4 The Second Law and Entropy Free Online FlashCards

What does the second law add to energy conservation?

The first law accounts for energy conservation; the second law specifies the direction of spontaneous processes and limits conversion of energy into useful work.

What does the Clausius statement say?

Heat does not flow spontaneously from a colder body to a hotter one.

Why can a refrigerator move heat from cold to hot?

A refrigerator moves heat from cold to hot by consuming work supplied from outside.

What kind of property is entropy?

Entropy is a thermodynamic state property, denoted by $S$. Its change between equilibrium states can be calculated along any reversible path connecting them.

How is entropy change calculated along a reversible path?

$\Delta S = \int_1^2 \frac{\delta Q_{\mathrm{rev}}}{T}$, where $T$ is absolute temperature in kelvins.

What is the entropy balance for a system exchanging heat at boundary temperature $T_b$?

$\Delta S_{\mathrm{sys}} = \int \frac{\delta Q}{T_b} + S_{\mathrm{gen}}$, with $S_{\mathrm{gen}} \ge 0$.

How does entropy generation differ for reversible and irreversible processes?

An irreversible process generates positive entropy; a reversible process generates none.

Can a system's entropy decrease without violating the second law?

The entropy of a system alone can decrease if it transfers enough entropy to its surroundings; the total entropy of an isolated system cannot decrease.

What defines a reversible process?

A reversible process is an ideal limit that can be reversed while leaving both the system and surroundings unchanged.

Name two common causes of irreversibility.

Friction, unrestrained expansion, mixing, and heat transfer across a finite temperature difference are common causes of irreversibility.

How is a heat engine's thermal efficiency defined?

A cyclic heat engine absorbs $Q_H$, produces work $W$, and rejects heat $Q_C$; its efficiency is $\eta = \frac{W}{Q_H} = 1 - \frac{Q_C}{Q_H}$.

What is the Carnot efficiency between hot and cold reservoirs?

The Carnot efficiency is $\eta_{\mathrm{max}} = 1 - \frac{T_C}{T_H}$, with reservoir temperatures in kelvins. Only an ideal reversible engine achieves it; real engines between the same reservoirs are less efficient.