# I Sustained Entropy?

#### TheoPan

Hello everyone!

I am an undergraduate student from Greece in my first semester of Mechanical Engineering, but I am fascinated with physics. I've been studying some physics books from my university's library and reached the chapter of entropy. I understand the 2nd law of thermodynamics but what would happen if we could sustain entropy at the same level over time? Is it possible? Would it be possible in the future?

Related Other Physics Topics News on Phys.org

#### Khashishi

A reversible reaction is one that does not increase the entropy. A machine only using reversible reactions will keep the same entropy. But this really limits what you can do.
An irreversible reaction increases entropy. Your body has a bunch of irreversible reactions, so entropy overall will increase.

#### Chestermiller

Mentor
A reversible reaction is one that does not increase the entropy. A machine only using reversible reactions will keep the same entropy. But this really limits what you can do.
An irreversible reaction increases entropy. Your body has a bunch of irreversible reactions, so entropy overall will increase.
Can you please state this more precisely, like, for example, with respect to the entropy of the universe rather than the entropy of a specific (closed) system?

#### Khashishi

I guess it depends on what the OP is asking. If they are asking if it's possible to keep the entropy of the whole universe at a fixed level, then the answer is no. But for a small, carefully designed closed system, the second law only prohibits the entropy from going down. It is allowed to stay at the same level. For a macroscopic sized system, there's always some non-idealities, so you can't really prevent entropy from increasing. As a MechE, you will certainly learn about adiabatic processes, which are idealizations, but they are useful to study since we can get close, and they teach us the fundamental limits on what is possible.

#### Chestermiller

Mentor
I guess it depends on what the OP is asking. If they are asking if it's possible to keep the entropy of the whole universe at a fixed level, then the answer is no. But for a small, carefully designed closed system, the second law only prohibits the entropy from going down. It is allowed to stay at the same level. For a macroscopic sized system, there's always some non-idealities, so you can't really prevent entropy from increasing. As a MechE, you will certainly learn about adiabatic processes, which are idealizations, but they are useful to study since we can get close, and they teach us the fundamental limits on what is possible.
I think that we have a bit of a terminology issue here. Physicists call a "closed system" one for which no mass enters or leaves, and no work or heat transfer takes place at the boundaries; however, this is what engineers refer to as an "isolated system." Engineers call a "closed system" one for which no mass enters or leaves, but for which work and heat transfer can take place at the boundaries. Under this engineering definition of a closed system, entropy can certainly decrease for certain processes.

Chet

"Sustained Entropy?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving