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Can you use a magnifying glass and moonlight to light a fire? (2016)

what-if.xkcd.com

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Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#71

Post by DenisM »

schoen wrote: post #51
DenisM wrote: post #26

Earlier quoted context omitted.

I can convert the optics to non-reversible by spreading a thin layer of dust on top. Now both systems are non-reversible and both allow higher temperatures? Or are we looking at a particular kind of non-reversibility?

I think the thermodynamics argument is that only non-reversible systems cause energy to flow from a lower-temperature source to a higher-temperature destination, not that all non-reversible systems do so. For example, if you set up a solar panel that directly feeds a resistor, you have a dissipative system that's unlikely to get hotter than the sun. However, a solar panel array and/or battery could conceivably power…

>if you set up a solar panel that directly feeds a resistor, you have a dissipative system that's unlikely to get hotter than the sun.

I guarantee you this can be done. Arc welding [1] goes to many thousands of degrees, up to 20,000C. A square mile of solar panels will most certainly give me enough energy to power an arc welder. In fact I will probably will get away with just 100kw of pwoer, so 500 panels give or take.

[1] https://hypertextbook.com/facts/2003/EstherDorzin.shtml

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#72

Post by Dylan16807 »

DenisM wrote: post #71
schoen wrote: post #51

Earlier quoted context omitted.

I think the thermodynamics argument is that only non-reversible systems cause energy to flow from a lower-temperature source to a higher-temperature destination, not that all non-reversible systems do so. For example, if you set up a solar panel that directly feeds a resistor, you have a dissipative system that's unlikely to get hotter than the sun. However, a solar panel array and/or battery could conceivably power…

>if you set up a solar panel that directly feeds a resistor, you have a dissipative system that's unlikely to get hotter than the sun. I guarantee you this can be done. Arc welding [1] goes to many thousands of degrees, up to 20,000C. A square mile of solar panels will most certainly give me enough energy to power an arc welder. In fact I will probably will get away with just 100kw of pwoer, so 500 panels give or tak…

Okay, sure, as long as you're not trying to say this contradicts the post. "A resistor" was just an example of how you could waste the power and not get much heat.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#73

Post by DenisM »

ALittleLight wrote: post #65
DenisM wrote: post #14

Thermodynamics argument seems iffy to me. I can cover the entire surface of the earth with solar panels to harvest the moon light and use the combined electricity to melt iron. If this is ok with thermodynamics then so is using lenses. The rest of the argument seems to be that light cannot be optically condensed to a single point as there will always be some dispersion due to diffraction, and the size and shape of th…

I was wrong about what I wrote here and am removing it.

>The analogous problem for you would be that you couldn't maintain your electric heat ray (or whatever) for a time equal to or greater than the time you spent collecting.

Yes, yes I can. 1000 solar panels (200 watts each) will power a an electric arc welder up to 20,000 C continuously for as along as the sun shines in the sky.

There is nothing here that collects energy across time, all energy is immediately imparted onto the electric arc.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#74

Post by School-Cotton »

newnewpdro wrote: post #47

I'm pretty sure if I were to go pick up a number of magnifying glasses and focus them on the same point using moonlight, the temperature at that point would increase with every additional magnifying glass. Am I to accept that the additional magnifying glasses would cease increasing the temperature once the temperature matched that of the moon's surface?

I don't know enough about physics to judge whether Randall's argument is correct, but in general it is possible for a sequence to be strictly increasing yet bounded.

For example, 1/2, 3/4, 7/8, 15/16, ...

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#75

Post by jarfil »

newnewpdro wrote: post #47

I'm pretty sure if I were to go pick up a number of magnifying glasses and focus them on the same point using moonlight, the temperature at that point would increase with every additional magnifying glass. Am I to accept that the additional magnifying glasses would cease increasing the temperature once the temperature matched that of the moon's surface?

You can't focus light from a single source with just magnifying glasses at different positions onto the same point. You can try it with the sun, get two magnifying glasses and try to focus them on the same point at once, it's not possible.

With the addition of some well placed mirrors though, that's possible.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#76

Post by Dylan16807 »

jarfil wrote: post #70
landryraccoon wrote: post #31

Earlier quoted context omitted.

That doesn't work because when the penny is hotter than the sun it radiates an equal amount of energy back to the sun. The sun is a blackbody, it radiates light because it's hot. Once the penny is the same temperature it will radiate back at the sun until the two are in equilibrium.

A penny at a temperature high enough to radiate the same amount of energy as the entire surface of the sun, would be at a far higher temperature than the sun.

If you surround the penny with sun with no gaps, all the light hits the penny.

If you add more sun, you add gaps, and the amount of light hitting the penny stays constant.

It never has more than one penny-surface-area of light hitting it, so it never has to get hotter than the sun.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#77

Post by schoen »

jarfil wrote: post #68
caf wrote: post #62

Earlier quoted context omitted.

What the thermodynamic principle says you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body. The solar panel system is different - it's a heat engine that takes Q units of heat energy from one body, transfers a portion of it Q₁ to a higher temperature body and transfers the remainder of it (Q - Q₁) to a lower temperature body (the immediate surroundings). That's…

> you can't do is to take Q units of heat energy from one body and transfer it all to a higher temperature body Except it's not "heat energy", it's electromagnetic energy that's getting transferred. If the target object is smaller than the source object, you can have an amount of energy that will increase the target's heat above that of the source.

I don't think it could remain above the source temperature in equilibrium, because it will radiate back toward the source!

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#78

Post by schoen »

DenisM wrote: post #73
ALittleLight wrote: post #65

Earlier quoted context omitted.

I was wrong about what I wrote here and am removing it.

>The analogous problem for you would be that you couldn't maintain your electric heat ray (or whatever) for a time equal to or greater than the time you spent collecting. Yes, yes I can. 1000 solar panels (200 watts each) will power a an electric arc welder up to 20,000 C continuously for as along as the sun shines in the sky. There is nothing here that collects energy across time, all energy is immediately imparted…

Yeah, maybe it was a red herring (a black-body herring?) to bring up batteries here.

The solar panel itself is already non-reversible, and so is the arc welder, right? That seems to be the important difference from the lens.

In that sense, we don't run either of them "for free" in the way that the xkcd piece describes.

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