Introduction to thermodynamics, Lecture notes of Thermodynamics

Download Introduction to thermodynamics and more Lecture notes Thermodynamics in PDF only on Docsity! Thermodynamics – 1 (203103257) Introduction - Thermodynamics is branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. Scope of thermodynamics - The science of thermodynamics was developed in the 19th century as a result of the need to describe basic operating principles of the newly invented steam engine and to provide a basis for relating the work produced to the heat supplied. Thus, the name itself denotes power generated from heat. From the study of steam engines, there emerged two of the primary generalization of science: the first and second law of thermodynamics. All of classical thermodynamics is implicit in these laws. Their statements are very simple, but their implications are profound. - The first law simply says that energy is conserved, meaning that it is neither created nor destroyed. It provides no definition of energy that is both general and precise. No help comes from its common informal use where the word has imprecise meanings. However, in scientific and engineering contexts, energy is recognized as appearing in various forms, useful because each form has mathematical definition as a function of some recognizable and measurable characteristics of the real world. Thus, kinetic energy is defined as a function of velocity, and gravitational potential energy as a function of elevation. - Conservation implies the transformation of one form of energy into another. Windmills have long operated to transform the kinetic energy of the wind into work that is used to raise water from land lying below sea level. The overall effect is to convert the kinetic energy of the wind into potential energy of water. Wind energy is now more widely converted to electrical energy. Similarly, the potential energy of water has long been transformed into work used to grind grain or saw lumber. Hydroelectric plants are now a significant source of electrical power. Applications of thermodynamics - Thermodynamics gives the foundation for heat engines, power plants, chemical reactions, refrigerators, and many more important concepts that the world we live in today relies on. Beginning to understand thermodynamics requires knowledge of how the microscopic world operates - It is used to stimulate energy, work and heat exchanges using thermodynamic rules. - Thermodynamics is a discipline of science that studies the various types of energy, their quantitative correlations, and the energy changes that occur throughout physical and chemical processes. - Thermal power plants, nuclear power plants, hydroelectric power plants, and power plants based on renewable energy sources such as solar, wind, geothermal, tides, and water waves are all studied in thermodynamics. Fundamental concepts System - A body of matter and/or radiation confined in space by walls, with defined permeabilities, which separates it from its surroundings [or say a quantity (set) of matter in a region (space) chosen for study]. Types of system Open system - A system that has the capacity to exchange both matter and energy with its surrounding. Closed system - A system that has the ability to exchange only energy and not matter with its surrounding. Isolated system - A system which can exchange neither matter nor energy with its surrounding. - Thermally isolated – when the enclosing walls are impervious to the flow of heat. - Mechanically isolated – when enclosed by rigid walls. Adiabatic system - A system which can exchange neither matter nor energy with its surroundings; but the work done can be exchanged with the surroundings. Homogeneous system - Where in a system the properties are the same throughout or properties vary smoothly without showing any surface of discontinuity. Heterogeneous system - This a system which consists of two or more distinct homogeneous regions or phases. There is a sudden change in properties at the phase boundaries. Boundary - The real or imaginary surface that separates the system from the surroundings is knowns as boundary. It may have zero thickness and may be in a fixed position; or else moving. Surrounding - The region outside the system (or say the matter not included in the system) is known as surrounding. Processes - A process occurs when the system undergoes a change in a state or an energy transfer at a steady. - A process may be non-flow in which a fixed mass within the defined boundary is undergoing a change of state. Types of processes Cyclic process - Any process or series of processes whose end states are identical is termed a cycle. - In cyclic process, the state function change is zero but path function change have value. Reversible process - Processes occur when there exists a driving force for a change of state between the parts of the system or between the system and the surroundings. If this driving force is finite, the process is irreversible and if it is infinitesimal in magnitude, the process is reversible. Power - The time rate of doing work. 1 HP = 745.7 W (or J/s) Heat - A quantity which is exchanged between bodies due to temperature difference existing between them. It is a path function and a form of energy in transit. (J) 1 cal = 4.1868 J 1 BTU = 1055.04 J (British thermal unit) Types of heat Latent heat - Energy absorbed or released by a substance during a change in it’s physical state (phase) that occurs without changing it temperature. Sensible heat - Energy moving from one system to another that changes the temperature rather than changing its phase> Energy - A quantity that can be stored in a system and can be exchanged between the system and the surroundings. The exchange of energy occurs either as heat or work. - Types of energy o Internal energy - Energy possessed by a body by the virtue of its molecular configuration and motion of molecules. o Macroscopic energy - Depends on the outer reference like kinetic energy and potential energy. o Potential energy - Energy possessed by a body by the virtue of its position over some arbitrary reference plane. [U = mgz] o Kinetic energy - Energy possessed by a body by the virtue of its motion. KE = [1/2] m u 2] o