Thermodynamics System and Processes MCQ Quiz - Objective Question with Answer for Thermodynamics System and Processes - Download Free PDF

Following four processes are involved

(i) Reversible adiabatic expansion, V → 2V

(ii) Reversible Isothermal compression, 2V → V

(iii) Reversible Isothermal expansion, V → 2V

(iv) Reversible Adiabatic compression, 2V → V

These four processes are presented on PV-diagram

Note that on same PV diagram

${\left(\frac{dP}{dV}\right)}_{\text{adiabatic }}=\gamma {\left(\frac{dP}{dV}\right)}_{\text{isothermal }}$

i.e. adiabatic curve is more steeper than isothermal.

In first step, Reversible adiabatic expression, V → 2V and pressure changes from P_i = P_A to P_B.

In second step, Reversible Isothermal compression, 2V → V, system moves from B to C with relatively lower slope.

In third step, Reversible Isothermal expansion, system retrace path CB and reaches back at B with pressure P_B.

In fourth step, it traces BA by following Reversible Adiabatic compression from 2V → V.

∴ Final pressure P_F = P_A = P_i.

Explanation:

Quasi-Static Process

• A quasi-static process, also known as a quasi-equilibrium process, is an idealized thermodynamic process that occurs infinitely slowly. This slow progression ensures that the system remains in near-perfect equilibrium at every stage of the process. In such processes, the state variables (like pressure, temperature, and volume) change so gradually that the system can be considered to pass through a series of equilibrium states.
• In reality, no process can be perfectly quasi-static as it would take an infinite amount of time to complete. However, engineers and scientists use the quasi-static approximation to simplify the analysis of thermodynamic systems. It allows for the use of equilibrium thermodynamics principles to describe the process.

Characteristics of a Quasi-Static Process:

• The system is always infinitesimally close to equilibrium.
• Changes in thermodynamic variables are gradual and reversible (if there are no irreversibilities).
• Heat and work interactions can be analyzed as they occur at boundary conditions that are well-defined.

In a quasi-static process, irreversibilities are considered negligible. This is because the slow nature of the process ensures that factors like friction, turbulence, and other sources of irreversibility have minimal impact on the system's behavior. This is a key assumption when analyzing quasi-static processes, as it simplifies calculations and allows for the application of reversible process equations.

Explanation:

Quasi-static Process

• A quasi-static process is a thermodynamic process that happens infinitely slowly, ensuring that the system remains in equilibrium at all times. This means that every intermediate state of the system during the process can be considered an equilibrium state. In practice, quasi-static processes are idealizations and cannot be perfectly achieved; however, they are critical for understanding thermodynamics and formulating equations for reversible processes.
• Quasi-static processes are characterized by the absence of rapid changes, which ensures that pressure, temperature, and other properties of the system remain uniform throughout. Such processes are often reversible, as there are no significant gradients or irreversibilities introduced during the process.

Reversible isothermal process:

• A reversible isothermal process is always quasi-static. In this process, the system undergoes a change while maintaining a constant temperature throughout. The process is conducted so slowly that the system stays in thermodynamic equilibrium at all times. Heat transfer occurs between the system and its surroundings in such a manner that the temperature remains constant, which requires the exchange of heat to be infinitesimally small and gradual.
• The reversibility of the process ensures that no entropy is generated within the system, and any change can be undone by reversing the conditions. For instance, in a reversible isothermal expansion or compression of an ideal gas, the system remains in equilibrium with its surroundings, and the process can be reversed by infinitesimally adjusting the external pressure.

Explanation:

The Greek word ‘therme’ translates to ‘heat’ in English. This word forms the root for various terms used in science and engineering, such as thermodynamics, thermal energy, and thermometer, which all relate to the concept of heat. Let us delve deeper into the concept of heat and its relevance in thermodynamics and other domains.

Understanding Heat:

Heat is a form of energy that is transferred between systems or objects with different temperatures. It flows from the system with a higher temperature to the one with a lower temperature until thermal equilibrium is achieved. Heat is a fundamental concept in thermodynamics, which is the branch of physics concerned with the relationships between heat, work, and energy.

Heat is measured in joules (J) in the International System of Units (SI), but other units, such as calories and British thermal units (BTUs), are also used in various contexts. It is important to note that heat is not the same as temperature. While temperature measures the average kinetic energy of particles in a substance, heat refers to the energy transfer due to a temperature difference.

Applications of Heat in Engineering:

Heat plays a pivotal role in various engineering disciplines, including thermal engineering, mechanical engineering, and chemical engineering. Some of the key applications of heat are:

• Power Generation: Heat energy is used in power plants to produce electricity. For example, in thermal power plants, heat generated by burning fossil fuels is used to convert water into steam, which drives turbines connected to generators.
• Refrigeration and Air Conditioning: Heat transfer is a critical process in refrigeration and air conditioning systems, where heat is removed from a space to cool it.
• Manufacturing Processes: Heat is extensively used in various manufacturing processes, such as welding, forging, casting, and metal cutting.
• Thermal Insulation: The study of heat transfer is crucial for designing effective thermal insulation materials to minimize heat loss or gain.
• Automotive Engineering: Heat management is essential in automotive engines to ensure efficient operation and prevent overheating.

