What prevents you from breaking thermodynamics with radiation and a clever arrangement of elliptical reflectors and heated objects of varying surface areas?
Basically, I was thinking of something in which one has a very small ‘target’, and a much larger (but still tiny relative to the whole system) ‘source’, both close to black body, and these objects at the two foci of a giant ellipsoidal reflector.
It seems that if the started at the same temperature, the source would radiate more than the target due to its larger surface area,and that radiation would hit the target, resulting in a temperature difference.
The amount of the source’s radiation that will actually reach the target is the same as the target’s emitted radation. Because the source is larger, there are steep angles it can emit at that do not pass through the target and only return to itself.
It looks to me like you’re tacitly assuming genuinely pointlike sources and targets when you say “that radiation would hit the target”, but then going back and reversing that assumption when you say the source would radiate more. The surface of the source is not exactly at the focal point, and so radiation from it does not necessarily pass exactly through the other focal point.
This is impossible to answer unless you give more information about your proposed set-up. At any rate, if you have any closed arrangement of heated objects, the radiation will always cause the objects to reach thermal equilibrium (same temperature) eventually. There is no way around this.
Are you describing an idea for an energy trap-type thing? It sounds like it would require perfect reflectors, which I don’t think actually exist. Most perpetual motion ideas tend to break because reality does not consist of spherical cows, or because energy is radiated out of the system and can’t be recaptured, is lost due to friction, etc.
(Apologies if I’ve completely misunderstood what you’re proposing.)
Most perpetual motion ideas tend to break because reality does not consist of spherical cows
That’s not true. The first and second laws of thermodynamics tend to be apply even with ideal objects. It’s true that an idealized pendulum is a perpetual motion machines of the third kind and they don’t exist because of friction, but the category exemplified by a pendulum is not interesting and isn’t what people usually mean by perpetual motion machine.
Moreover, any amount of friction will eventually stop a pendulum, whereas something that is supposed to generate a definite amount of energy or decrease entropy by a definite amount will not be stopped by an arbitrarily small amount of friction or an arbitrarily small deviation from sphericality. But in practice you can make things arbitrarily close to perfect.
We don’t need clever arrangements of reflectors for perpetual motion machines. Things like souls, angels, demons, disembodied minds, etc. are all perpetual motion machines. So if they actually exist, all we would have to do is capture them and harness them for unlimited energy, like Shang Tsungs soulnado to solve our energy problems.
What prevents you from breaking thermodynamics with radiation and a clever arrangement of elliptical reflectors and heated objects of varying surface areas?
Bears
OK, what reference am I missing this time?
Google to the rescue.
TL;DR: something hotter than the source will radiate back more than it absorbs.
Which specific arrangement do you have in mind, and why would it break thermodynamics?
Basically, I was thinking of something in which one has a very small ‘target’, and a much larger (but still tiny relative to the whole system) ‘source’, both close to black body, and these objects at the two foci of a giant ellipsoidal reflector.
It seems that if the started at the same temperature, the source would radiate more than the target due to its larger surface area,and that radiation would hit the target, resulting in a temperature difference.
The amount of the source’s radiation that will actually reach the target is the same as the target’s emitted radation. Because the source is larger, there are steep angles it can emit at that do not pass through the target and only return to itself.
It looks to me like you’re tacitly assuming genuinely pointlike sources and targets when you say “that radiation would hit the target”, but then going back and reversing that assumption when you say the source would radiate more. The surface of the source is not exactly at the focal point, and so radiation from it does not necessarily pass exactly through the other focal point.
OHHHHHHHH...… I get it. Thank you.
The thermodynamics police. (e.g. Detective Lieutenant Noether)
This is impossible to answer unless you give more information about your proposed set-up. At any rate, if you have any closed arrangement of heated objects, the radiation will always cause the objects to reach thermal equilibrium (same temperature) eventually. There is no way around this.
Are you describing an idea for an energy trap-type thing? It sounds like it would require perfect reflectors, which I don’t think actually exist. Most perpetual motion ideas tend to break because reality does not consist of spherical cows, or because energy is radiated out of the system and can’t be recaptured, is lost due to friction, etc.
(Apologies if I’ve completely misunderstood what you’re proposing.)
That’s not true. The first and second laws of thermodynamics tend to be apply even with ideal objects. It’s true that an idealized pendulum is a perpetual motion machines of the third kind and they don’t exist because of friction, but the category exemplified by a pendulum is not interesting and isn’t what people usually mean by perpetual motion machine.
Moreover, any amount of friction will eventually stop a pendulum, whereas something that is supposed to generate a definite amount of energy or decrease entropy by a definite amount will not be stopped by an arbitrarily small amount of friction or an arbitrarily small deviation from sphericality. But in practice you can make things arbitrarily close to perfect.
We don’t need clever arrangements of reflectors for perpetual motion machines. Things like souls, angels, demons, disembodied minds, etc. are all perpetual motion machines. So if they actually exist, all we would have to do is capture them and harness them for unlimited energy, like Shang Tsungs soulnado to solve our energy problems.