Intro to Thermal Aspects Introduction Ch. 1

Question 1

What are the three types of thermodynamic systems?

Answer 1

Closed system: No mass crosses boundaries (control mass).
Open system: Mass crosses boundaries (control volume).
Isolated system: No mass, heat, or work crosses boundaries.

Question 2

What does the prefix “iso” indicate in thermodynamic processes?

Answer 2

“Iso” indicates a constant property:
Isothermal: constant temperature
Isobaric: constant pressure
Isochoric: constant volume

Question 3

What is a thermodynamic cycle?

Answer 3

A series of processes where the end-point conditions are identical to the initial conditions.

Question 4

What are thermodynamic properties and how are they classified?

Answer 4

Properties describing a substance’s state. Classified as:

Intensive: Independent of mass (e.g., pressure, temperature)
Extensive: Dependent on mass (e.g., total volume)
Extensive properties per unit mass (e.g., specific volume) become intensive.

Question 5

What is thermal capacity and the difference between sensible heat and latent heat?

Answer 5

Thermal capacity: Ability of a substance to hold heat.
Sensible heat: Heat absorbed to raise temperature to boiling point.
Latent heat: Heat required to convert liquid to vapor at constant temperature and pressure.

Question 6

What is vapor quality and superheated vapor?

Answer 6

Vapor quality: Ratio of vapor mass to total mass in a liquid-vapor mixture.
Superheated vapor: Saturated vapor with added heat, raising temperature above boiling point.

Question 7

What is the state or phase of a substance?

Answer 7

The state is defined by macroscopic properties like temperature and pressure, indicating the phase (solid, liquid, gas).

Question 8

What happens during a state change in a substance?

Answer 8

When heat is added or removed, a substance undergoes a state change (e.g., solid to liquid), with temperature remaining constant until the change is complete.

Question 9

What are key points in a temperature–volume (T–v) diagram for water?

Answer 9

A–B: Heating liquid to saturation (x=0)
B–C: Vaporization (x=0 to 100%)
C–D: Superheating vapor
E–F: Critical point (identical liquid and vapor)
H–I: Heating without phase change

Question 10

What is internal energy and specific internal energy?

Answer 10

Internal energy: Molecular energy of a system.
Specific internal energy: Energy per unit mass (u), influenced by temperature change (cvdT).

Question 11

What is enthalpy and specific enthalpy?

Answer 11

Enthalpy: Total energy per unit mass of a substance.
Specific enthalpy: Energy per unit mass (h), influenced by temperature change (cpdT).

Question 12

What is entropy?

Answer 12

Entropy: Measure of molecular disorder in a substance at a given state, defined as the ratio of heat added to absolute temperature.

Question 13

What is a pure substance?

Answer 13

A substance with homogeneous and constant chemical composition throughout its phases (e.g., liquid, vapor).

Question 14

What defines an ideal gas?

Answer 14

An ideal gas is described by its volume, pressure, and temperature, following the equation of state 𝑃𝑣=𝑅𝑇

Question 15

Why are ideal gases useful in thermodynamics?

Answer 15

Ideal gases simplify calculations with a single gas constant
𝑅=𝑅∗/𝑀, applicable across a wide range of temperatures and pressures.

Question 16

What is the compressibility factor (Z) for real gases?

Answer 16

The compressibility factor 𝑍 measures how closely real gases behave like ideal gases, with 𝑍=𝑃𝑣/𝑅𝑇

Question 17

What are the laws describing gas behavior?

Answer 17

Boyle’s law (P1V1 = P2V2), Charles’ law (V1/T1 = V2/T2), and the combined gas law (P1V1/T1 = P2V2/T2) describe gas relationships under varying conditions.

Question 18

How does entropy change in ideal gases?

Answer 18

Entropy change Δ𝑆 in ideal gases depends on temperature and pressure changes, governed by Δ𝑆=𝑐𝑣ln⁡(𝑇2/𝑇1)+𝑅ln⁡(𝑣2/𝑣1)

Question 19

What is energy and power?

Answer 19

Energy is the capacity for work (thermal, mechanical, electrical, chemical). Power is the rate of energy transfer per unit time.

Question 20

What is heat?

Answer 20

Heat is thermal energy transferred between substances due to temperature differences, essential for processes like cooking and heating.

Question 21

What is work?

Answer 21

Work is energy transferred by pressure or force, categorized into shaft work (mechanical energy used in pumps, compressors, turbines) and flow work (energy transferred by fluid entering or leaving a system).

Question 22

What is the first law of thermodynamics?

Answer 22

The first law of thermodynamics states that energy in a closed system remains constant, meaning it cannot be created or destroyed. It includes internal energy, kinetic energy, and potential energy, and states that energy change is independent of the path taken by the system.

Question 23

What is the second law of thermodynamics?

Answer 23

The second law of thermodynamics states that energy cannot be completely converted into work with 100% efficiency. It’s encapsulated in two statements: the Kelvin-Plank statement, which prohibits a heat engine from operating with 100% efficiency, and the Clausius statement, which forbids a process that transfers heat from a cooler body to a hotter one without external work. Additionally, it asserts that the entropy of the universe always increases, leading to irreversible processes in closed systems.

Question 24

What is reversibility in thermodynamics?

Answer 24

Reversibility in thermodynamics refers to a theoretical process where both a system and its surroundings can be returned to their initial states without any net change in the universe. It implies the absence of irreversibilities such as friction, heat transfer losses, and mechanical or electrical effects. Real processes are irreversible, meaning they involve some degree of irreversibility that leads to the destruction of useful energy, reducing efficiency compared to ideal reversible processes.

Question 25

What is exergy?

Answer 25

Exergy is the maximum amount of work that can be extracted from a system as it reaches equilibrium with its surroundings. It represents the potential of a system to cause change and is defined relative to a specified reference environment’s temperature, pressure, and composition. Unlike energy, exergy is not conserved and is consumed due to irreversibilities in processes.