Correct Option Analysis:

The correct option is:

Option 4: Heat

The Greek word ‘therme’ literally means ‘heat,’ which aligns perfectly with the definition and its applications in various scientific and engineering fields. This term is foundational in understanding concepts related to thermal energy and heat transfer, making it the correct answer.

Important Information:

Analysis of Other Options:

Let us analyze why the other options are incorrect:

Option 1: Energy

While heat is a form of energy, the Greek word ‘therme’ specifically refers to heat, not energy in general. Energy is a broader term encompassing various forms such as kinetic energy, potential energy, electrical energy, and more. Therefore, this option is incorrect.

Option 2: Power

Power is the rate at which work is done or energy is transferred. It is measured in watts (W) in the SI system. While power is related to energy and heat, the term ‘therme’ does not directly translate to power. Hence, this option is incorrect.

Option 3: Work

Work refers to the transfer of energy that occurs when a force is applied to an object, causing it to move. It is a different concept from heat, even though both are related in the field of thermodynamics. The Greek word ‘therme’ does not mean work, making this option incorrect.

Option 5: [Blank]

This option is left blank and does not provide any meaningful answer. Therefore, it is not a valid choice.

Summary:

The Greek word ‘therme’ specifically means heat, which is a form of energy associated with the transfer of thermal energy between systems or objects. Understanding this term is essential for grasping concepts in thermodynamics and various engineering applications. The other options—energy, power, and work—are related but do not accurately represent the meaning of ‘therme.’

Explanation:

The Efficiency of the Carnot Cycle

The Carnot cycle is a theoretical thermodynamic cycle that defines the maximum possible efficiency that any heat engine can achieve operating between two temperatures. It serves as a benchmark for the performance of all real-world heat engines. The efficiency of the Carnot cycle is given by the ratio of the useful work done by the engine to the heat energy supplied to it. Mathematically, it is expressed as:

Efficiency (η): $\frac{\mathrm{W}\mathrm{o}\mathrm{r}\mathrm{k}\mathrm{d}\mathrm{o}\mathrm{n}\mathrm{e}}{\mathrm{H}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}$Work Done (W)Heat Supplied (Qh)" id="MathJax-Element-1-Frame" role="presentation" style="position: relative;" tabindex="0"> Work Done (W)Heat Supplied (Qh) $\frac{\text{Work Done (W)}}{\text{Heat Supplied (Q\(_h\))}}$

This relationship highlights that the efficiency is directly proportional to the work output of the engine and inversely proportional to the heat energy supplied during the process.

• Carnot cycle efficiency depends upon the absolute temperature range of operation.

$\eta =1-\frac{T_L}{T_H}$

$\eta =\frac{T_H-T_L}{T_H}$

where TH is the hot reservoir temperature and TL is the cold reservoir temperature.

Explanation:

The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

This law is the basis for the temperature measurement.

• By replacing the third body with a thermometer, the Zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.
• The thermometer is based on the principle of finding the temperature by measuring the thermometric property.

Concept:

If the balloon containing the ideal gas is initially kept in an evacuated and insulated room. Then if the balloon ruptures and the gas fills up the entire room, the process is known as free or unrestrained expansion.

Now if apply the first law of thermodynamics between the initial and final states.

$Q=(u_2-u_1)+W$

In this process, no work is done on or by the fluid, since the boundary of the system does not move. No heat flows to or from the fluid since the system is well insulated.

$u_2-u_1=0\Rightarrow u_2=u_1$

Enthalpy is given as 

h = u + Pv

For ideal gases, as we know, internal energy and enthalpy are a function of temperature only, so if internal energy U remains constant, temperature T also remains constant which means enthalpy also remains constant.

So, during the free expansion of an ideal gas, both internal energy and enthalpy remain constant.

Concept:

Gibbs phase Rule

P + F = C + Non-compositional variable

If numbers of Non-compositional variable is given then we should put that number, otherwise it is 2

i.e.

P + F = C + 2

P = No. of phases

F = Degrees of freedom

C = No. of components

Calculation:

Given that C = 2, P = 1

P + F = C + 1 (because non-compositional variable is only temperature)

⇒ 1 + F = 2 + 1

Explanation:

Celsius scale 

• In this scale, LFP (ice point) is taken 0° and UFP (steam point) is taken 100°.
• The temperature measured on this scale all in degree Celsius (° C).

Fahrenheit scale 

• This scale of temperature has LFP as 32° F and UFP as 212° F .
• The change in temperature of 1° F corresponds to a change of less than 1° on the Celsius scale.

Kelvin scale 

• The Kelvin temperature scale is also known as the thermodynamic scale. The triple point of water is also selected to be the zero of the scale of temperature.
• The temperatures measured on this scale are in Kelvin (K).

