Engineering Questions with Answers - Multiple Choice Questions
Aerodynamics – A Brief Review of Thermodynamics – 1
1 - Question
The equation of state for a perfect gas is p = δRT, where R is______ (for air at standard conditions).
a) 287 kJ/kg.K
b) 8314 kJ/kg
c) 8.314 kJ/kgK
d) 287 J/kg.K
Explanation: The universal gas constant R in the equation of gas has different values for different gases and at different conditions. For air at standard conditions, the correct value is 287 J/kg.K (check units).
2 - Question
Which is the wrong result for a calorically perfect gas?
c) cp and cp are functions of T
d) For T < 1000 K, specific heats are constant
Explanation: The calorically perfect gases have constant specific heats at low temperatures (T < 1000 K). cp and cp are nothing else but specific heat constants at constant pressure and temperature respectively. Internal energy and enthalpy can be given in terms of the specific heats i.e. e=cv T and h=cpT.
3 - Question
Specific heat for gas is almost constant in the case of_______
a) Space vehicle
b) High-temperature flow
c) Air in a desert
d) Chemically reacting flow at high- speeds
Explanation: Instances of very high temperature, chemically reacting flow at high-speeds do not have constant specific heats. Space vehicles fall in this category. The air in any desert would not have temperatures higher than 1000K and thus fall in the category of calorically perfect gas (i.e. constant specific heat).
4 - Question
Which is not a state variable/function in the given options according to thermodynamics?
c) Functions dependent on path
d) Internal energy
Explanation: By definition, state functions do not depend on the path and only depend on the initial and final state of the gas. Temperature, enthalpy and internal energy are all path independent i.e. state functions.
5 - Question
The incorrect formula for the specific heat of an ideal gas in terms of specific heat ratio of a gas and universal gas constant is_____
Explanation: The specific heat ratio of a gas is defined as the ratio cpcv and the difference in cp and cv is defined as the universal gas constant. From these, the following relations for the specific heat of an ideal gas are obtained cp=γRγ−1 and cv=Rγ−1.
6 - Question
For air, at 35°C what is the total enthalpy? (cp=1005 JkgK).
b) 309.54 J
c) 309.54 kJ
d) 8977 J
Explanation: The specific enthalpy (h) is given as h=cpT where T is in Kelvin. This is the enthalpy per unit mass. For total enthalpy of the air, we need to multiply h by the molecular mass of air i.e. H=29h which gives total enthalpy equal to 8977 J, irrespective of the process.
7 - Question
The first law of thermodynamics is proved by experiments. Select the correct choice for it.
a) de is an exact differential
b) dq is an exact differential
c) dw is an exact differential
d) The relation has been proved theoretically
Explanation: The first law in thermodynamics is an empirical relation and has been verified by experiments. It has never been violated. Exact differential depends only on final and initial points. de is an exact differential while dq and dw are not exact differentials.
8 - Question
Identify the incorrect source for the corresponding variables of the first law.
a) Absorbed radiation: dq
b) Squeezing of volume: dw
c) Heating: de
d) Displacement of system boundary: dq
Explanation: The change in dq or dw brings a change in de. Displacement of system boundary or squeezing of system volume are sources of dw. While heating and absorbing of radiation by mass in the system are sources of dq, therefore de.
9 - Question
Which is the process in which no heat exchange is there between system and surroundings and in which no dissipative phenomena occur?
a) Adiabatic process
b) Reversible process
c) Isentropic process d) Work
Explanation: A reversible process is one in which no dissipative phenomena occur. An adiabatic process is one in which no heat exchange is there between the system and the surroundings. A process that is both adiabatic and reversible is called an isentropic process
10 - Question
For a reversible process, which is not the correct statement?
a) No presence of viscosity
b) dw = -pdv
c) dq – pdv = de
d) Mass diffusion occurs
Explanation: Reversible process has no dissipative effects i.e. viscosity and mass diffusion are absent. Also, work in a reversible process is given as dw = -pdv. Therefore, the first equation of thermodynamics can be written as dq – pdv = de.