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# 物理代写|热力学代写Thermodynamics代考|MECH3720 System & Surroundings

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## 物理代写|热力学代写Thermodynamics代考|System & Surroundings

As discussed in Section 5.1, the molecular state energy $E$ is conserved (constant over time) if the system is isolated. An isolated system is one that has neither mechanical nor thermal contact with its surroundings – it might as well be “alone in the universe.”

Of course, this is not a very realistic picture. Generally speaking, contact between system and surroundings allows for changes not only of the molecular state of the system (as in Section 5.1), but also its thermodynamic state-and that of the surroundings as well.

It can be convenient to consider the surroundings explicitly, as in Figure 7.1. Treated as its own system, the surroundings has its own thermodynamic variables and quantities, denoted with a ‘sur’ subscript. Together, the system plus surroundings form the total system, whose quantities adopt a ‘tot’ subscript.

The picture above resembles the subsystems picture of Section 4.3, but with some key differences. Whereas subsystems are usually treated “equally,” the system and its surroundings are not. For instance, the surroundings are usually much larger than the system, as in the case of a heat bath (see Section 9.1).

Another difference is the requirement that the total system must be isolated. Arguments that apply only to isolated systems (such as energy conservation) may then be referred to the total system-allowing the system itself to be more general. Of course, the precise division into “system” and “surroundings” is flexible.

## 物理代写|热力学代写Thermodynamics代考|Thermodynamic Change

If “nothing ever happened” (see Talking Heads excerpt on p. 9), life would be pretty boring. This is the case with thermodynamic systems in equilibrium, so long as there is no change in external factors (Definition 4.1, p. 26). What happens when external factors do change? In most cases, this gives rise to a change of thermodynamic state. One obvious consequence of such a change is that at least one of the two independent variables must change its value.

We will often refer to the initial thermodynamic state as “state A,” and denote the initial values of thermodynamic quantities with an ‘ $i$ ‘ subscript. Likewise, “state B,” and ‘ $f$ ‘ subscripts, are used for the final state. The change in the value of a generic quantity $X$, under the thermodynamic change, is then given as
$$\Delta X=\left(X_f-X_i\right) .$$
Note that most thermodynamic quantities – not just the variables – may be expected to change their values under a thermodynamic change. The surroundings quantities also undergo their own change.

## 物理代写|热力学代写Thermodynamics代考|Thermodynamic Change

$$\Delta X=\left(X_f-X_i\right) .$$

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