![]() ![]() Substituting in the energy equation and simplifying: However, since the pressure is constant throughout the process: Enthalpy is not a fundamental property, however is a combination of properties and is defined as follows:Īs an example of its usage in closed systems, consider the following constant pressure process: We will find also that a new property called Enthalpy will be useful both for Closed Systems and in particular for Open Systems, such as the components of steam power plants or refrigeration systems. In the case studies that follow we find that one of the major applications of the closed system energy equation is in heat engine processes in which the system is approximated by an ideal gas, thus we will develop relations to determine the internal energy for an ideal gas. Since specific internal energy is a property of the system, it is usually presented in the Property Tables such as in the Steam Tables. The third component of our Closed System Energy Equation is the change of internal energy resulting from the transfer of heat or work. Positive forms of shaft work, such as that due to a turbine, will be considered in Chapter 4 when we discuss open systems. We note that work done by the system on the surroundings (expansion process) is positive, and that done on the system by the surroundings (compression process) is negative.įinally for a closed system Shaft Work (due to a paddle wheel) and Electrical Work (due to a voltage applied to an electrical resistor or motor driving a paddle wheel) will always be negative (work done on the system). It is sometimes convenient to evaluate the specific work done which can be represented by a P-v diagram thus if the mass of the system is m we have finally: Adiabatic (no heat flow to or from the system during the process). ![]() Isochoric or Isometric(constant volume process).Isothermal(constant temperature process).Recall in Chapter 1 that we introduced some typical process paths of interest: Note that work done is a Path Function and not a property, thus it is dependent on the process path between the initial and final states. This is shown in the following schematic diagram, where we recall that integration can be represented by the area under the curve. We normally deal with a piston-cylinder device, thus the force can be replaced by the piston area A multiplied by the pressure P, allowing us to replace Adx by the change in volume dV, as follows: By convention positive work is that done by the system on the surroundings, and negative work is that done by the surroundings on the system, Thus since negative work results in an increase in internal energy of the system, this explains the negative sign in the above energy equation.īoundary work is evaluated by integrating the force F multiplied by the incremental distance moved dx between an initial state (1) to a final state (2). In all cases we assume a perfect seal (no mass flow in or out of the system), no loss due to friction, and quasi-equilibrium processes in that for each incremental movement of the piston equilibrium conditions are maintained. In this course we are primarily concerned with Boundary Work due to compression or expansion of a system in a piston-cylinder device as shown above. ![]()
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