THERMODYANAMICS
Thermodynamics studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. Roughly, "thermo", or heat means "energy in transit" and dynamics relates to "movement"; thus, in essence thermodynamics studies the movement of energy (or heat) and how energy instills movement.
The starting point for most thermodynamic considerations are the laws of thermodynamics, which postulate that energy can be exchanged between physical systems as heat or work. They also postulate the existence of a quantity named entropy, which is well defined for any system. In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of system and surroundings. A system is composed of particles, whose average motions define its properties, which in turn are related to one another through equations of state. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes.
With these tools, thermodynamics describes how systems respond to changes in their surroundings. This can be applied to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, chemical engineering. aerospace engineering, mechanical engineering, cell biology, biomedical engineering, and materials science to name a few.
Thermodynamic Systems
An important concept in thermodynamics is the "system", Everything in the universe except the system is known as "surroundings". A system is the region of the universe under study. A system is separated from the remainder of the universe by a boundary which may be imaginary (virtual) or real, but which by convention delimits a finite volume. The possible exchanges of work, heat, or matter between the system and the surroundings take place across this boundary. Boundaries are of four types: fixed. movable, real, and imaginary.
Basically, the "boundary" is simply an imaginary dotted line drawn around the volume of something in which there is going to be a change in the internal energy of that something. Anything that passes across the boundary that effects a change in the internal energy of that something needs to be accounted for in the energy balance equation. For an engine, a fixed boundary means that the piston is locked at its position; as such, a constant volume process occurs. In that same engine, a movable boundary allows the piston to move in and out. For closed systems, boundaries are real while for open systems, boundaries are often imaginary (or virtual). There are five dominant classes of systems:
1. Isolated Systems - matter and energy may not cross the boundary.
2. Adiabatic Systems - heat may not cross the bound ary.
3. Diathermic Systems - heat may cross boundary.
4. Closed Systems - matter may not cross the bound ary.
5. Open Systems - heat, work, and matter may cross the boundary (often called a control volume in this case),
As time passes in an isolated system, internal differences in the system tend to even out and pressures and temperatures tend to equalize, as do density differences. A system in which all equalizing processes have gone practically to completion, is considered to be in a state of thermodynamic equilibrium which means that the system's properties are, by definition, unchanging in time. Systems in equilibrium are much simpler and easier to understand than systems which are not in equilibrium. Often, when analysing a thermodynamic process, it can be assumed that each intermediate state in the process is at equilibrium. This will also considerably simplify the situation. Thermodynamic processes, which develop so slowly as to allow each intermediate step to be an equilibrium state, are said to be reversible processes.
Thermodynamic processes
A thermodynamic process may be defined as the energetic evolution of a thermodynamic system proceeding from an initial state to a final state. Typically, each thermodynamic process is distinguished from other processes, in energetic character, according to what parameters, as temperature, pressure, or volume, etc. are held fixed. Furthermore, it is useful to group these processes into pairs as mentioned below.
● An isobaric process occurs at constant pressure.
● An isochoric process, or isometric/ isovolumetric process, occurs at constant volume.
● An isothermal process occurs at a constant temperature.
● An adiabatic process occurs without loss or gain of heat.
● An isentropic process (reversible adiabatic process) occurs at constant entropy.
● An isenthalpic process occurs at a constant enthalpy.It is also known as a throttling process or wire drawing.
● A steady state process occurs without a change in the internal energy of the system.
The Laws of Thermodynamics
In thermodynamics, there are four laws of very general validity, and as such they do not depend on the details of the interactions or the systems being studied. The four laws are:
● Zeroeth law of thermodynamics, stating that ther modynamic equilibrium is an equivalence relation. If two thermodynamic systems are separately in ther mal equilibrium with a third, they are also in thermal equilibrium with each other.
● First law of thermodynamics, is about the conservation of energy. It states that the change in the internal energy of a closed thermodynamic system is equal to the sum of the amount of heat energy supplied to the system and the work done on the system.
● Second law of thermodynamics, is about entropy and states that the total entropy of any isolated thermodynamic system tends to increase over time. approaching a maximum value.
● Third law of thermodynamics, is about absolute zero temperature and states that as a system asymp totically approaches absolute zero of temperature (273 °C or 0 K), all processes virtually cease and the entropy of the system asymptotically approaches a minimum value; also stated as: "the entropy of all systems and of all states of a system is zero at absolute zero" or equivalently "it is impossible to reach the absolute zero of temperature by any finite number of processes"
Thermodynamic Potentials
These are energetic quantities called thermodynamic potentials, and are the quantitative measures of the stored energy in the system and can be derived from the energy balance equations of a thermodynamic system. The five most well known potentials are:
◆ Internal energy
◆ Helmholtz free energy
◆ Enthalpy
◆ Gibbs free energy
◆ Grand potential
Where A is the Helmholtz free energy, U is the internal energy of the system. T is the absolute temperature (on Kelvin scale, K), S is the entropy, H is the enthalpy, Pis the pressure of the system, V is the volume, 1 is the chemical potential, and N is the number of particles in the system.
Potentials are used to measure energy changes in systems as they evolve from an initial state to the final state. The potential used depends on the constraints of the system, such as constant temperature or pressure. Internal energy is the internal energy of the system, enthalpy is the internal energy of the system plus the energy related to pressure-volume work, and Helmholtz and Gibbs energy are the energies available in a system to do useful work when the temperature and volume or the pressure and temperature are fixed, respectively.
