Basic concepts of thermodynamics
This article is intended to prepare for the study of thermodynamics, here are given general information that does not require memorization, but is mandatory for understanding
Thermodynamics
There are only three basic laws in thermodynamics on which this science is based. Thermodynamics is a fundamental subject that studies the processes of energy conversion. There are no exceptions in the laws of thermodynamics, as, for example, in chemistry. In order to work with thermodynamics, it is necessary to study the basic concepts that are used to explore everything that surrounds us. Thermodynamics is used to study some of course, the first thing to do is to decide what we will study.
Thermodynamic system
So, the first concept, thermodynamic system is an object of study, part of space, bounded by a physical or imaginary boundary, which will be subjected to study. If we undertake to study the exchange of energy in a person, then the whole person is a thermodynamic system. If we study the exchange of heat with the sun, then the sun is a thermodynamic system. If we study the exchange the heat between the stove and the house, then the stove and the house are a thermodynamic system.
System boundaries
It is important to indicate where the system ends, the boundary of the thermodynamic system is a physical or imaginary object, for example, if we study coffee in a thermocup as a thermodynamic system, then coffee is a system, and the inner walls of the mug and the lower part of the lid are the boundaries of the system. We can also study the section of the pipe through which the liquid flows: the inner surface of the pipe and the ones defined by us the (imaginary) boundaries of the site will be the boundary of the thermodynamic system.
Environment
Everything that is outside the system boundary is the environment. It is assumed that the environment does not change over time, i.e. all its parameters are constant regardless of what happens in the system (for example, the system can give or receive heat from the environment, which does not affect the environment in any way).
Types of thermodynamic system
Open system
An open system is one that exchanges matter and energy with the environment. This can be, for example, a combustion chamber in an engine or a steam turbine.
Closed system
In a closed system, neither a change in the mass of a substance nor an exchange of a substance with the environment is possible. For example, vacuum packaging - while the packaging is intact, the mass of the product inside is constant (unless, of course, there are complex chemical processes that are not of interest in this situation).
Adiabatic system
It can be open or closed, but in any case there is no exchange of energy in the form of heat.
Properties of the thermodynamic system
A thermodynamic system is described by a set of quantities that do not depend on the previous ones system states. Each state can be described by thermodynamic quantities. Any characteristic of the system is intensive or additive. The intensive characteristic (from Latin intensive - belonging to an object) does not depend on the size system, i.e. its value is the same for the whole system (or part of the system) regardless of size, for example temperature or concentration. Additive characteristic (from Latin - the ability to join) depends on the size of the selected object and for the whole system will be the sum of the values for all its elements, for example, mass.
Specific volume and density ν, ρ
ν = V/m specific volume
ρm = m/V density
Pressure P
P = F/A force normal per unit area
The pressure is distinguished between absolute and manometric. Manometric pressure is an excess pressure in relation to to atmospheric (for vacuum, the manometric pressure is negative). Absolute pressure is the pressure relative to a complete vacuum.
Pressure measurement units:
Pascal (Pa) = 1 N/m2
Bar (bar) = 105 N/m2 = 105 Pa
Atmosphere (atm) = 1.013 bar = 760 mm Hg
Pounds per square inch - lbs or psi; 6894.8 Pa
Thermodynamic exchange what is it like?
Thermodynamic systems exchange energy by changing their properties. The following forms of energy are distinguished: mechanical, electrical, magnetic, thermal, chemical and nuclear.
Energy is exchanged in two ways: work (macroscopic exchange) and heat (microscopic exchange). With the help of the work, the additive parameters of the system are changed. By means of heat , the internal the energy of the system.
Thermodynamic equilibrium
We say that the system is in hydrodynamic equilibrium when the resulting interaction any kind of energy or substance between the system and the environment is zero.
Equation of state of an ideal gas
An ideal gas implies the following assumptions: molecules are represented as the collisions between them are absolutely elastic (i.e., no heat is released during the collision) and there are no attractive and repulsive forces between the molecules. Such a model is only suitable for theoretical calculations and for real gas, it becomes much more complicated.
Equation of state of an ideal gas
P V = N R T
where P is the pressure [Pa], V is the volume of gas [m3], N is the number of moles of gas, R is the universal gas constant (a constant equal to the expansion work of one mole of an ideal gas in an isobaric process with increasing temperature at 1 K), T is the absolute temperature [K]
General view of the equation of state:
P = P (T,ν)
Processes in thermodynamics
To simplify calculations, idealized systems and types of processes are used in thermodynamics.
Quasi-static process
Such a process in which each subsequent state of the system is in equilibrium is called quasi-static. This means that all changes in the system occur slowly enough for that there would be no transients. Imagine that you are brewing coffee in French press: if you press the piston slowly, the coffee will settle down if You will try to push the piston sharply with great force, first of all, in total You will need more energy, and secondly, all the coffee will pour out, because of the sharp increasing the pressure and incompressibility of the liquid. A quasi-static process is a process slow, in which all parts of the system are in the same state.
Reversible and irreversible processes
Reversible is a process that can occur in the opposite direction to normal development, restoring the energy exchanges that have occurred. Such a process is quasi-static and cannot be real. In a reversible process, there are no nonequilibrium forces between the system and the environment. Irreversible process - any real process is called.
Polytropic process
Another model of an ideal system that corresponds to the equation "Pvn = cte" (for n=cte). It is used for systems whose behavior is similar to that of an ideal gas. Such a process describes the behavior of the gas well enough and the result is close to reality.
Special cases of a polytropic process are often considered: isobaric (constant pressure), isothermal (constant temperature), adiabatic (no heat exchange with the environment) and isochoric (constant volume).
