JSSC- JE Mechanical Engg. Question Bank 3 (Chapter – 5) 2nd Law of Thermodynamics

Mechanical

What is a heat source in thermodynamics?

• A reservoir that supplies heat to a thermodynamic system
• A reservoir that always receives heat
• A device that converts work into pressure
• A system that contains only cold water

Explanation:

• A heat source is a body or reservoir capable of supplying heat to another body without appreciable change in its own temperature.

What is a heat sink?

• A reservoir that receives heat from a thermodynamic system
• A reservoir that supplies all the heat
• A device that produces heat without work
• A reservoir having zero temperature

Explanation:

• A heat sink is a low-temperature reservoir that absorbs rejected heat from a heat engine or other thermodynamic device.

What is a heat reservoir?

• A body capable of absorbing or supplying heat at nearly constant temperature
• A body that can only supply work
• A body whose temperature changes rapidly during heat transfer
• A device used only for measuring pressure

Explanation:

• A heat reservoir is an idealized body with sufficiently large thermal capacity so that heat transfer occurs without appreciable temperature change.

Which reservoir supplies heat to a heat engine?

• High-temperature reservoir
• Low-temperature reservoir
• Cold reservoir only
• Vacuum reservoir

Explanation:

• A heat engine receives heat from a high-temperature source, converts part of it into work, and rejects the remaining heat to a low-temperature sink.

Which reservoir receives the rejected heat from a heat engine?

• Low-temperature reservoir
• High-temperature reservoir
• Work reservoir
• Pressure reservoir

Explanation:

• The heat rejected by a heat engine is transferred to a low-temperature reservoir called the sink.

What is a heat engine?

• A device that converts part of heat supplied into work
• A device that converts all work into heat only
• A device that transfers heat from cold to hot without work
• A device that stores heat permanently

Explanation:

• A heat engine operates cyclically, receives heat from a high-temperature source, produces work, and rejects some heat to a low-temperature sink.

What are the three essential energy interactions of a heat engine?

• Heat input, work output and heat rejection
• Only heat input
• Only work input and heat input
• Only heat rejection

Explanation:

• A cyclic heat engine receives heat Q_H, produces work W and rejects heat Q_L.

Which statement correctly represents the energy balance of a cyclic heat engine?

• W = Q_H − Q_L
• W = Q_H + Q_L
• W = Q_L − Q_H
• W = Q_HQ_L

Explanation:

• For a cyclic heat engine, the net work output equals heat supplied minus heat rejected.

What is the thermal efficiency of a heat engine?

• Ratio of net work output to heat supplied
• Ratio of heat rejected to work output
• Ratio of heat supplied to heat rejected
• Ratio of work input to heat supplied

Explanation:

• Thermal efficiency is η = W/Q_H = (Q_H − Q_L)/Q_H.

Can the thermal efficiency of a heat engine be 100%?

• No, for a cyclic heat engine operating between finite-temperature reservoirs
• Yes, for every heat engine
• Yes, whenever heat input is large
• Yes, for all reversible engines

Explanation:

• The second law requires some heat rejection to a low-temperature reservoir, so a heat engine cannot convert all supplied heat into work.

What is a refrigerator?

• A device that removes heat from a low-temperature region using work input
• A device that converts heat completely into work
• A device that supplies heat without work
• A device that rejects heat only to a cold reservoir

Explanation:

• A refrigerator uses work input to transfer heat from a low-temperature space to a high-temperature surroundings.

What is a heat pump?

• A device that supplies heat to a high-temperature region using work input
• A device that produces work from heat only
• A device that removes heat without work
• A device that operates without heat transfer

Explanation:

• A heat pump transfers heat from a low-temperature source to a high-temperature space, with work supplied to the system.

What is the coefficient of performance of a refrigerator?

• Ratio of heat removed from the refrigerated space to work input
• Ratio of work output to heat supplied
• Ratio of heat rejected to heat supplied
• Ratio of work input to heat removed

Explanation:

• For a refrigerator, COP_R = Q_L/W.

