Is the rate of entropy of a closed system constant?

[kfmfe04]

If I am within that closed system, can I make the rate of entropy of the entire system any faster or slower overall? Does the existence of life within the system decrease or increase the overall system entropy rate?

 

[zhermes]

What do you mean by the 'rate' of entropy? What kind of system are you talking about, and how is life involved?

 

[kfmfe04]

If you put a glass of water with ice cubes in a room, the ice cubes will melt at a certain rate and eventually, all the water will evaporate into the air, reaching full entropy. The rate that these events happen is the rate of entropy.

In a larger system, you may place an imaginary sun and Earth without any living things. This system will have its own rate of entropy.

If, say, there were living things in this system, would the rate of entropy for the entire system go up, down, or stay the same?

 

[Phrak]

I think you are asking about the rate of entropy increase. The rate can vary. You only have to imagine a system consisting of a box having hot gas on one side and cold gas on the other with the two separated by a thermal partition between. Put the partition on a timer that will open when a clock counts down to zero. Before the clock times-out the rate of increase will change slowly as heat leaks across the partition. When the partition is opened, it increases much faster.

 

[kfmfe04]

I am concerned about the change of entropy with respect to time for a closed system.

So Phrak's example is an appropriate illustration; the change of entropy with respect to time will increase after you open the partition.

In particular, I am curious as to whether this measure will go up, down, or stay the same when we introduced living organisms into a closed system.

As I understand it, living organisms have negative entropy, but require energy input to stay alive. Will the extra entropy caused by the energy taken in by living organisms more than offset that negative entropy maintained by keeping the living organism alive?

I'm curious about change of entropy for the entire closed system, without living things and with living things.

 

[Phrak]

Living organisms reduce their rate of entropy increase at the expense of an increase in the entropy of their environment.

 

[kfmfe04]

Do the measurements even out, or is the net an increase in the rate of entropy for the closed system?

 

[Bob S]

For millions of years, homo sapiens used energy at the metabolic rate, 2000 Kcal/day, or ~3 million BTU per year. just for food. When homo sapiens discovered fire, and began using it to cook, provide warmth, and ward off carnivorous animals, the energy consumption rate per capita began to increase. So with fire, the rate of entropy increase per capita began to increase faster than the population growth. Now in the United States, the total annual energy consumption rate is over 300 million BTU per capita, or 100 x metabolic (Maybe 5% of this is renewable), and the population growth is ~1% per year. About 99% of this energy usage is due to our intelligence in using available natural non-renewable energy sources. So did our intelligence increase or decrease the rate of entropy increase?

Bob S

 

[Phrak]

I never thought to run the numbers, Bob. I concur using the annual US consumption of 100 * 10^18 Joules and the human daily consumption of 10 mega Joules.

 

[Phrak]

I'm curious about change of entropy for the entire closed system, without living things and with living things.

It really depends upon the situation. Put some wood and air in a closed environment. Without living organisms it will take a long time for the wood to oxidize. Add sunlight and it will proceed faster. Instead, add fire and it's a lot faster. Add termites without the fire or sunlight and it will be somewhere in between adding fire and being left alone in the dark.

 

[kfmfe04]

It really depends upon the situation. Put some wood and air in a closed environment. Without living organisms it will take a long time for the wood to oxidize. Add sunlight and it will proceed faster. Instead, add fire and it's a lot faster. Add termites without the fire or sunlight and it will be somewhere in between adding fire and being left alone in the dark.

This is a very interesting point.

Say human beings are introduced into an environment where forest fires are a naturally occurring phenomenon due to lightning strikes. Effective fire-fighting would probably result in a net slowdown in the rate of entropy.

Thanks for that great example, Phrak.

 

[Phrak]

Thanks for that great example, Phrak.

You're welcome. But there's a cavet to my scenario. Where did the free oxygen on planet Earth come from to begin with? It came from the living organisms. It came from organisms such as green plants we see today that expire oxygen to obtain carbon.

So my example is somewhat tainted by the requirement that living organisms had to set-up the free oxygen environment to begin with. It couldn't be too hard to set-up a better example, though I can't think of one right off.

 

[kfmfe04]

I have two more questions:

1. Let's think about the tree case... ...where we introduce non-sentient organisms into a closed system.

Can we expect the enthalpy increase to be more than offset by the additional entropy needed to keep the trees alive?

2. Does the thermodynamics definition of entropy include non-classical forms of potential power like nuclear power?

Case A: Naturally decaying universe
Case B: Sentient beings using nuclear power to build interesting things

Which one will reach universal heat death first?

 

[Bob S]

Does the thermodynamics definition of entropy include non-classical forms of potential power like nuclear power?

The world's first "classical" (meaning pre-homo sapiens) nuclear reactor was the one in Gabon, about 2 billion years ago. The Oklo reactor ran at about 100 kW for a few hundred thousand years. This nuclear fission reaction caused a small local bump in the the world's rate of entropy increase. See

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

Bob S