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Maxwell's daemon

This isn't technically homework help but arising from my own interest in physics... but whatever I can find on the topic is too technical. Just correct me when I'm wrong. This is Maxwell's daemon:

It is a sacrosanct and inviolable fact of this world that organised stuff becomes disorganised. Life becomes death, heat becomes cold, etc. Never the other way round. Right? You leave a glass of tea on the dresser, it's not going to become hotter.

Except...

We know that the reason it's hot is because the atoms in it are moving fast. The faster they move, the hotter. But they don't ALL move at the same speed X per second; some move a bit faster, and some a bit slower.

So say you hooked up two boxes A and B, both filled with gas Y at X degrees Fahrenheit and therefore also atoms moving at speed X per second on average. There's a wall between the boxes with a close-able pinhole in it, enough to let exactly one atom pass through. This is controlled by a computer. If it senses an atom moving faster than X towards box A, it lets it through; if it senses an atom moving slower than X towards box B, it also lets it through; and both of these on the condition that the number of atoms in boxes A and B remain about equal.

Basically you'll get a "free lunch" in the sense that box A will begin to spontaneously heat up. Right? That much I've been able to gather from Maxwell's writings.

Only here's the problem: the computer needs power to function and to open and close the pinhole. And I'll lay you ten to one that whatever power the computer uses is ten or a hundred times more than whatever you can save by using this gadget. The Universe, being its usual hateful self towards all its inhabitants, refuses to allow even this moderate example of a "free lunch" and thus the energy-saving device is, in practice, an impossibility.

Did I bugger up somewhere or is that the main thrust of the Maxwell's Daemon exercise?

Reply 1

lerjj

Original post by honeywhite

That would be pretty much correct on all counts by my knowledge. It's a suggestion of a possible way to break the 2nd law of thermodynamics, you gave the most common failing of it (might be others, can't remember).

Reply 2

honeywhite

Original post by lerjj

Oh that's funny because I came up with that last bit all by myself---reading Maxwell's oddly-spelled variant of English was one thing (still, more legible than Burns by a country mile), but it was only when I was "getting cold chicken from the icebox" (as Alfred Hitchcock would say) that I realised---hey, Maxwell isn't saying anything about power requirements!

Reply 3

lerjj

Original post by honeywhite

Yep, the general fix is that the demon in some way needs to increase the entropy more than he decreases it (obviously). And this is where Maxwell gets stuck as far as I know- because there isn't so far a way of doing it (and based on the Demon, there presumably never will be). Well done for coming up with it on your own though!

Reply 4

Joinedup

Original post by honeywhite

That's a perfectly legitimate spelling

so there

Reply 5

honeywhite

Original post by Joinedup

That's a perfectly legitimate spelling

so there

I know "daemon" is a perfectly legitimate spelling; it is my preferred one as well---but Maxwell had a few things he spelled wrong. Then again, Robert Burns was even worse---he spelled "foot" as "fit" for gawd's sake, and the publishers didn't even deign to fix it.

Reply 6

natninja

Original post by honeywhite

The Maxwell's Demon example actually uses no energy - it's a thought experiment. The crunch however is that entropy and information are linked and so it doesn't decrease the entropy of the system as you have to have knowledge about the system for it to work. I know it's a bit of a fudgy answer but I hope it's ok for you. So unless you already had complete knowledge of the system it would be impossible to do without having to measure the speeds of the particles which would involve energy.

Reply 7

honeywhite

Original post by natninja

I know it's a thought experiment, but any real-world application WOULD use power, so that's where I started to think. On the other hand, if the computer would be made such that it uses no power except when storing and retrieving information (which is what I think you were alluding to above), it would STILL probably use more power than what it would save.

Reply 8

natninja

Original post by honeywhite

I'm not even alluding to that. Look up information theory.

Reply 9

honeywhite

Original post by natninja

I'm not even alluding to that. Look up information theory.

If it's got equations and stuff in it, I won't. Physics hurts my head and it hurts my arms (from writing all that stuff down). I feel like Stevie H in the movie "Hawking".

Reply 10

natninja

Original post by honeywhite

If it's got equations and stuff in it, I won't. Physics hurts my head and it hurts my arms (from writing all that stuff down). I feel like Stevie H in the movie "Hawking".