Basic Laws of Thermodynamics MCQS

 Unit 1: Basic Laws of Thermodynamics

Thermodynamics Questions and Answers – First Law for a Closed System

1. Energy has different forms which include
a) heat
b) work
c) all of the mentioned
d) none of the mentioned

Answer: c
Explanation: Basic fact about energy.

2. Work input is directly proportional to heat and the constant of proportionality is called
a) joule’s equivalent
b) mechanical equivalent of heat
c) all of the mentioned
d) none of the mentioned

Answer: c
Explanation: True for a closed system undergoing a cycle.

3. The value of constant of proportionality, J, has the value
a) 1
b) 0
c) -1
d) infinity

Answer: a
Explanation: In the S.I. system, both heat and work are measured in the derived unit of energy, the Joule.

4. It was Joule who first established that heat is a form of energy, and thus laid the foundation of the first law of thermodynamics.
a) true
b) false

Answer: a
Explanation: Prior to Joule, heat was considered to be an invisible fluid flowing from a body of higher calorie to a body of lower calorie.

5. Which of the following represents the energy in storage?
a) heat
b) work
c) internal energy
d) none of the mentioned

Answer: c
Explanation: Energy in storage is internal energy or the energy of the system.

6. By first law of thermodynamics,
a) Q=ΔE-W
b) Q=ΔE+W
c) Q=-ΔE-W
d) Q=-ΔE+W

Answer: b
Explanation: Q-W is the net energy stored in system and is called internal energy of system.

Thermodynamics Questions and Answers – Specific Heat at Constant Volume and Pressure and Control Volume

1. The specific heat of a substance at constant volume is defined as the rate of change of ___ with respect to ___
a) specific internal energy, temperature
b) work, pressure
c) specific internal energy, pressure
d) heat, temperature

Answer: a
Explanation: cv=∂u/∂T at constant volume.

2. Heat transferred at constant _____ increases the _____ of a system.
a) pressure, increases
b) volume, increases
c) both of the mentioned
d) none of the mentioned

Answer: c
Explanation: At constant pressure, (dQ)=dh and at constant volume, Q=Δu.

3. Specific heat of a substance at constant volume is a property of the system.
a) true
b) false

Answer: a
Explanation: Since T,v and u are the properties of the system, specific heat at a constant volume is a property of the system.

4. The specific heat of a substance at constant pressure is defined as the rate of change of ___ with respect to ___
a) work, pressure
b) enthalpy, temperature
c) enthalpy, pressure
d) heat, temperature

Answer: b
Explanation: cp=∂h/∂T at constant pressure.

5. The heat capacity at constant pressure Cp
a) m/cp
b) cp/m
c) mcp
d) none of the mentioned

Answer: c
Explanation: Cp=(mass*specific heat at constant pressure).    

6. Specific heat of a substance at constant pressure is a property of the system.
a) true
b) false

Answer: a
Explanation: cp is a property of a substance just like cv.

7. When there is mass transfer across the system boundary, the system is called
a) isolated system
b) closed system
c) open system
d) none of the mentioned

Answer: c
Explanation: Basic definition of an open system.

8. If a certain mass of steam is considered as the thermodynamic system, then the energy equation becomes
a) Q=ΔKE + ΔPE – ΔU + W
b) Q=ΔKE + ΔPE – ΔU – W
c) Q=-ΔKE – ΔPE + ΔU + W
d) Q=ΔKE + ΔPE + ΔU + W

Answer: d
Explanation: Q=ΔE + W and E=KE + PE + U.

9. The surface of the control volume is known as the control surface.
a) true
b) false

Answer: a
Explanation: This is same as the system boundary of the open system.

10. Steady flow means that the rates of flow of mass and energy across the control surface
a) varies
b) remains constant
c) depends on the control surface
d) none of the mentioned

Answer: b
Explanation: In a steady flow rate of flow remains constant.

Thermodynamics Questions and Answers – Enthalpy

1. The enthalpy of a substance(denoted by h), is defined as
a) h=u-pv
b) h=u+pv
c) h=-u+pv
d) h=-u-pv

Answer: b
Explanation: This is a basic definition for enthalpy.

