Ask a physicist anything. (2)

[Beechwell]

I mean a box that light or heat cannot escape from. When the battery is drained, all that remains is heat (where's the light gone?) which, on a larger scale, can be used to heat water that can be used to, I dunno, power a hydro-wotsit that charges a battery...

The problem of your box that now contains all the energy in the form of heat is that "heat" basically means "completely random movements of the molecules (and within the molecules) of that box". You can't just transform all this spread out and undirectional energy back into a single, accessible form (like an electrical current or directed movement of the whole system (at least not witout investing additional evergy).
i don't really know how to explain it any better without becoming comletely scientifically unsound.

 

[Wiccan_Child]

Is perpetual motion/energy plausible?

No.

I'm not giving this up, it's got to be possible.

Mathematics says otherwise.

If energy cannot be created or destroyed then surely it can be harnessed no? Somehow? I mean, come on, what happens when you put a small bulb on a battery? The battery's energy is transferred to the bulb and eventually lost as heat and light. What if that happens in a heat/light proof box, eh? Okay I fully understand that the entropy of an isolated system which is not in equilibrium will tend to increase over time but that's just human understanding, we need to think outside the box.

Entropy is "just human understanding"? It has been mathematically proven that the entropy of a closed system will increase to a maximum. The proof starts with such a general system that no matter what box you think outside of, you're still bound by the Second Law of Thermodynamics.

"The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation" - Sir Arthur Stanley Eddington, The Nature of the Physical World

I mean a box that light or heat cannot escape from. When the battery is drained, all that remains is heat (where's the light gone?) which, on a larger scale, can be used to heat water that can be used to, I dunno, power a hydro-wotsit that charges a battery...

Which is physically impossible.

To charge the battery, you would have to lower the entropy of a closed system: heat would have to be drawn from the box's walls and into the battery, and it would have to be done in such a way that the chemical substrate ionises. This would lower the entropy of the system, so we know it's impossible.

In short, you can't draw useful energy from the ambient temperature, since it would require more energy that you could ever get back.

 

[Wiccan_Child]

The problem of your box that now contains all the energy in the form of heat is that "heat" basically means "completely random movements of the molecules (and within the molecules) of that box".

That's temperature (or a manifestation thereof). 'Heat' is the energy that makes them move, while temperature is the movement itself. That's why two objects with the same amount of heat can have different temperatures: one doesn't like to vibrate as much, thus making it have a lower temperature per joule.

 

[Beechwell]

That's temperature (or a manifestation thereof). 'Heat' is the energy that makes them move, while temperature is the movement itself. That's why two objects with the same amount of heat can have different temperatures: one doesn't like to vibrate as much, thus making it have a lower temperature per joule.

Ah, true, sorry. I was more trying to convey the general idea (hopefully correctly?) rather than being physically exact.

 

[Wiccan_Child]

Ah, true, sorry. I was more trying to convey the general idea (hopefully correctly?) rather than being physically exact.

It's not fantastically important, but it's best to be careful with these things. And if someone learns something, all the better.

 

[canehdianhotstuff]

I would just like to tell you that the 2nd law of thermodynamics has been broken...just a little though.

Second law of thermodynamics "broken" - 19 July 2002 - New Scientist

 

[Cabal]

I would just like to tell you that the 2nd law of thermodynamics has been broken...just a little though.

Second law of thermodynamics "broken" - 19 July 2002 - New Scientist

That entire article seems a bit broken, largely due to this sentence:

They found that the change in entropy was negative over time intervals of a few tenths of a second, revealing nature running in reverse. In this case, the bead was gaining energy from the random motion of the water molecule - the small-scale equivalent of the cup of tea getting hotter."

Hardly sounds like a closed system they were looking it - so what's the big deal, of course entropy can be reversed locally?

 

[Wiccan_Child]

That entire article seems a bit broken, largely due to this sentence:



Hardly sounds like a closed system they were looking it - so what's the big deal, of course entropy can be reversed locally?

I think the point is that all the energy can be randomly concentrated into one corner of the system (say). This isn't just a local decrease in the entropy of an open system, but a universal decrease in a closed one.

 

[catzrfluffy]

Thread is dead, baby.​

 

[Wiccan_Child]

Thread is dead, baby.​

*pumps fresh life into it*

Not physics-y, but they recently found octopuses that can collect coconut shells for makeshift shelter:

http://www.youtube.com/watch?v=1DoWdHOtlrk

 

[pgp_protector]

Love this one.

http://www.youtube.com/watch?v=HEheh1BH34Q

 

[catzrfluffy]

http://www.youtube.com/watch?v=5OrCCKDUMos​

 

[Wiccan_Child]

Ferrofluids!

http://www.youtube.com/watch?v=me5Zzm2TXh4

 

[pgp_protector]

Ferrofluids!

http://www.youtube.com/watch?v=me5Zzm2TXh4

I so wish I had the spair coin to play with that stuff

That and non newtonian fluids

http://www.youtube.com/watch?v=3zoTKXXNQIU

 

[Wiccan_Child]

You can make non-Newtonian fluids yourself with cornstarch (whatever that is...).

 

[pgp_protector]

You can make non-Newtonian fluids yourself with cornstarch (whatever that is...).

Sorry, yea I know that Cornstarch + water

Even the Ferrofluids can be made cheaply, but the magnets to drive them for effects like the video cost a bit more than I can afford right now

 

[Wiccan_Child]

Sorry, yea I know that Cornstarch + water

Even the Ferrofluids can be made cheaply, but the magnets to drive them for effects like the video cost a bit more than I can afford right now

Plus those smooth, grooved towers made of awesome and win. Can't just get them at a car boot sale.

 

[Cabal]

You can make non-Newtonian fluids yourself with cornstarch (whatever that is...).

*whispers* cornflour.....

 

[catzrfluffy]

*or arrowroot*

hmm...what to do with that left-over pencil nib?...
http://www.youtube.com/watch?v=s9eE2jFoioM&feature=related

 

[TerranceL]

Love this one.

http://www.youtube.com/watch?v=HEheh1BH34Q

Always loved that video. Thank you very much for posting this and reminding me of it.