This equation, which relates the microscopic details, or microstates, of the system (via W) to its macroscopic state (via the entropy S), is the key idea of statistical mechanics. By “microscopic states”, we mean the exact states of all the molecules making up the system.Įntropy = (Boltzmann’s constant k) x logarithm of number of possible states What disorder refers to is really the number of microscopic configurations, W, that a thermodynamic system can have when in a state as specified by certain macroscopic variables ( volume, energy, pressure, and temperature). In statistical physics, entropy is a measure of the disorder of a system. In thermodynamics and statistical physics, entropy is a quantitative measure of disorder, or of the energy in a system to do work. Therefore the resulting volume is 2 m 3 x 1.67 = 3.34 m 3 and ∆V = 3.34 m 3 – 2 m 3 = 1.34 m 3. Since at this enthalpy the steam have density of 1.31 kg/m 3, it is obvious that it has expanded by about 2.2/1.31 = 1.67 (+67%). When we use simply Q = H 2 − H 1, then the resulting enthalpy of steam will be:įrom steam tables, such superheated steam (15812/4.4 = 3593 kJ/kg) will have a temperature of 828 K (555☌).
Since at this condition the steam has density of 2.2 kg/m 3, then we know there is about 4.4 kg of steam in the piston at enthalpy of 2912 kJ/kg x 4.4 kg = 12812 kJ. Using steam tables we know, that the specific enthalpy of such steam (500 kPa 500 K) is about 2912 kJ/kg. Calculate the final temperature, if 3000 kJ of heat is added. For ideal gas αT = 1 and therefore:Įxample: Frictionless Piston – Heat – Enthalpy Calculate the final temperature, if 3000 kJ of heat is added.Ī frictionless piston is used to provide a constant pressure of 500 kPa in a cylinder containing steam ( superheated steam) of a volume of 2 m 3 at 500 K. Where C p is the heat capacity at constant pressure and α is the coefficient of (cubic) thermal expansion. There are expressions in terms of more familiar variables such as temperature and pressure: This energy storage on the atomic level includes energy associated with electron orbital states, nuclear spin, and binding forces in the nucleus.Įnthalpy is represented by the symbol H, and the change in enthalpy in a process is H 2 – H 1. On the other hand, energy can be stored in the chemical bonds between the atoms that make up the molecules. It is the thermodynamic quantity equivalent to the total heat content of a system.
It is the energy contained within the system, excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. The SI unit of enthalpy is the joule (J). For a variable-pressure process, the difference in enthalpy is not quite as obvious.Įnthalpy in Extensive Units Extensive and intensive properties of medium in the pressurizer.Įnthalpy is an extensive quantity, it depends on the size of the system, or on the amount of substance it contains. At constant pressure, the enthalpy change equals the energy transferred from the environment through heating (Q = H 2 – H 1) or work other than expansion work. It is due to the fact, it simplifies the description of energy transfer.
It is so useful that it is tabulated in the steam tables along with specific volume and specific internal energy. The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements at constant pressure. In many thermodynamic analyses the sum of the internal energy U and the product of pressure p and volume V appears, therefore it is convenient to give the combination a name, enthalpy, and a distinct symbol, H.
#What is entropy plus#
The enthalpy is defined to be the sum of the internal energy E plus the product of the pressure p and volume V. In thermodynamics, the enthalpy is the measure of energy in a thermodynamic system. What is Enthalpy Four common thermodynamic potentials.
0 kommentar(er)