2. In a constant volume process, internal energy change is equal to
a) heat transferred
b) work done
c) zero
d) none of the mentioned

Answer: a
Explanation: In a constant volume process, there is no work other than the pdV work

3. For an ideal gas, enthalpy becomes
a) h=u-RT
b) h=-u-RT
c) h=u+RT
d) h=-u+RT

Answer: c
Explanation: For an ideal gas, pv=RT.

4. Enthalpy is an intensive property of a system.
a) true
b) false

Answer: a
Explanation: Enthalpy is an intensive property measured mostly in kJ/kg.

5. Heat transferred at constant pressure _____ the enthalpy of a system.
a) decreases
b) increases
c) first decreases then increases
d) first increases then decreases

Answer: b
Explanation: At constant pressure, (dQ)=dh where h=u+pv is the specific enthalpy of the system.

6. The enthalpy of an ideal gas depends only on the temperature.
a) true
b) false

Answer: a
Explanation: This is because the internal energy of an ideal gas depends only on the temperature.

7. Total enthalpy of a system H is given by
a) H=h/m
b) H=m/h
c) H=mh
d) none of the mentioned

Answer: c
Explanation: Total enthalpy equals (mass*enthalpy) of substance.

8. The enthalpy and internal energy are the function of temperature for
a) all gases
b) steam
c) water
d) ideal gas

Answer: d
Explanation: The enthalpy of an ideal gas depends only on the temperature because the internal energy of an ideal gas depends only on the temperature.

9. Change in enthalpy of a system is due to heat supplied at
a) constant volume
b) constant pressure
c) both at constant volume and pressure
d) none of the mentioned

Answer: b
Explanation: Change in enthalpy occurs when heat is given to a system at constant pressure.

10. At constant pressure
a) pdv=d(pv)
b) dQ=du+d(pv)
c) dQ=d(u+pv)
d) all of the mentioned

Answer: d
Explanation: For a constant pressure process, dQ=du+pdv.

Thermodynamics Questions and Answers – Second Law of Thermodynamics

1. Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the net power output for this heat engine.
a) 30 MW
b) 40 MW
c) 50 MW
d) 60 MW

Answer: a
Explanation: Net power output = 80 – 50 MW = 30 MW.

2. Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat rejection to a nearby river is 50 MW, determine the thermal efficiency for this heat engine.
a) 47.5 %
b) 27.5 %
c) 37.5 %
d) none of the mentioned

Answer: c
Explanation: The thermal efficiency of heat engine = net work output / heat input
= 30/80 = 0.375 = 37.5 %.

3. A car engine with a power output of 50 kW has a thermal efficiency of 24 percent. Determine the fuel consumption rate of this car if the fuel has a heating value of 44,000 kJ/kg . 
a) 0.00273 kg/s
b) 0.00373 kg/s
c) 0.00473 kg/s
d) 0.00573 kg/s

Answer: c
Explanation: Q = 50/0.24 = 208.3 kW,
hence fuel consumption rate = 208.3 kW / 44000 kJ/kg = 0.00473 kg/s.

4. The food compartment of a refrigerator is maintained at 4°C by removing heat from it at a rate of 360 kJ/min. If the required power input to the refrigerator is 2kW, determine the coefficient of performance of the refrigerator.

a) 4
b) 3
c) 2
d) 1

Answer: b
Explanation: COP = (360/2)(1/60) = 3.

5. The food compartment of a refrigerator is maintained at 4°C by removing heat from it at a rate of 360 kJ/min. If the required power input to the refrigerator is 2kW, determine the rate of heat rejection to the room that houses the refrigerator.
a) 450 kJ/min
b) 460 kJ/min
c) 470 kJ/min
d) 480 kJ/min

Answer: d
Explanation: Q = 360 + (2)(60/1) = 480 kJ/min.

