In going from i to f in each of the three processes of FIGURE Q21.1, is work done by the system $(W<0\text{,}{W}_{\text{s}}>0)$, is work done on the system $(W>0\text{,}{W}_{\text{s}}<0)$, or is no net work done?

FIGURE Q21.1

Rank in order, from largest to smallest, the amount of work $(W_{\text{s}}){}_{\text{A}}$ to $(W_{\text{s}}){}_{\text{D}}$ done by the gas in each of the cycles shown in FIGURE Q21.2. Explain.

FIGURE Q21.2

Could you have a heat engine with $\eta >1\text{?}$ Explain.

FIGURE Q21.4 shows the pV diagram of a heat engine. During which process or processes is (a) heat added to the gas, (b) heat removed from the gas, (c) work done on the gas, and (d) work done by the gas?

FIGURE Q21.4

Rank in order, from largest to smallest, the thermal efficiencies ${\eta }_1$ to ${\eta }_4$ of the four heat engines in FIGURE Q21.5. Explain.

FIGURE Q21.5

FIGURE Q21.6 shows the thermodynamic cycles of two heat engines. Which heat engine has the larger thermal efficiency? Or are they the same? Explain.

FIGURE Q21.6

A heat engine satisfies $W_{\text{out}}=Q_{\text{net}}$. Why is there no $\text{Δ}{E}_{\text{th}}$ term in this relationship?

Do the energy-transfer diagrams in FIGURE Q21.8 represent possible heat engines? If not, what is wrong?

FIGURE Q21.8

Do the energy-transfer diagrams in FIGURE Q21.9 represent possible refrigerators? If not, what is wrong?

FIGURE Q21.9

It gets pretty hot in your apartment. In browsing the Internet, you find a company selling small “room air conditioners.” You place the air conditioner on the floor, plug it in, and—the advertisement says—it will lower the room temperature up to $10\text{°F}$. Should you order one? Explain.

The first and second laws of thermodynamics are sometimes stated as “You can’t win” and “You can’t even break even.” Do these sayings accurately characterize the laws of thermodynamics as applied to heat engines? Why or why not?