Question 1

Why is the net entropy change zero in a Carnot cycle?

Accepted Answer

In the Carnot cycle, heat is exchanged only during two reversible isothermal processes, where the ratio of heat to temperature is identical for both. The two adiabatic processes involve no heat exchange. Since entropy change equals heat divided by temperature, the entropy changes from the isothermal processes cancel each other, resulting in zero net entropy change for the complete cycle.

Question 2

How does the Carnot cycle demonstrate that entropy is a state function?

Accepted Answer

Any reversible cyclic process can be decomposed into many Carnot cycles. By breaking a closed path into two independent routes between two points and reversing one path's integral, we show that entropy change between those points is identical regardless of path taken. This path independence proves entropy is a state function, like internal energy, depending only on initial and final states.

Question 3

What does it mean that entropy is a state function?

Accepted Answer

As a state function, entropy has a unique value for any given system state. This means a single entropy value completely describes the thermodynamic condition of a system. However, only entropy changes are defined; absolute entropy values require a reference state. Once a reference entropy is established, absolute values for all other states can be calculated.

Question 4

How do reversible and irreversible processes differ in entropy change?

Accepted Answer

Reversible processes produce entropy changes that depend only on initial and final states. Irreversible processes generate greater entropy changes than reversible ones between the same states, causing systems to evolve differently. An irreversible cycle results in net entropy increase for both the system and surroundings, forming the basis for the entropy statement of the second law of thermodynamics.

Question 5

Why does entropy increase in irreversible cycles?

Accepted Answer

Irreversible processes inherently generate more entropy than reversible ones operating between identical states. This increased entropy reflects the spontaneous, uncontrolled nature of irreversible changes. The net entropy increase in irreversible cycles for both system and surroundings is a fundamental observation that leads to the entropy statement of the second law of thermodynamics.

Question 6

Can any reversible cycle be represented as a combination of Carnot cycles?

Accepted Answer

Yes, any reversible cyclic process can be shown mathematically to be equivalent to a sum of many Carnot cycles. This generalization is powerful because it allows us to apply Carnot cycle properties to all reversible cycles. Since the net entropy change in each Carnot cycle is zero, the total entropy change for any reversible cycle is also zero.

Question 7

How is entropy change calculated between two points on a p-V diagram?

Accepted Answer

For any two points on a p-V diagram, entropy change is independent of the path connecting them because entropy is a state function. You can calculate entropy change along any reversible path between those points and obtain the same result. This property simplifies thermodynamic calculations by allowing choice of the most convenient path for computation.

可逆过程中的熵变

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