6. A heat pump is used to meet the heating requirements of a house and maintain it at 20°C. On a day when the outdoor air temperature drops to 2°C, the house is estimated to lose heat at a rate of 80,000 kJ/h. If the heat pump under these conditions has a COP of 2.5, determine the power consumed by the heat pump.
a) 32000 kJ/h
b) 33000 kJ/h
c) 34000 kJ/h
d) 35000 kJ/h

Answer: a
Explanation: W = Q/COP = 80000 kJ/h / 2.5 = 32000 kJ/h.

7. A heat pump is used to meet the heating requirements of a house and maintain it at 20°C. On a day when the outdoor air temperature drops to 2°C, the house is estimated to lose heat at a rate of 80,000 kJ/h. If the heat pump under these conditions has a COP of 2.5, determine the rate at which heat is absorbed from the cold outdoor air.
a) 32000 kJ/h
b) 48000 kJ/h
c) 54000 kJ/h
d) 72000 kJ/h

Answer: b
Explanation: The rate at which heat is absorbed = 80000 – 32000 = 48000 kJ/h.

8. An air-conditioner provides 1 kg/s of air at 15°C cooled from outside atmospheric air at 35°C. Estimate the amount of power needed to operate the air-conditioner.
a) 1.09 kW
b) 1.19 kW
c) 1.29 kW
d) 1.39 kW

Answer: d
Explanation: Q = m*cp*(temperature change) = 20.08 kW
COP = (15+273)/(35-15) = 14.4
hence power needed = 20/14.4 = 1.39 kW.

9. A cyclic machine, as shown below, receives 325 kJ from a 1000 K energy reservoir. It rejects 125 kJ to a 400 K energy reservoir and the cycle produces 200kJ of work as output. Is this cycle reversible, irreversible, or impossible?
a) reversible
b) irreversible
c) impossible
d) none of the mentioned

Answer: c
Explanation: The Carnot efficiency = 1 – (400/1000) = 0.6 and real efficiency = (300/325) = 0.615 which is greater than the Carnot efficiency hence cycle is impossible.

10. In a cryogenic experiment you need to keep a container at -125°C although it gains 100 W due to heat transfer. What is the smallest motor you would need for a heat pump absorbing heat from the container and rejecting heat to the room at 20°C?
a) 97.84 kW
b) 98.84 kW
c) 99.84 kW
d) 95.84 kW

Answer: a
Explanation: COP = 1.022 and thus power required = 100/1.022 = 97.84 kW.

11. A car engine operates with a thermal efficiency of 35%. Assume the air-conditioner has a coefficient of performance of 3 working as a refrigerator cooling the inside using engine shaft work to drive it. How much fuel energy should be spend extra to remove 1 kJ from the inside?
a) 0.752 kJ
b) 0.952 kJ
c) 0.852 kJ
d) none of the mentioned

Answer: b
Explanation: W = thermal efficiency * Q(fuel) thus Q(fuel) = 1/(0.35*3) = 0.952 kJ.

Thermodynamics Questions and Answers – Cyclic Heat Engine

1. The first law of thermodynamics doesn’t tell us whether a thermodynamic process is feasible or not.
a) true
b) false

Answer: a
Explanation: The second law of thermodynamics provides criterion as to the probability of a process.

2. According to Joule’s experiments,
a) heat can be completely converted into work
b) work can be completely converted into heat
c) both heat and work are completely interchangeable
d) all of the mentioned

Answer: b
Explanation: Work transfer -> internal energy increase -> heat transfer.

3. Which of the following is true?
a) work is a high grade energy
b) heat is a low grade energy
c) complete conversion of low grade energy into high grade energy in a cycle is impossible
d) all of the mentioned

Answer: d
Explanation: These facts are in accordance with Joule’s work and underlies the work of Carnot.

4. In a cyclic heat engine there is
a) net heat transfer to the system and net work transfer from the system
b) net heat transfer from the system and net work transfer to the system
c) depends on the conditions of cycle
d) none of the mentioned

Answer: a
Explanation: This is the basic concept of cycle heat engine.

