Summarizing the Introduction to the Second Law of Thermodynamics

Summary

• The two expressions of the second law of thermodynamics are: (i) Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction; and (ii) It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.
• Irreversible processes depend on path and do not return to their original state. Cyclical processes are processes that return to their original state at the end of every cycle.
• In a cyclical process, such as a heat engine, the net work done by the system equals the net heat transfer into the system, or \(W={Q}_{\text{h}}–{Q}_{\text{c}}\phantom{\rule{0.25em}{0ex}}\) , where \({Q}_{\text{h}}\) is the heat transfer from the hot object (hot reservoir), and \({Q}_{\text{c}}\) is the heat transfer into the cold object (cold reservoir).
• Efficiency can be expressed as \(\text{Eff}=\cfrac{W}{{Q}_{\text{h}}}\), the ratio of work output divided by the amount of energy input.
• The four-stroke gasoline engine is often explained in terms of the Otto cycle, which is a repeating sequence of processes that convert heat into work.

Glossary

irreversible process

any process that depends on path direction

second law of thermodynamics

heat transfer flows from a hotter to a cooler object, never the reverse, and some heat energy in any process is lost to available work in a cyclical process

cyclical process

a process in which the path returns to its original state at the end of every cycle

Otto cycle

a thermodynamic cycle, consisting of a pair of adiabatic processes and a pair of isochoric processes, that converts heat into work, e.g., the four-stroke engine cycle of intake, compression, ignition, and exhaust