Rankine scale

• This scale of temperature has LPF as 492° R and UFP as 672° R.
• Interval of this scale is according to Fahrenheit.
• The temperature measured on this scale are in Rankine (R)

All these temperatures are related to each other by the following relationship

$\frac{C}{100}=\frac{F-32}{180}=\frac{R-492}{180}=\frac{K-273}{100}$

Additional Information

Relationship between the Celsius and Fahrenheit scale is –

$\frac{F-32}{9}=\frac{C}{5}$

$\Rightarrow C=\frac{5}{9}(F-32)$

Concept:

The volume of the sphere is:

$V=\frac{4}{3}\times \pi \times r^3=\frac{\pi }{6}\times D^3$

D = diameter of the balloon

Calculation:

Given:

Pressure in the balloon (P)

P ∝ D²

P = C × D² … (1)

$Volumeofasphere=\frac{4}{3}\pi r^3=\frac{\pi }{6}{D}^3$

Now, we can write

V = K × D³

$D={\left(\frac{V}{K}\right)}^{\frac{1}{3}}$

Substitute D value in equation (1),

$P=C\times {\left({\left(\frac{V}{K}\right)}^{\frac{1}{3}}\right)}^2\Rightarrow P{V}^{-\frac{2}{3}}=\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}$

Explanation:

According to Zeroth Law, if system A is in thermal equilibrium with system C, and system B is thermal equilibrium with systems C, then system A is in thermal equilibrium with system B.

Now, two systems are said to be in (mutual) thermal equilibrium if, when they are placed in thermal contact (basically, contact that permits the exchange of energy between them), their state variables do not change.

Concept:

Point function:

• The thermodynamic properties which depend on the end state only (independent of the path followed) are known as point functions like temperature, pressure, density, volume, enthalpy, entropy, etc.

Path function: 

• The thermodynamic properties which depend on the end states, as well as the path followed, are known as path functions like heat and work.

Additional Information

Thermodynamic property: 

• A thermodynamic property is any property that is measurable, and whose value describes a state of a system.
• Some e.g of the thermodynamic property are pressure, temperature, viscosity, and density. 
• Properties are point functions i.e. these do not depend on the path followed. ​

Intensive Property: 

• These are the properties of the system which are independent of the mass under consideration. For e.g. Pressure, Temperature, density, etc.

Extensive Properties:

• The properties which depend on the mass of the system under consideration. For e.g Internal Energy, Enthalpy, Volume, Entropy, etc.

Explanation:

• Intensive Property: These are the properties of the system which are independent of mass under consideration. For e.g. Pressure, Temperature, density, composition, viscosity
• Extensive Properties: The properties which depend on the mass of the system under consideration. For e.g Internal Energy, Enthalpy, Mass, Volume, Entropy

​Important Points

All specific properties are intensive properties. For e.g. specific volume, specific entropy etc.

Concept:

The compressibility factor the ratio of actual volume to the volume predicted by the ideal gas law at a given temperature and pressure. It is used to quantify the deviation of real behaviour from the ideal gas behaviour.

$z=\frac{v}{\left(\frac{RT}{P}\right)}=\frac{Pv}{RT}$

Calculation:

Given:

P = 1500 kPa, ν = 2.75 m³/kmol, Z = 0.95, R = 8.314 J/K/mol

Z = $\frac{Pv}{RT}$

T = $\frac{Pv}{RZ}$ =  $\frac{1500\times 2.75}{8.314\times 0.95}$= 522.26 K $\approx$ 522 K

T = 522 - 273 = 249 ^∘C 

Additional Information

• For an ideal gas, Z = 1 at all temperatures and pressures.
• Whereas for real gases Z may be > 1 or < 1.
• The farther away Z is from unity, the more the gas deviates from ideal-gas behaviour.

Don't mark 522 K as an answer as the final answer asked is in degree Celcius 249 ∘C.

Explanation:

Temperature: It is the measure of the degree of hotness and coldness of a body. The SI unit of temperature is Kelvin (K).

The major temperature scales are:

• Celsius scale: It is also known as the centigrade scale, and most commonly used scale. It is defined from assigning 0° C to 100° C of freezing and boiling point of water at 1 atmospheric pressure.
• Fahrenheit scale: The temperature scale which is based on 32° for the freezing point of water and 212° for the boiling point of water and the interval between the two range divided into 180 equal parts is called Degree Fahrenheit scale.
• Kelvin scale: It is the base unit of temperature, denoted with K. There are no negative numbers on the Kelvin scale as the lowest is 0 K.

The relation between Celsius and Kelvin is:

°C + 273.15 = K

The relation between degree Fahrenheit and degree Celsius is given by:

$\frac{{}^{\circ }F-32}{9}=\frac{{}^{\circ }C}{5}$

Kelvin scale is an absolute scale and it is also known as the absolute thermodynamic scale.

Before 1954 temperature scales were based on two reference points. e.g. Degree Celsius and Fahrenheit scale.

After 1954 the scale and temperature measurement has been based on a single reference point. i.e the Tripple point of water is used as a single reference point.

According to the internationally accepted convention $1K=(\frac{1}{273.16})$ th of the triple point of water.