What is the coefficient of performance of a heat pump?

• Ratio of heat delivered to the heated space to work input
• Ratio of work output to heat rejected
• Ratio of heat removed to work output
• Ratio of heat supplied to heat removed

Explanation:

• For a heat pump, COP_HP = Q_H/W.

What is the relationship between COP of a heat pump and refrigerator?

• COP_HP = COP_R + 1
• COP_HP = COP_R − 1
• COP_HP = COP_R
• COP_HP = 1/COP_R

Explanation:

• Since Q_H = Q_L + W, dividing by W gives COP_HP = COP_R + 1.

Can the COP of a refrigerator be greater than 1?

• Yes
• No, never
• Only for an ideal heat engine
• Only when work input is zero

Explanation:

• COP is not an efficiency. A refrigerator can have a COP greater than 1 because it transfers heat rather than converting all input work into heat removal.

If a refrigerator removes 300 kJ of heat using 100 kJ of work, what is its COP?

• 3
• 0.33
• 2
• 4

Explanation:

• COP_R = Q_L/W = 300/100 = 3.

A heat engine receives 1000 kJ of heat and rejects 400 kJ. What is its work output?

• 600 kJ
• 1400 kJ
• 400 kJ
• 250 kJ

Explanation:

• W = Q_H − Q_L = 1000 − 400 = 600 kJ.

For the heat engine receiving 1000 kJ and rejecting 400 kJ, what is its thermal efficiency?

• 60%
• 40%
• 140%
• 25%

Explanation:

• η = W/Q_H = 600/1000 = 0.60 = 60%.

Which statement is the Kelvin-Planck statement of the second law?

• No cyclic heat engine can convert all the heat received from a single reservoir completely into work
• Heat naturally flows from cold to hot
• All heat can be converted into work
• Entropy of every system is always zero

Explanation:

• The Kelvin-Planck statement establishes that a cyclic heat engine cannot produce a net work output while exchanging heat with only one reservoir.

What does the Kelvin-Planck statement imply about a heat engine?

• Some heat must be rejected to a low-temperature reservoir
• All heat supplied must become work
• No work can ever be produced
• The engine must operate at zero temperature

Explanation:

• A heat engine cannot convert all supplied heat into work; a portion must be rejected to a sink.

Which statement is the Clausius statement of the second law?

• No cyclic device can transfer heat from a colder body to a hotter body without external work input
• Heat can always flow from cold to hot naturally
• A heat engine can convert all heat into work
• Entropy is always zero

Explanation:

• The Clausius statement says that heat cannot, by itself, flow from a low-temperature reservoir to a high-temperature reservoir in a cyclic process.

Which device requires work input to transfer heat from a low-temperature reservoir to a high-temperature reservoir?

• Refrigerator or heat pump
• Heat engine
• Steam turbine
• Boiler without external input

Explanation:

• Refrigerators and heat pumps use external work to transfer heat from a lower temperature to a higher temperature.

What is the relationship between the Kelvin-Planck and Clausius statements?

• They are equivalent statements of the second law
• They contradict each other
• They apply only to the first law
• They apply only to fluid mechanics

Explanation:

• Violation of one statement would imply violation of the other; therefore, the two statements are thermodynamically equivalent.

What is the absolute or thermodynamic temperature scale based on?

• The second law and reversible heat-engine behavior
• The freezing point of mercury only
• The boiling point of water only
• Atmospheric pressure only

Explanation:

• The thermodynamic temperature scale is independent of the properties of any particular thermometric substance and can be established using reversible heat engines.

What is the SI unit of thermodynamic temperature?

• Kelvin (K)
• Degree Celsius
• Joule
• Pascal

Explanation:

• The kelvin is the SI base unit of thermodynamic temperature.

What is the relation between Celsius and Kelvin temperature scales?

