BeginnerTopic 25 of 105Thermodynamics & Fluids
Entropy
A measure of disorder and irreversibility; entropy of an isolated system never decreases.
In depth
dS≥δQ/T; for reversible processes dS=δQ_rev/T. Entropy generation S_gen≥0 measures irreversibility; isolated systems never decrease in entropy. Statistically S=k ln W. Used to evaluate efficiency and process feasibility.
Key points
- Entropy balance for control volumes
- Ideal gas Δs=c_p ln(T₂/T₁)-R ln(P₂/P₁)
- Adiabatic reversible is isentropic
ExampleMixing hot and cold water increases total entropy.
What is Entropy?
A measure of disorder and irreversibility; entropy of an isolated system never decreases.
Why is it important to understand entropy?
Entropy measures irreversibility and gives the direction of processes, so it is central to every analysis of losses and efficiency.
What does entropy measure?
The degree of energy dispersal and the irreversibility of a process. It increases in irreversible processes in isolated systems.
Does entropy always increase?
In an isolated system the total entropy never decreases, but a local system can lose entropy if the surroundings gain more.
How is Entropy used in practice?
Typical applications include calculating the entropy generated by friction in a valve, checking whether a proposed process could occur in an isolated system and finding the entropy change of a gas during an irreversible expansion.
What should I learn before Entropy?
Study "Carnot cycle" first (topic 24 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Entropy?
Entropy balance for control volumes. Ideal gas Δs=c_p ln(T₂/T₁)-R ln(P₂/P₁). Adiabatic reversible is isentropic.
Can you give an example of Entropy?
Mixing hot and cold water increases total entropy.
What comes after Entropy?
The next topic is "T-s diagrams", which builds directly on this one.
How difficult is Entropy?
It is classed as Beginner in Thermodynamics & Fluids (topic 25 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.