Microscopic energy - Related to the molecular behavior of the system; independent of the outer reference. • Translational energy - Molecules moves throughout a space/system with some speed. • Rotational energy - Kinetic energy due to the rotation of an object. • Vibrational energy - Energy in a molecule (or say a system) due to the vibrations of its atoms. • Spin energy - Energy associated with paired electrons sharing one orbital and its effects on the molecules surrounding it. Sensible energy (Ɛ) = Ɛspin + Ɛrotational + Ɛtranslational + Ɛvibrational U = N Ɛ Where, U – internal energy N – number of molecules in the system E = U + PE + KE = N Ɛ + mgz + [1/2] m u 2] Temperature - The temperature measures the degree (or say intensity) of hotness or coldness of a body. - The zeroth law of thermodynamics forms the basis for the measurement of temperature. - The zeroth law allows us to build thermometers which are devices that indicate the change in temperatures by the changes in some physical properties of the thermometer fluid. - Such properties are called the thermometric properties. - The thermometric properties include: i. Volume of gases and liquids [thermometers] ii. Pressure of gases at constant volume [constant volume gas thermometers] iii. Electrical resistance of solids [thermistors] iv. Electromotive forces of two dissimilar metals [thermocouples] v. Intensity of radiation [pyrometers] Ideal gas temperature scale - Here the ideal gas suffices to treat an ideal gas as one in which distance between the molecules is so large that the inter-molecular forces are negligible and the volume occupied by the molecules is only a negligible fraction of the total volume. It follows from kinetic theory that for such a fluid the product of pressure and volume varies linearly with temperature. This is stated mathematically by the ideal gas equation. Heat capacity - The heat capacity of a substance is the quantity of heat to be supplied to affect a temperature rise of one degree. dQ = C dT where C is known as heat capacity of the substance. - Heat capacity of unit mass of a substance is also known as specific heat of the substance. - The heat capacity depends on the way in which heat is supplied. - When heat is supplied to a system at constant volume, the system is unable to do any work and the quantity of heat required is given by; dQ = Cv dT (constant volume) where Cv is known as the heat capacity at constant volume. Thus, 𝐶𝑉 = ( 𝜕𝑄 𝜕𝑇 ) v - As per the equation of first law of thermodynamics for non-flow process, for a constant volume process dU = dQ, so that we can write the above equation as; 𝐶𝑉 = ( 𝜕𝑈 𝜕𝑇 ) - If heat is supplied to a substance at constant pressure, it is free to expand doing work against the constant pressure. A part of the heat supplied to the system is utilized for the work of expansion and more heat will be required to raise the temperature than that required in a constant volume process for the same temperature change. The amount of heat required is related to the temperature rise as dQ = Cp dT (constant pressure) where Cp is called the heat capacity at constant pressure. Thus, 𝐶𝑝 = ( 𝜕𝑄 𝜕𝑇 ) For a constant pressure process, the equation of first law of thermodynamics for non-flow process becomes, dU = dQ – P dV or dQ = dU + P dV = dH ∴ 𝐶𝑝 = ( 𝜕𝐻 𝜕𝑇 ) Enthalpy - The internal energy required to generate a system and the amount of energy that is required to make room for it by establishing its pressure and volume and displacing its environment (or say measurement of energy in a thermodynamic system; state function). H = U + PV - In differential form, dH = dU + d(PV) The change in enthalpy for a mechanically reversible, non – flow process at constant pressure is equal to the heat supplied. dH = dU + P dV + V dP Substituting [dU = dQ -dW] from the first law of thermodynamics in the equation above. dH = dQ – dW + P dV + v dP Now, [dW = P dV] for a reversible non flow process [V dP = 0] for constant pressure process. dH = dQ i.e., a system heated at constant pressure with only expansion work occurring, the change in enthalpy is equal to the heat supplied. Whereas, for a process occurring at constant volume, work of expansion is zero and the differential equation reveals that the change in internal energy is equal to the heat supplied. dU = dQ (for a constant volume process). Equilibrium state and phase rule Steady state - A system which is interacting with the surroundings, is said to have attained a steady state condition when the properties at a specified location in the system do not vary with time. - A system in the steady state exchanges mass, heat or work with the surroundings, even when exhibiting time variance for the properties. Equilibrium state - A system is said to be in a state of equilibrium if the properties (i.e., Properties that are on macroscopic scale and does not exclude the probability of individual molecules having different values for the properties) are uniform throughout and they do not vary with time. - Thermal equilibrium – no heat exchange between various points within the system and the temperature is uniform throughout. - Mechanical equilibrium – the pressure is uniform throughout the system. - Chemical equilibrium – no transfer of matter from one part to other part of the system due to no chemical potential change with time. - Thermodynamic equilibrium – in addition to the absence of heat and work exchange, there would be no mass transfer between the phases, no diffusion of mass within the phase and no chemical reaction between the constituents. Therefore, a state of equilibrium implies a state at rest. [When a system satisfies all the three given conditions above for a system]. Phase rule - The state of an equilibrium system consisting of pure fluid is uniquely determined by specifying any two intensive variables. - The number of independent variables necessary to define the state of equilibrium uniquely is known as the number of degrees of freedom. This number will be different for varying equilibrium states. - Gibbs phase rule; F = C – P + 2 [or F = 2 – M + N] - Where, F – the number of degree of freedom of the system C(N) – the number of components in the system P (M) – the number of phases in the system