5. Boiler, turbine, condenser and pump together constitute a heat engine.
a) true
b) false

Answer: a
Explanation: It is an example for a cyclic heat engine

6. In a heat engine cycle, which of the following process occurs?
a) heat is transferred from furnace to boiler
b) work is produced in turbine rotor
c) steam is condensed in condenser
d) all of the mentioned

Answer: d
Explanation: These are the basic processes occurring in a heat engine cycle comprising of furnace, boiler condenser and a turbine.

7. The function of a heat engine cycle is to _____ continuously at the expense of _____ to the system.
a) heat input, produce work
b) produce work, heat input
c) can be both of the mentioned
d) none of the mentioned

Answer: b
Explanation: Net work and heat input are of primary interest in a cycle.

8. Efficiency of a heat engine is defined as
a) total heat output / net work input
b) total heat input / net work output
c) net work output / total heat input
d) net work input / total heat output

Answer: c
Explanation: Basic definition of efficiency.

9. A thermal energy reservoir is a large body of
a) small heat capacity
b) large heat capacity
c) infinite heat capacity
d) none of the mentioned

Answer: c
Explanation: Basic fact about TER.

10. Processes inside a thermal energy reservoir are quasi-static.
a) true
b) false

Answer: a
Explanation: The changes taking place in TER are very slow and minute.

11. A TER which transfers heat to system is called ____ and one which receives heat is called ____
a) source, sink
b) sink, source
c) sink, sink
d) source, source

Answer: a
Explanation: A source transfers heat while a sink receives heat.

12. Which if the following statements are true for a mechanical energy reservoir(MER)?
a) it is a large body enclosed by an adiabatic impermeable wall
b) stores work as KE or PE
c) all processes within an MER are quasi-static
d) all of the mentioned

Answer: d
Explanation: These are some important features of an MER.

Thermodynamics Questions and Answers – Refrigerator and Heat Pump

1. Which device maintains a body at a temperature lower than the temperature of the surroundings?
a) PMM1
b) PMM2
c) refrigerator
d) heat pump

Answer: c
Explanation: This is the main function of a refrigerator.

2. What does a refrigerant do?
a) absorbs the heat leakage into body from surroundings
b) evaporates in the evaporator
c) absorbs latent heat of vaporization form the body which is cooled
d) all of the mentioned

Answer: d
Explanation: Refrigerant is required for the proper functioning of a refrigerator.

3. Coefficient of performance(COP) is defined as
a) heat leakage/work input
b) work input/heat leakage
c) latent heat of condensation/work input
d) work input/latent heat of condensation

Answer: a
Explanation: Coefficient of performance is the performance parameter used in a refrigerator cycle.

4. Which device maintains a body at a temperature higher than the temperature of the surroundings?
a) PMM1
b) PMM2
c) refrigerator
d) heat pump

Answer: d
Explanation: This is the main function of a heat pump.

5. In a heat pump, there is heat leakage from the body to the surroundings.
a) true
b) false

Answer: a
Explanation: This is just opposite to a refrigerator.

6. What is the relation between COP of heat pump and refrigerator?
a) COP of pump=COP of refrigerator – 1
b) COP of pump=COP of refrigerator + 1
c) COP of pump=COP of refrigerator – 2
d) COP of pump=COP of refrigerator + 2

Answer: b
Explanation: This relation comes from the COP of pump and refrigerator.

    

7. Heat leakage from a heat pump to surroundings is always greater than work done on pump.
a) true
b) false

Answer: a
Explanation: (Heat leakage from a heat pump to surroundings)=(COP of refrigerator + 1)*(work done on pump).

8. Which of the following statements are true?
a) a heat pump provides a thermodynamic advantage over direct heating
b) COP for both refrigerator and pump cannot be infinity
c) work input for both refrigerator and pump is greater than zero
d) all of the mentioned

Answer: d
Explanation: W is the electrical energy used to drive the pump or refrigerator which cannot be zero.

9. Kelvin-Planck’s and Clausius’ statements are
a) not connected to each other
b) virtually two parallel statements of second law
c) violation of one doesn’t violate the other
d) none of the mentioned

Answer: b
Explanation: Kelvin-Planck’s and Clausius’ statements are equivalent in all aspects.

