# I Entropy: second law for systems with zero input net energy

1. Jun 12, 2016

### haushofer

Dear all,

I'm trying to think about applying the second law of thermodynamics to a system which is not isolated, but has an energy flowing inwards and an equal (!) energy flowing outwards, such that the total energy does not change (total energy flux is zero). Can we still apply the second law in this case? And where can I find a reference (here or elsewhere) which treats this case?

2. Jun 12, 2016

### hilbert2

If you compress an ideal gas isothermally, there is an influx of work and equal outflux of heat and the internal energy of the gas doesn't change. ( http://hyperphysics.phy-astr.gsu.edu/hbase/therm/entropgas.html )

Of course if a closed system is in thermal equilibrium with its surroundings, it's constantly exchanging molecular kinetic energy (heat) with the surroundings, but there is no net flow of heat to either direction.

3. Jun 12, 2016

### haushofer

Ah, yes, of course, that's a familiar example. Thanks! My thermodynamics is a bit rusty, but I'm reviewing some for applications to cosmology (gravitating systems).

4. Jun 12, 2016

### mfig

Yes. Here is a place to start.

5. Jun 12, 2016

### hilbert2

Do we mean the same thing with terms "isolated", "closed", and "open"? An isolated system doesn't exchange either energy or matter with its surroundings. A closed system can exchange heat but not matter. An open system can exchange both heat and matter with the rest of the universe.

6. Jun 12, 2016

### haushofer

Yes. So I'm referring to the second law for closed systems instead of isolated ones.