Thermodynamics - 1 - Mechanical Engineering MCQ

Thermal efficiency is defined as 
Where Q₂ is the heat rejected from the system and Qt is the heat added to the system.
Here Q₂ = 50 J 
Q₁ = 80 J

Availability is the maximum useful work that can be extracted from the system working between two reservoirs.

Availability of Heat = 

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.

Clausius statement is related to refrigerator and heat pump whereas gas turbines works on Brayton Cycle.

Absolute zero (0 K) is a state at which the enthalpy and entropy of a system reach their minimum value, taken as 0.

In mathematical term,

S = k ln W

Where, S is entropy

k is Boltzmann constant

W is thermodynamic probability

When W = 1, it represents the greatest order, S = 0. This occurs only at T = 0 K.

This statement is also known as Third law of thermodynamics.

Entropy is a property which is a measure of energy dispersion in a system or the irreversibility. Entropy transfer is associated with heat transfer. If the heat is added to the system, then its entropy increases and if heat is lost from the system, its entropy decreases.

Efficiency of first engine (E₁), 

Efficiency of second engine

As efficiency of both engine is same,

So, η₁ = η₂

⇒ T² = 2000 × 50

Short Trick:

When efficiency of both engines are equal, then intermediate temperature

Efficiency of Carnot engine (η_E) = 50%

Short trick:

If a Carnot heat engine which is operating between two temperature limits is converted to heat pump then 

T₁ = 2000 K

T₂ = 500 K

Since, the actual efficiency is more than the Carnot efficiency, so this engine has an impossible cycle.

dQ = dW + ΔU (As per first law)

For the cyclic process,

ΔU = 0

∴∫dW=∫dQ∴∫dW=∫dQ

∴ W = 20 - 30 - 5 + 10

= -5 kJ

A Carnot cycle consists of the four processes:

1. Reversible isothermal heat addition
2. Isentropic (reversible adiabatic) expansion
3. Reversible isothermal heat rejection
4. Isentropic compression of the gas

The pressure and temperature at which three phases of a pure substance coexist is called triple point. The triple point is merely the point of intersection of the sublimation and vaporization curves. It has been found that on a ‘p-T’ diagram the triple point is represented by a point and on a ‘p-v’ and T-s diagram it is a line, and on a ‘u-v’ diagram it is a triangle. In the case of ordinary water, the triple point is at a pressure of 4.58 mm Hg and a temperature of 0.01^OC.

γ_monoatomic> γ_diatomic> γ_triatomic

As efficiency is directly proportional to the ratio of specific heat (γ), so monoatomic will have the highest efficiency.

First law of thermodynamics:

δQ=ΔU+δWδQ=ΔU+δW

(-3000) = ΔU + (-9000)

ΔU = 6000 kJ

As there is 9000 kJ work done on the system, no work will be done by the system. Energy will be stored in the system as internal energy.

Correct Answer :- C

Explanation : Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities in places where the pressure is relatively low.

When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate a shock wave that is strong very close to the bubble, but rapidly weakens as it propagates away from the bubble.

Therefore, If pressure at any point in the liquid approaches the vapor pressure, liquid starts vaporising and creates pockets or bubbles of dissolved gases and vapours. This phenomenon is Cavitation.

Work done, W = P₂V₂ - P₁V₁

Correct Answer :- c

Explanation : A series of operations, which takes place in a certain order and restores the initial conditions at the end, is known as thermodynamic cycle.

The specific heat of solids and liquids and the specific heats (both at constant volume and constant pressure) of ideal gases are essentially functions of temperature only. At T = 0K i.e, at the absolute zero thermodynamic temperature one should have the values of specific heats as zero putting T = 0K in the expressions for specific heats.

This strictly means at an absolute temperature of 0K we would need no energy at all to raise the temperature of unit mass of a substance by unit degree rise in temperature. But as soon as the temperature rises by even an infinitesimal amount, we have some value of the specific heat and it increases with temperature and hence more and more energy would be needed to achieve a similar rise in temperature as temperature increases.

Joule Thomson Coefficient:-

For an ideal gas, μ = 0, because ideal gases neither warm not cool upon being expanded at constant enthalpy.

A thermometer works on the principle of Zeroth law of Thermodynamics.

According to Zeroth Law of thermodynamics, if two bodies are in thermal equilibrium with a third body then the two bodies are also in thermal equilibrium with each other.

Thermometer are based on the principle of finding the temperature by measuring the thermometric property.

The measurement of temperature is performed by the thermometer by using mercury, which - when heated - expands, and contracts in response to a fall in temperature. This causes the length of the liquid column to be longer or shorter depending on the temperature.