10. If one of the Kelvin-Planck’s or Clausius’ statement is violated, then other is also violated.
a) true
b) false

Answer: a
Explanation: This shows the equivalence of Kelvin-Planck’s and Clausius’ statements.

Thermodynamics Questions and Answers – Kelvin-Planck Statement and Clausius’ Statement of Second Law

1. According to Kelvin-Planck statement, it is ____ for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at ____
a) impossible, single fixed temperature
b) possible, changing temperature
c) impossible, changing temperature
d) possible, single fixed temperature

Answer: a
Explanation: This is the basic definition of Kelvin-Planck statement.

2. If heat rejected from the system Q2 is zero, then
a) net work=Q1 and efficiency=1.00
b) heat is exchanged only with one reservoir
c) it violates the Kelvin-Planck statement
d) all of the mentioned

Answer: d
Explanation: Such a heat engine is called a perpetual motion machine of the second kind(PMM2).

3. A PMM2 is possible.
a) true
b) false

Answer: b
Explanation: A PMM2 is impossible because it violates the Kelvin-Planck statement.

4. A heat engine has to exchange heat with ___ energy reservoir at ___ different temperatures to produce net work in a complete cycle.
a) one, one
b) one, two
c) two, two
d) none of the mentioned

Answer: c
Explanation: This is required to produce power.

5. The second law is not a deduction of the first law.
a) true
b) false

Answer: a
Explanation: The first law is a separate law of nature.

6. The continual operation of a machine that creates its own energy and thus violates the first law is called
a) PMM2
b) PMM1
c) PMM0
d) none of the mentioned

Answer: b
Explanation: This is a basic fact about PMM1.

7. Which of the following is true?
a) heat always from a high temperature body to a low temperature body
b) heat always from a low temperature body to a high temperature body
c) heat can flow from both low to high and high to low temperature body
d) none of the mentioned

Answer: a
Explanation: The reverse process never occurs spontaneously.

8. According to Clausius statement
a) it is impossible to construct a device than can transfer heat from a cooler body to a hotter body without any effect
b) it is impossible to construct a device than can transfer heat from a hotter body to a cooler body without any effect
c) it is possible to construct a device than can transfer heat from a cooler body to a hotter body without any effect
d) none of the mentioned

Answer: a
Explanation: To transfer heat from a cooler body to a hotter body, some work must be expended.

9. If the second law were not true
a) a ship could be driven by extracting heat from the ocean
b) run a power plant by extracting heat from the air
c) both of the mentioned
d) none of the mentioned

Answer: c
Explanation: Both of the above possibilities do not violate the first law but do violate the second law.

10. The operation of a machine that utilizes the internal energy of only one TER, thus violating second law is called
a) PMM0
b) PMM1
c) PMM2
d) none of the mentioned

Answer: b
Explanation: PMM2 violates the second law.

Thermodynamics Questions and Answers – Reversibility, Irreversibilty and causes of Irreversibilty

1. A reversible process is performed in such a way that
a) at the conclusion of process, both system and surroundings can be restored to their initial states without producing any change
b) it should not leave any trace to show that the process had ever occurred
c) it is carried out infinitely slowly
d) all of the mentioned

Answer: d
Explanation: These are some basic concepts of a reversible process.

2. A reversible process coincides with a quasi-static process.
a) true
b) false

Answer: a
Explanation: A reversible process is carried out very slowly and every state it passes through is an equilibrium state.

3. Irreversibility of a process may be due to
a) lack of equilibrium during the process
b) involvement of dissipative effects
c) both of the mentioned
d) none of the mentioned

Answer: c
Explanation: These two are the major causes of irreversibility.

4. A heat transfer process approaches reversibility as the temperature difference between two bodies approaches
a) infinity
b) zero
c) -1
d) 1

Answer: b
Explanation: For heat transfer to be reversible, heat must be transferred through an infinitesimal temperature difference.

5. All actual heat transfer processes are
a) irreversible
b) take place through a finite temperature difference
c) both of the mentioned
d) none of the mentioned

Answer: c
Explanation: An infinitesimal temperature difference is not easy to attain.

