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Thermodynamics


Systems

A thermodynamic system is that part of the universe that is under consideration. A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the environment. A useful classification of thermodynamic systems is based on the nature of the boundary and the flows of matter, energy and entropy through it.
    There are three kinds of system depending on the kinds of exchanges taking place between a system and its environment:

  1. Isolated System - It does not exchange heat, matter or work with the environment. An example of an isolated system would be an insulated container, such as an insulated gas cylinder. In reality, a system can never be absolutely isolated from its environment, because there is always at least some slight coupling, even if only via minimal gravitational attraction. Figure 02 shows the essence of classical thermodynamics: In a system isolated from the outside world, heat
    Isolated System within a gas of temperature, T2, will flow in time, t, toward a gas of temperature, T1, where T2 > T1 and T = T2 - T1, thus the system's total energy E is constant (via the first law of thermodynamics), while its free energy F decreases, and its entropy S rises (via the second law of thermodynamics), until finally T 0 at equilibrium.

    Figure 02 Isolated System [view large image]

    Some literatures refer the isolated system as closed system, while the other systems are lumped together as open system.

  2. Closed System - It exchanges energy (heat and work) but not matter with the environment. A greenhouse is an example of a closed system exchanging heat but not work with its environment. Another example is the heat engine shown in Figure 03. It is defined as a device that converts heat energy into mechanical energy or more exactly a system which
    Closed System operates with only heat and work passing across its boundaries. As work is done on the gas inside the chamber, the temperature and pressure increase and some heat will be transferred out of the system. When heat is transferred to the system, the gas expands, it does work on the surroundings and the temperature and pressure decrease.

    Figure 03 Closed System
    [view large image]

  3. Open System - It exchanges energy (heat and work) and matter with the environment. A boundary allowing matter exchange is called permeable. It's possible for an open system to import order and export disorder, locally increasing order. What the Second Law says is that in such a transaction more disorder than order will be created. It does not, however, forbid the creation of pockets of order. What happens is that disorder in the entire system will increase even though individual open systems within it might become more ordered. As shown in Figure 04, in a thermodynamically open system, energy (in the form of radiation or matter) can enter the system from the outside environment, thereby increasing the system's total energy, E, over the course of time, t. Such energy flow can lead to an increase, a decrease, or no net change at all in the entropy, S, of the system. Even so, the net entropy of system and its environment would
    Open System Photosynthesis still increase according to the second of thermo- dynamics. The ocean would be an example of an open system. Another good example would be the photosynthesis in plants as shown in Figure 05. Infusion of energy and exchange of matter are taking place inside the chloroplast resulting in the production of glucose, which is in a higher energy level. The system becomes nonequilibrium and will decay to the more stable form in the long run.

    Figure 04 Open System [view large image]

    Figure 05 Photosynthesis
    [view large image]

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