Proof: S (or entropy) is a valid state variable | Thermodynamics | Physics | Khan Academy

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Proof that S (or entropy) is a valid state variable. Created by Sal Khan.
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[@terrymorse]

@patilnikh "in "adiabatic" how is temperature NOT ADDED or LOST?????" I think your question is how can temperature change in an adiabatic process. That's easy. "Adiabatic" just means no heat added to or removed from the system. There are other ways than heat to change a system's internal energy and temperature. Work, for example: an expanding gas moving a piston does work, and the temperature of the gas decreases as the piston moves.

[@gatesofhorn]

It is hard to be convinced that entropy is a state variable, by just showing that delta S is zero for a reversible process like Carnot cycle. In general, delta S may not be zero even for a cyclic process; moreover Q itself is not a state variable. Please dont name this video as a 'proof'.

[@pedrocarvalho6587]

My thoughts exactly! I know its been ten years since you commented but I need to understand why entropy can be treated as a state variable for any process in order to understand the TdS equations, I cant find the answer anywhere. Do you know the answer?

[@Thermospecialist]

When showing the Carnot cycle in terms of entropy, I rather would use the TS-diagram. Chemists are not aware of it, so they use the PV-diagram. Not wrong, but more complicated. Btw, change of entropy is a measure for the dispersion of energy: deltaS = deltaQ/T

[@KiroStun]

U should always be the same, a lesson of life.

[@s0m0c]

Gracias.

[@AndrewCedricThe2nd]

@patilnikh hahahahahaa

[@medwatt]

Why is Q2 not negative since it is being lost from the system ??

[@sciencenerd7639]

Just the fact that Sal is able to make me understand things about thermodynamics is phenomenal.

[@RK-bj7fi]

Infinite thanks to you Sir for providing such a thorough video, and to Khan Academy for freely publishing such helpful education. Though this proof does not prove the idea for any random cycle, it is much clearer than what my book has failed to deliver this elegantly. This is a saver.

[@mikeb5174]

10:45 you said you had VD lol anyways thank you thank you. you clarified what the book was TRYING to say. thanks Khan.

[@mayukhintesarislam306]

Thanxxxx!

[@Iamatheist1234]

@Annergize it's fine to call it state variable, state function as long as they are path-independent. Pressure, T and specific volume are technically the properties of the system, it would be kind of weird to call it state-variables since they are USED to define a state(two independent intensive properties).

[@carldepp]

zerooo!!! haha,... think about applying @ el camino city college. our profs are sucks!!

[@Thermospecialist]

Quite right!