Thread: Thermodynamics 2nd law question

Hello, I hope this hasn't been discussed before - there's no search function on this forum so I couldn't search.

I have some queries about the 2nd law if anyone is kind enough to answer.

When I was doing engineering at uni, I remember that the 2nd law applies to isolated systems.

My ultimate question is:

Can the 2nd law also ultimately apply to open systems too?

The best I've seen so far is that: The universe can be classed as a closed system so OVERALL, entropy increases over time even though the universe is filled with cases of decreasing entropy... but they are comparatively short lived.

What is a system? How can it be classified?

I.e. the universe is a system... a battery.. a human body, ... a planet.... a species of life... can these be described as systems?

The reason I ask is, for example, that if a planet can be described as a system, then it would be a open system as it is open to everything around it... heat, meteorites, etc.. can it not be said that entropy increases over time there too?

Or a human body.. the body grows old and dies... but it is an open system, (if it can be classed as a system) it will burn out... die and decay... can that be called an increase in entropy?

Thank you.. I hope someone can enlighten me.
I've searched on the net but it hasn't answered this in a very clear cut way.

While I can't answer your question, I can tell you there's a search function. It's at the top-right of the page between the FAQ and the Memberlist.

Oops silly me - I just looked for a text box.
Thanks.


I'm looking now but so far.... i've not quite found exactly what I'm looking - although other issues of interest have been addressed - so the question it still open...

Originally Posted by sromag

Hello, I hope this hasn't been discussed before - there's no search function on this forum so I couldn't search.

I have some queries about the 2nd law if anyone is kind enough to answer.

When I was doing engineering at uni, I remember that the 2nd law applies to isolated systems.

My ultimate question is:

Can the 2nd law also ultimately apply to open systems too?

The best I've seen so far is that: The universe can be classed as a closed system so OVERALL, entropy increases over time even though the universe is filled with cases of decreasing entropy... but they are comparatively short lived.

What is a system? How can it be classified?

I.e. the universe is a system... a battery.. a human body, ... a planet.... a species of life... can these be described as systems?

The reason I ask is, for example, that if a planet can be described as a system, then it would be a open system as it is open to everything around it... heat, meteorites, etc.. can it not be said that entropy increases over time there too?

Or a human body.. the body grows old and dies... but it is an open system, (if it can be classed as a system) it will burn out... die and decay... can that be called an increase in entropy?

Thank you.. I hope someone can enlighten me.
I've searched on the net but it hasn't answered this in a very clear cut way.

A closed system is a region in space across the boundary of which neither mass, nor energy passes. So no work is done on or by a closed system. Closed systems are typical of those usually studied in chemistry.

An open system is one in which energy and mass are permitted to cross the boundary. Thermodynamics as studied by mechanical engineers usually involved open systems, since their ultimate objective involves building machines that do work. Open systems also obey thermodynamics, but one has to keep track of the work and mass that cross the boundary.

One can decrease the entropy of a system, but to do that one must do work on it. That of course requires an open system.

Thanks for the reply.

SO in essence... you could say that an open system increases in entropy over time ... IF no work is done to decrease it...... as such it acts like a closed system.


So a human body for example... could that be classed a system?.. (an open one)

I wouldn't know where to begin to look for the entropy on a human being.

Originally Posted by sromag

Hello, I hope this hasn't been discussed before - there's no search function on this forum so I couldn't search.

I have some queries about the 2nd law if anyone is kind enough to answer.

When I was doing engineering at uni, I remember that the 2nd law applies to isolated systems.

My ultimate question is:

Can the 2nd law also ultimately apply to open systems too?

The best I've seen so far is that: The universe can be classed as a closed system so OVERALL, entropy increases over time even though the universe is filled with cases of decreasing entropy... but they are comparatively short lived.

What is a system? How can it be classified?

I.e. the universe is a system... a battery.. a human body, ... a planet.... a species of life... can these be described as systems?

The reason I ask is, for example, that if a planet can be described as a system, then it would be a open system as it is open to everything around it... heat, meteorites, etc.. can it not be said that entropy increases over time there too?

Or a human body.. the body grows old and dies... but it is an open system, (if it can be classed as a system) it will burn out... die and decay... can that be called an increase in entropy?

Thank you.. I hope someone can enlighten me.
I've searched on the net but it hasn't answered this in a very clear cut way.

If you call the universe big bang how do you know it close ?

Well from what I remember.... you can't really have a closed system... The universe is one of the closest things you can get.

No heat/mass/energy from the outside enters or leaves.

That's how.

And if you ask me how do I know that nothing enters nor leaves the universe... then you're asking the wrong person. You'd need to talk to a physicist....but I've never heard anyone mention it... and I would think that's a pretty big thing.

Anyway... that's off topic.

there are three types of systems.

open - closed - isoated

in a closed system the only thing that cannot cross the boundary is matter.

energy , work , light , can cross the boundary.

in a isolated system nothing can cross the boundary.

nothing can cross the boundary.

our universe is considered to be the closest thing to an isolated system
but theres no such thing as a isolated system , because its not possible
to have a isolated system.

Thanks ninus...

So what are the limitations of a system? In other words.... Does a system have to be a single unit... such as .... an engine - a spinning top - maybe a human body - or even something like a solar system or galaxy??

OR

Can it include ... say, .. a species of animal - a family of people?

And also...
Is it correct to say that an open system increases in entropy over time ... IF no work is done to decrease it...... as such it acts like a closed system.

sounds a similar question to what 'information' means/is in the world of physics.....

Does that mean that the answer is too big?

Surely a system has quite definite finite boundaries