## Change in Entropy of Universe

$\Delta S = \frac{q_{rev}}{T}$

Ju-Wei Wang 1I
Posts: 31
Joined: Fri Sep 29, 2017 7:05 am

### Change in Entropy of Universe

When do you know that you can set delta Suniv equal to zero? Is it just for reversible processes?

Anika_Patel_1G
Posts: 32
Joined: Fri Sep 29, 2017 7:06 am

### Re: Change in Entropy of Universe

Delta S universe is equal to 0 only when the system us at equilibrium. So yes, only when a reversible process is taking place. When delta S universe is positive, the process is spontaneous.

804899546
Posts: 51
Joined: Sat Jul 22, 2017 3:00 am

### Re: Change in Entropy of Universe

Can you explain why delta S universe can be zero during a reversible process, but is always positive for a spontaneous process? I thought it was always positive no matter what.

Curtis Wong 2D
Posts: 62
Joined: Sat Jul 22, 2017 3:00 am

### Re: Change in Entropy of Universe

Delta S of the universe is equal to 0, because it's essentially Delta S of the system + Delta S of the surroundings. The system experiences an increase in entropy in a reversible process, but since its a reversible process, the Delta S of the surroundings experience a decrease in entropy as well. Thus, Delta S of the universe is equal to 0.

Now in a spontaneous irreversible process, the Delta S of the surrounding will be equal to 0, because it cannot go back to that state. It would require energy to go back to that initial state. So The Delta S of the universe will be just the Delta S of the system.

Problem #47 is a good example showcasing this.