PMM1)
b. Perpetual Motion Machine of the Second kind (PMM2)
c. Perpetual Motion Machine of the Third kind (PMM3)
d. none of the above
Answer  Explanation 

ANSWER: Perpetual Motion Machine of the First kind (PMM1)

Explanation:
The first law of thermodynamics states that the energy can neither be created nor be destroyed. It can only gets transformed from one form to another form. Perpetual Motion Machine is the machine which violates the law of thermodynamics. A machine, which can supply mechanical work continuously without consumption of any energy, violates the first law of thermodynamics. Thus this machine is the Perpetual Motion Machine of the First kind (PMM1). When a machine violates the second law or third law of thermodynamics then this machine is called as PMM2 or PMM3 respectively. Perpetual Motion Machine is a hypothetical concept.


3)   Which among the following statements are correct?
(1) Energy is an extensive property
(2) Specific energy is an extensive property
(3) Energy is a point function
(4) Heat capacity is an extensive property


a. Statement (1), (2) and (3)
b. Statement (1), (3) and (4)
c. Statement (2), (3) and (4)
d. Statement (1), (2), (3) and (4)
Answer  Explanation 

ANSWER: Statement (1), (3) and (4)

Explanation:
We know that, when a system changes from state 1 to state 2, the change in internal energy (ΔE) of the state 2 is same as that of the state 1. Therefore value of internal energy of the system is independent of the path followed by the system. It has a fixed value along the path, therefore energy is the point function [statement (3) is correct]. But energy changes with mass of the body, therefore it is an extensive property [statement (1) is correct]. Specific energy is the energy of the system per unit mass of the system, therefore it will become intensive property [statement (2) is wrong]. Heat capacity is the product of specific heat and mass of the body. It depends on mass of the system, therefore heat capacity is an extensive property.


4)   Total amount of energy in the universe is

a. increasing
b. decreasing
c. constant
d. none of the above
Answer  Explanation 

ANSWER: constant

Explanation:
The first law of thermodynamics states that the energy can neither be created nor be destroyed. It can only get transformed from one form to another form. The universe consists of the system as well as the surrounding together. Energy can only be transferred from system to surrounding or surrounding to system in various forms, but it can never be destroyed or created. Thus the total amount of energy in the universe is constant. We cannot produce a device which can supply mechanical work without consuming any energy.


5)   Which of the following is/are mode/s of storage of internal energy in a system?

a. Macroscopic energy mode
b. Microscopic energy mode
c. both a. and b.
d. none of the above
Answer  Explanation 

ANSWER: both a. and b.

Explanation:
There are two modes in which energy can be stored in a system, macroscopic energy mode and microscopic energy mode.
In macroscopic energy mode, the kinetic energy and the potential energy of a system is considered. Let us consider a fluid element of mass m and a center of mass velocity is V. The macroscopic kinetic energy Ek of the fluid element is,
Ek = ½ mV2

If this fluid element is elevated to a certain height h from and arbitrary datum, the macroscopic potential energy Ep of the element,
Ep = mgh

The microscopic energy mode consists of an energy stored in molecular and atomic structure of the system. This energy consists of molecular translational kinetic energy, rotational kinetic energy, vibrational kinetic energy, chemical energy, electrical energy and nuclear energy.
If e represents the energy of one molecule then,

ε = εtrans + εrot + εvib + εchem + εelectronic + εnuclear

and if N is the total number of molecules in the system, the total microscopic internal energy will be,

U = Nε

Thus the total internal energy of the system will be,
E = Ek + Ep + U


6)   Which of the following is NOT a path function?

a. Internal energy
b. Heat energy
c. Work energy
d. none of the above
Answer  Explanation 

ANSWER: Internal energy

Explanation:
In thermodynamics, energy can be in two forms, energy in transit and energy in storage. Energy in transit is a path function as the transfer of this energy through the boundaries of the system depends on the path which is followed by the system in the process. But energy in storage does not cross the boundaries of the system; hence it is a point function. Heat transfer as well as work transfer between the system and surrounding depends upon the path by which the process is occurred. Therefore heat energy and work energy are the path functions. Energy in storage is the internal energy. The change in internal energy (ΔE) remains constant, no matter which path is followed by a system to undergo a change of a certain state. Thus internal energy is a point function or state function.


7)   Consider the below diagram of heat transfer and work transfer for a system. What will be the first law equation for the below system?

a. (Q1 – Q2) = ΔE – ( W2 + W3 – W1 )
b. (Q1 + Q2) = ΔE + ( W2 – W3 + W1 )
c. (Q1 – Q2) = ΔE + ( W2 + W3 – W1 )
d. none of the above
Answer  Explanation 

ANSWER: (Q1 – Q2) = ΔE + ( W2 + W3 – W1 )

Explanation:
When a system undergoes cycle, then the algebraic sum of all energy transfer across the boundaries is zero. But when a system undergoes a change of state in which both heat transfer and work transfer are involved, the net energy transfer is stored and collected within the system. If Q is the amount of heat transferred to the system and W is the amount of work transferred from the system during the process, then the net energy (Q – W) is stored in the system. This energy is neither heat nor work but it is called as internal energy(ΔE).
Q – W = ΔE
Therefore,
Q = ΔE + W


8)   A system changes its state from state 1 to state 2 through path A and returns from state 2 to state 1 through path B. Path C is also an alternate returning path from state 2 to state 1 as shown in figure. What will be the effect on change in internal energy of the system(ΔE), if the system undergoes these different paths A, B and C?

a. ΔE of path C will be greater then ΔE of path B and path A
b. ΔE of path A and path C are equal but ΔE of path B is different than rest
c. ΔE of all the paths are equal
d. none of the above
Answer  Explanation 

ANSWER: ΔE of all the paths are equal

Explanation:
According to the description of path given, through the path A and path B system undergoes cycle,
Writing the first law equation for path A,
QA = ΔEA + WAand for path B,
QB = ΔEB + WB

The processes A and B together constitute a cycle, for which
(∑ W)cycle = (∑ Q)cycle
WA + WB = QA + QB
QA – WA = WB – QB
ΔEA = – ΔEB

Similarly, when the system returns from state 2 to state 1 through path C,
ΔEA = – ΔEC

Therefore,
ΔEB = ΔEC

Therefore above equations show that the change in internal energy between two states of the system is the same, no matter which path may system follow.


9)   When the heat transfer into a system is more than the work transfer out of the system, then

a. the internal energy of the system remains constant
b. the internal energy of the system decreases
c. the internal energy of the system increases
d. none of the above
Answer  Explanation 

ANSWER: the internal energy of the system increases

Explanation:
When a system undergoes cycle, then the algebraic sum of all energy transfer across the boundaries is zero. But when a system undergoes a change of state in which both heat transfer and work transfer are involved, the net energy transfer is stored and collected within the system. If Q is the amount of heat transferred to the system and W is the amount of work transferred from the system during the process, then the net energy (Q – W) is stored in the system. This energy is neither heat nor work but it is called as internal energy. As the heat transfer is more that work transfer then the internal energy (ΔE) increases.
Q – W = ΔE