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Thermodynamics · Quick Reference
Second Law of Thermodynamics Cheat Sheet
In one line: the second law of thermodynamics explains why some processes happen naturally in only one direction — heat flows from hot to cold, never the reverse, without outside help.
Key Ideas
1The Second Law Statement. Heat spontaneously flows from hotter objects to colder ones, never the reverse, without external work being done. Equivalently, the total entropy of an isolated system never decreases.
2Entropy. A measure of disorder or randomness in a system; natural processes tend to increase the total entropy of the universe.
3Why Some Processes Are Irreversible. Many everyday processes (like an egg breaking, or perfume dispersing in a room) are easy to do but essentially impossible to reverse spontaneously, reflecting the second law's directionality.
4Heat Engines and Efficiency Limits. The second law implies no heat engine can be 100% efficient -- some energy must always be 'wasted' as heat expelled to a cooler reservoir.
5Refrigerators and Heat Pumps. These devices move heat from cold to hot regions, but only by doing external work (input energy) -- they don't violate the second law, since they aren't spontaneous processes.
Worked Examples
Why can't a heat engine convert 100% of absorbed heat into useful work?
The second law of thermodynamics forbids perfect heat-to-work conversion in a cyclic process
A room's perfume disperses evenly over time, but never spontaneously concentrates back into the bottle. What does this illustrate?
The second law of thermodynamics -- entropy naturally increases in isolated systems
Does a refrigerator violate the second law of thermodynamics by moving heat from its cold interior to the warmer kitchen?
No, because it's not a spontaneous process -- external work (electricity) drives the heat transfer
Practice Yourself
What does the second law say about heat flow direction (without external work)?
Heat flows spontaneously only from hot to cold
What does 'entropy' measure?
Disorder or randomness in a system
Can any real heat engine be 100% efficient?
No, the second law forbids it
Does entropy of an isolated system tend to increase or decrease over time?
Increase (or stay the same in a reversible process)