![]() 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. Devices where mechanical work is used to remove heat from a system, for example refrigerators and air conditioning systems, can be understood in the same way, moving round a loop on the pressure-volume diagram in the opposite direction.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. Similar diagrams (but with curved sides, not straight) are used to understand the behaviour of different types of "heat engines" such as steam engines, gasoline and diesel car engines, jet aircraft engines, etc. ![]() Since the starting and ending conditions at point X are the same, the amount of heat energy added to the system during the cycle is the same as the amount of mechanical work done by the gas. The net amount of heat added to the system is converted into mechanical work when the gas pressure changes the volume of the system, moving from Y to Z and then from W back to X. Don't try to over-think this (because as the previous paragraph said, this diagram doesn't correspond to any simple physical device), but going round the complete cycle, the amount of heat added is not the same as the amount of heat removed. The rectangular shape of the pressure-volume diagram doesn't correspond to anything that is physically simple, but you should be able to see that as you go from Y to Z and the volume increases, the gas pressure would decrease unless you continue to add more heat energy to the system and keep the pressure constant as the gas expands.įor the other two sides, Z to W and back to X, the gas is losing heat energy. One way to make it happen would to add heat energy to the gas and raise its temperature. Something must be happening to the gas to cause that. The volume is constant, but the pressure increases. Think about going from X to Y on the diagram. ![]()
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