Thermodynamics - 3 - Mechanical Engineering MCQ

The critical temperature is the highest temperature at which vapour and liquid phases can coexist in equilibrium. For gases, the critical temperature is the one above which a gas cannot be liquefied regardless of the pressure applied. Also, note that dryness fraction is not defined at the critical point

The universe is formed by system and surrounding and considered as an isolated system.

i) The energy of the world (universe) is constant.

ii) The entropy of the world tends towards a maximum.

iii) The entropy increase of an isolated system is a measure of the extent of irreversibility of the process undergone by the system.

In actual practice the reversible isentropic process never really occurs, it is only an ideal process. In actual practice whenever there is a change in the state of the system the entropy of the system increases. The entropy of the universe increases because energy never flows uphill spontaneously.

If in any process, if the entropy of the universe decreases, then the process will be an impossible process.

In properties of vapour, we require at least two independent properties to determine the state of the vapour. Both pressure and temperature are dependent properties so, if we know only pressure and temperature, we can’t determine the properties of vapour.

The Carnot COP of a refrigerator working between the temperature limit of TH and TL is given as

Kindly note that the COP of Carnot heat pump or refrigerator depends only on the upper and lower temperature limits and COP is independent of the working fluid.

Change of entropy in a reversible process:

Entropy changes for a closed system:

For constant volume:

For constant pressure:

For constant temperature process:

For adiabatic process (Reversible):

dQ = 0 ⇒ dS = 0

Calculation:

Here T₁ = T₂

So the process is isothermal

The universe is formed by system and surrounding and considered as an isolated system. The entropy of the universe always tends towards a maximum. The entropy increase of an isolated system is a measure of the extent of irreversibility of the process undergone by the system.

There are two statements of second law of thermodynamics.

Clausius statement: No device can operate on a cycle and produce effect that is solely the heat transfer from a lower-temperature body to a higher-temperature body

Kelvin Plank statement: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work

Equivalence of two statements: If some device violates one statement, it also violates the other statement, and vice versa.

From the first law of thermodynamics

dQ = dU + dW

dQ = -800 J (As the heat is rejected from the system)

dW = -2000 J (as work is supplied to system)

Change in internal energy = dQ – dW = -800 – (-2000) = 1200 J

Change in specific internal energy = 1200/4 = 300 J/kg  (mass is 4 kg)

Kelvin – Planck statement states “it is impossible to construct an engine, which is operating in a cycle produces no other effect except to external heat from a single reservoir and do equivalent amount of work.

A PMM II is the heat engine which produces the work equal to the heat supplied. Thus, it violates the second law of thermodynamics and It is a hypothetical machine.

On the other hand, Perpetual motion machine of type-I violates the first law of thermodynamics.

temperature, when the absolute pressure remains constant." Mathematically,

Or “All perfect gases change in volume by 1 / 273 th of its original volume at 0° C for every 1° C change in temperature, when the pressure remains constant."

Boyles law: The absolute pressure of a given mass of a perfect gas varies inversely as its volume, when the temperature remains constant." Mathematically,

As per Gibb’s phase rule,

F = C – P + 2

Where F = Degree of freedom

C = Number of chemical components

P = No of Phases

F = 1 – 3 + 2 = 0

Matter around us exists in three different states – solid, liquid and gas. They are distinguished by their intermolecular attraction force, molecular arrangement and the mobility of the system.

Particles in a:

• Gas vibrate and move freely at high speeds.
• Liquid vibrate, move about, and slide past each other.
• Solid vibrate (jiggle) but generally do not move from place to place.

The process of heating up of an automobile can be assumed as isobaric. Because the pressure inside the car is constant. So out of given options the option b is correct.

W = Q₁ – Q₂ = T₁ΔS – T₂ΔS = (T₁ – T₂) ΔS = (300 – 150) × 1 = 150 kJ

For free expansion dW = 0

No heat interaction takes place

Hence dQ = 0

From first law

dQ = du + dW

du = 0

Closed System: There is no mass transfer across the system boundary but energy transfer takes place into or out of the system.

Open System: In this type of system both mass and energy transfer takes place across the boundary of system.

Isolated System: In this type of system neither mass nor energy crosses the boundary of the system. So it is a system with fixed mass and fixed energy.