• T(K) = t(°C) + 273.15
• T(K) = t(°C) − 273.15
• T(K) = 273.15t
• T(K) = t(°C)/273.15

Explanation:

• Absolute temperature in kelvin is obtained by adding 273.15 to the Celsius temperature.

Why is absolute temperature used in thermodynamic efficiency equations?

• Because thermodynamic temperature has an absolute zero and is independent of the thermometric substance
• Because Celsius temperature is always zero at room temperature
• Because Kelvin has no units
• Because pressure is measured in kelvin

Explanation:

• The thermodynamic scale provides an absolute temperature measure suitable for fundamental thermodynamic relationships.

For a reversible Carnot engine, efficiency depends on which temperatures?

• Source and sink absolute temperatures
• Only source temperature
• Only sink temperature
• Pressure and volume only

Explanation:

• Carnot efficiency depends only on the absolute temperatures of the high-temperature and low-temperature reservoirs.

What is the Clausius integral for a reversible cycle?

• ∮ δQ/T = 0
• ∮ δQ/T > 0
• ∮ δQ/T < 0
• ∮ δQ/T = 1

Explanation:

• For a reversible cycle, the cyclic integral of heat transfer divided by absolute temperature is zero.

What inequality applies to the Clausius integral for an irreversible cycle?

• ∮ δQ/T < 0
• ∮ δQ/T = 0
• ∮ δQ/T > 0
• ∮ δQ/T = 1

Explanation:

• For an irreversible cycle, the Clausius inequality states that ∮δQ/T is less than zero.

What does the Clausius inequality lead to?

• The concept of entropy
• The first law only
• The ideal gas constant
• Newton’s law of motion

Explanation:

• The Clausius inequality provides the foundation for defining entropy as a thermodynamic property.

What is entropy?

• A thermodynamic property related to energy dispersal and irreversibility
• A measure of pressure only
• A measure of volume only
• A form of mechanical work

Explanation:

• Entropy is a thermodynamic state property that quantifies energy dispersal and is central to the analysis of reversible and irreversible processes.

What is the SI unit of specific entropy?

• kJ/kg·K
• kJ/kg
• kPa
• m³/kg

Explanation:

• Specific entropy is measured in energy per unit mass per unit absolute temperature, commonly kJ/kg·K.

For a reversible process, what is the differential relation for entropy change?

• ds = δQ_rev/T
• ds = TδQ
• ds = δW/T
• ds = Pdv

Explanation:

• For a reversible process, the differential change in specific entropy is given by ds = δq_rev/T.

Is entropy a path function or a state function?

• State function
• Path function
• Process variable only
• Mechanical variable

Explanation:

• Entropy depends only on the initial and final states, making it a thermodynamic state property.

For an isolated system, what happens to entropy during an irreversible process?

• It increases
• It decreases
• It always remains zero
• It becomes negative

Explanation:

• For an isolated system, entropy cannot decrease; it increases for an irreversible process and remains constant for a reversible process.

What is the entropy change of a system undergoing a reversible adiabatic process?

• Zero
• Positive
• Negative
• Infinite

Explanation:

• A reversible adiabatic process has no heat transfer and is therefore isentropic, so entropy remains constant.

For an ideal gas, what is the entropy change between two states using temperature and volume?

• Δs = C_v ln(T₂/T₁) + R ln(v₂/v₁)
• Δs = C_p ln(T₂/T₁)
• Δs = R(T₂ − T₁)
• Δs = C_v(v₂ − v₁)

Explanation:

• For an ideal gas, entropy change can be calculated using Δs = C_v ln(T₂/T₁) + R ln(v₂/v₁).

For an ideal gas, what is the entropy change between two states using temperature and pressure?

• Δs = C_p ln(T₂/T₁) − R ln(P₂/P₁)
• Δs = C_v ln(P₂/P₁)
• Δs = R ln(T₂/T₁)
• Δs = C_p(P₂ − P₁)

Explanation:

• For an ideal gas, entropy change can also be expressed as Δs = C_p ln(T₂/T₁) − R ln(P₂/P₁).

