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# Thermodynamic entropy definition clarification

Clarifying that the thermodynamic definition of Entropy requires a reversible system. Created by Sal Khan.

## Want to join the conversation?

• I dont really undertand what Entropy means :S
• Entropy is essentially a measure of the unusable energy that the system has (a more technical definition is that it is the measure of energy dispersion)
• why he took friction as heat added to the system with positive sign. shouldn it be minus becaouse while system is expanding systems itself is gonna do this work and loose energy for friction forces?
• Friction always adds heat to a system because you are creating energy by the friction process. During the expansion the work is being done as the piston moves up, in a 'non-friction piston' there would be no Q_f because we assume the expansion is able to do this with no loss in energy. In a 'friction piston' we take accountability of the Q_f because the piston is rubbing against the side creating heat and works against the expansion process so the piston wouldn't reach the same height in a 'non-friction piston' when the same amount of pebbles are removed.

Since the equations relate to internal energy the piston friction is adding energy to the system (friction against the expansion), where the work is losing energy to the system (raising the piston).
• Why does the W cancel out at ? If friction was there when the system expanded how can the work done on the system be the same as the work it did on the surrounding before?
• It appears to me that he is looking at the same isotherm, so even if you go back in the other direction ( from A to B, or from B to A) the area under the curve, or the work, will be the same, so they will cancel out.
• at he says the system generates heat and so it will be given to surrounding...so it must be -Qf right...
• Ya I thought of the same thing..
Shouldn't it be dU=Q-(W=Qf) ?
i.e. Q goes into doing displacement work + frictional work or heat
• Has entropy been proven?
• Entropy in an isolated system increases or remains constant. Energy tends to degrade into its lowest state -- heat. Yes, entropy is real. Heat is relative to absolute zero (0ºK) -- the absence of heat. Entropy tends to be linear and never decreases; in this way we can see it as time (particles from the creation of the universe or cosmic background radiation). Therefore, the universe tends to move toward disorder --entropy. The heat your body is producing from cellular respiration proves it.
• In , Sal divided all the terms by the same T. However, the first isothermal process should have a different process from the other (T1, then T2, where T1>T2), based on what I know from his previous videos. So why did he state that the temperature was constant the whole time (although i can understand graphically that he did go from state 2 to 1 using the same path he used when he went from state 1 to 2, and hence, the temperature should stay the same, but why did he draw it that way?)
• what is the difference between quasi-static process & reversible process?
• A reversible process equals a quasistatic proces where you have no change in entropy.
So you can have reversible quasistatic processes and irreversible quasistatic processes.
(1 vote)
• I keep hearing that entropy is always increasing that is because Q is increasing in dS=dQ/T. But suppose when I cool something off isn't it releasing heat? And thus isn't entropy decreasing? Because then dQ would be negative
• Entropy is always increasing in the universe. If you cool off something it is just releasing that heat/energy.