6. Free expansion is irreversible.
a) true
b) false

Answer: a
Explanation: It can be demonstrated by the second law.

7. Which of the following can be a cause of irreversibility?
a) friction, viscosity
b) inelasticity
c) electrical resistance, magnetic hysteresis
d) all of the mentioned

Answer: d
Explanation: These effects are known as dissipative effects.

8. The continual motion of a movable device in the complete absence of friction is known as
a) PMM2
b) PMM3
c) PMM1
d) PMM0

Answer: b
Explanation: This is not possible since lubrication cannot be completely eliminated.

9. The friction present in moving devices makes a process reversible.
a) true
b) false

Answer: b
Explanation: Friction lakes the process irreversible.

10. Which of the following is irreversible?
a) stirring work
b) friction work in moving devices
c) current flowing through a wire
d) all of the mentioned

Answer: d
Explanation: All these processes includes a particular cause of irreversibility.

11. The external irreversibility occurs at the system boundary.
a) true
b) false

Answer: a
Explanation: This mainly includes heat interaction with the surroundings due to a finite temperature gradient.

Thermodynamics Questions and Answers – Carnot Theorem, Carnot Cycle and Reversed Heat Engine

1. Carnot cycle is a reversible cycle.
a) true
b) false

Answer: a
Explanation: A reversible cycle is an ideal hypothetical cycle in which all processes are reversible.

2. A reversible cycle has following processes.
a) 4 isothermal processes
b) 4 adiabatic processes
c) 2 isothermal and 2 adiabatic processes
d) none of the mentioned

Answer: c
Explanation: Two reversible isotherms and two reversible adiabatics constitute a Carnot cycle.

3. The correct sequence of the processes taking place in a carnot cycle is
a) adiabatic -> adiabatic -> isothermal -> isothermal
b) adiabatic -> isothermal -> adiabatic -> isothermal
c) isothermal -> isothermal -> adiabatic -> adiabatic
d) isothermal -> adiabatic -> isothermal -> adiabatic

Answer: d
Explanation: Carnot cycle consists if these four processes in succession.

4. The reversed heat engine takes heat from a ___ temperature body, then discharges it to a ___ temperature body and ___ an inward flow of network.
a) high, low, receives
b) low, high, receives
c) high, low, gives
d) low, high, gives

Answer: b
Explanation: In reversed heat engine, the magnitude of energy transfers remains same and only directions change.

5. Example of reversed heat engine is
a) heat pump
b) refrigerator
c) both of the mentioned
d) none of the mentioned

Answer: c
Explanation: Heat pump and refrigerator are the types of reversed heat engine.

6. According to Carnot’s theorem, all heat engines operating between a given constant temperature source and sink, none has a higher efficiency than a reversible engine.
a) true
b) false

Answer: a
Explanation: This is the statement of Carnot’s theorem 

7. The efficiency of all reversible heat engines operating between the same heat reservoirs is
a) same
b) independent of the nature of working substance
c) independent of the amount of working substance
d) all of the mentioned

Answer: d
Explanation: This statement is a corollary of Carnot’s theorem.

8. Efficiency of a reversible heat engine is given by
a) 1-(T1/T2)
b) 1-(T2/T1)
c) (T1/T2)-1
d) (T2/T1)-1

Answer: b
Explanation: Efficiency=1-(Q2/Q1) and T2,T1 are temperatures at which heat is rejected and received.

9. For a reversible refrigerator, Coefficient of Performance is given by
a) T2/(T1-T2)
b) T1/(T1-T2)
c) T2/(T2-T1)
d) T1/(T2-T1)

Answer: a
Explanation: For a reversible refrigerator, (Q1/Q2)=(T1/T2).

10. For a reversible heat pump, COP is given by
a) T2/(T1-T2)
b) T1/(T1-T2)
c) T2/(T2-T1)
d) T1/(T2-T1)

Answer: b
Explanation: For a reversible heat pump we have, (Q1/Q2)=(T1/T2).