For an ideal gas undergoing an isothermal process, what is the entropy change?

• Δs = R ln(v₂/v₁)
• Δs = C_v ln(T₂/T₁)
• Δs = 0 for every isothermal process
• Δs = C_p(T₂ − T₁)

Explanation:

• For an ideal gas at constant temperature, the temperature term is zero, leaving Δs = R ln(v₂/v₁).

For an ideal gas undergoing a constant-volume process, what is the entropy change?

• Δs = C_v ln(T₂/T₁)
• Δs = R ln(v₂/v₁)
• Δs = C_p ln(P₂/P₁)
• Δs = 0 in every case

Explanation:

• At constant volume, v₂/v₁ = 1, so the volume term vanishes and Δs = C_v ln(T₂/T₁).

For an ideal gas undergoing a constant-pressure process, what is the entropy change?

• Δs = C_p ln(T₂/T₁)
• Δs = C_v ln(T₂/T₁)
• Δs = R ln(v₂/v₁)
• Δs = 0 for every process

Explanation:

• At constant pressure, the pressure term is zero, so entropy change is Δs = C_p ln(T₂/T₁).

What is the Carnot cycle?

• An ideal reversible cycle operating between two temperature limits
• An irreversible cycle using one reservoir
• A cycle consisting only of isobaric processes
• A cycle operating without heat transfer

Explanation:

• The Carnot cycle is an ideal reversible heat-engine cycle consisting of two isothermal and two reversible adiabatic processes.

How many processes are present in a Carnot cycle?

• Four
• Two
• Three
• Five

Explanation:

• The Carnot cycle consists of two reversible isothermal processes and two reversible adiabatic processes.

Which processes constitute a Carnot cycle?

• Two isothermal and two adiabatic processes
• Four isothermal processes
• Two isobaric and two isochoric processes
• Four adiabatic processes

Explanation:

• The Carnot cycle alternates between reversible isothermal heat-transfer processes and reversible adiabatic expansion/compression processes.

During the isothermal expansion of a Carnot engine, heat is supplied at what temperature?

• High temperature T_H
• Low temperature T_L
• Absolute zero
• Room temperature only

Explanation:

• During the high-temperature isothermal expansion, the working substance absorbs heat from the high-temperature reservoir at T_H.

During which process does heat rejection occur in a Carnot engine?

• Low-temperature isothermal compression
• High-temperature isothermal expansion
• Adiabatic expansion
• Adiabatic compression

Explanation:

• Heat is rejected to the low-temperature reservoir during the reversible isothermal compression process.

What happens during the adiabatic expansion of a Carnot engine?

• Temperature decreases without heat transfer
• Temperature increases with heat input
• Heat is rejected at constant temperature
• Pressure remains constant

Explanation:

• During reversible adiabatic expansion, no heat is transferred and the working fluid does work, causing its temperature to decrease.

What happens during the adiabatic compression of a Carnot engine?

• Temperature increases without heat transfer
• Temperature decreases with heat rejection
• Heat is supplied isothermally
• Pressure becomes zero

Explanation:

• During reversible adiabatic compression, work is done on the working fluid, increasing its temperature without heat transfer.

What is the efficiency of a Carnot engine operating between temperatures T_H and T_L?

• η = 1 − T_L/T_H
• η = 1 + T_L/T_H
• η = T_L/T_H
• η = T_H/T_L

Explanation:

• Carnot efficiency is η = 1 − T_L/T_H, where temperatures must be expressed on an absolute scale.

A Carnot engine operates between 600 K and 300 K. What is its efficiency?

• 50%
• 25%
• 75%
• 33.3%

Explanation:

• η = 1 − 300/600 = 0.5 = 50%.

