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📘 Lesson 8 of 10 · Thermodynamics

↩️ Second Law of Thermodynamics

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.

Course progress: 80%

01 Key Concepts

The 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.

Entropy

A measure of disorder or randomness in a system; natural processes tend to increase the total entropy of the universe.

Why 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.

Heat 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.

Refrigerators 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.

02 Solved Examples

Example 1 Why can't a heat engine convert 100% of absorbed heat into useful work?
  1. The second law requires that some heat always be expelled to a colder reservoir.
  2. This 'wasted' heat is unavoidable in any real cyclic heat engine.
Answer: The second law of thermodynamics forbids perfect heat-to-work conversion in a cyclic process
Example 2 A room's perfume disperses evenly over time, but never spontaneously concentrates back into the bottle. What does this illustrate?
  1. The dispersed state has much higher entropy (more disorder) than the concentrated state.
  2. Natural processes trend toward higher total entropy.
Answer: The second law of thermodynamics -- entropy naturally increases in isolated systems
Example 3 Does a refrigerator violate the second law of thermodynamics by moving heat from its cold interior to the warmer kitchen?
  1. The refrigerator requires external electrical work to force this heat transfer.
  2. The second law only forbids heat moving from cold to hot spontaneously (without external work).
Answer: No, because it's not a spontaneous process -- external work (electricity) drives the heat transfer

03 Practice Questions

1What does the second law say about heat flow direction (without external work)?
Heat flows spontaneously only from hot to cold
2What does 'entropy' measure?
Disorder or randomness in a system
3Can any real heat engine be 100% efficient?
No, the second law forbids it
4Does entropy of an isolated system tend to increase or decrease over time?
Increase (or stay the same in a reversible process)
5Why doesn't a refrigerator violate the second law?
It uses external work (electricity) to force heat from cold to hot, rather than doing so spontaneously

📄 Second Law of Thermodynamics — Downloadable Worksheet

10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.

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