What happens to Carnot efficiency if the sink temperature is reduced while source temperature remains constant?

• Efficiency increases
• Efficiency decreases
• Efficiency becomes zero
• Efficiency remains unchanged

Explanation:

• Since η = 1 − T_L/T_H, reducing T_L increases the Carnot efficiency.

What happens to Carnot efficiency if the source temperature increases while sink temperature remains constant?

• Efficiency increases
• Efficiency decreases
• Efficiency becomes zero
• Efficiency remains unchanged

Explanation:

• Increasing T_H reduces the ratio T_L/T_H, thereby increasing Carnot efficiency.

What is the maximum possible efficiency of a heat engine operating between two fixed temperatures?

• Carnot efficiency
• 100% in every case
• 50% in every case
• Zero

Explanation:

• The Carnot cycle establishes the maximum theoretical efficiency for a heat engine operating between specified temperature limits.

What is PMM-2?

• A perpetual motion machine of the second kind
• A machine that violates only Newton’s law
• A machine with zero mechanical motion
• A machine that violates the first law only

Explanation:

• PMM-2 is a hypothetical machine that would violate the second law of thermodynamics by allowing complete conversion of heat from a single reservoir into work or equivalent forbidden operation.

Why is PMM-2 impossible?

• It violates the second law of thermodynamics
• It violates only the conservation of mass
• It requires no working fluid
• It violates Newton’s first law

Explanation:

• PMM-2 would contradict the Kelvin-Planck or Clausius statement of the second law, so such a machine cannot exist.

What would a PMM-2 heat engine accomplish if it were possible?

• Convert heat from a single reservoir completely into work in a cycle
• Convert work completely into heat
• Transfer heat from hot to cold naturally
• Produce no work from heat

Explanation:

• A PMM-2 would violate the Kelvin-Planck statement by converting heat received from a single reservoir entirely into work in a cyclic process.

Which fundamental law is violated by a PMM-2?

• Second law of thermodynamics
• First law only
• Law of conservation of mass
• Newton’s second law

Explanation:

• PMM-2 is impossible because its operation contradicts the second law of thermodynamics.

What is the fundamental difference between PMM-1 and PMM-2?

• PMM-1 violates the first law, while PMM-2 violates the second law
• Both violate only the first law
• Both violate only the second law
• PMM-1 violates the second law and PMM-2 violates the first law

Explanation:

• PMM-1 proposes creation of energy and violates the first law, whereas PMM-2 proposes an impossible thermodynamic operation and violates the second law.

Which of the following is impossible according to the second law?

• A cyclic heat engine converting all supplied heat into work
• A heat engine rejecting some heat
• A refrigerator consuming work
• A heat pump transferring heat to a warmer region with work input

Explanation:

• The second law prohibits complete conversion of heat from a single reservoir into work by a cyclic heat engine.

What is the entropy change of the universe for a reversible process?

• Zero
• Positive
• Negative
• Infinite

Explanation:

• For a reversible process, the entropy generated is zero, so the total entropy change of the system and surroundings is zero.

What is the entropy change of the universe for an irreversible process?

• Positive
• Zero
• Negative
• Always infinite

Explanation:

• Irreversibilities generate entropy, so the entropy of the universe increases for an irreversible process.

Which condition is necessary for a Carnot engine to have zero efficiency?

• T_H = T_L
• T_H > T_L
• T_H = 2T_L
• T_L = 0 K only

Explanation:

• From η = 1 − T_L/T_H, if the source and sink temperatures are equal, efficiency becomes zero.

Can a Carnot engine operate with a sink temperature of 0 K in an ideal mathematical limit?

• Its efficiency would approach 100%
• Its efficiency would become zero
• Its efficiency would become negative
• Its efficiency would always remain 50%

Explanation:

• From the Carnot relation, as T_L approaches 0 K, efficiency approaches 100%. Absolute zero, however, cannot be reached by a finite sequence of thermodynamic processes.