4 ms·
I was wondering about this too. After all, human beings have produced temperatures that were hotter than the sun's surface and almost all of the energy that we
by schoen 8y ago
I was wondering about this too. After all, human beings have produced temperatures that were hotter than the sun's surface and almost all of the energy that we used to do that ultimately came from the sun (except for very small components that came from nuclear reactions on Earth and geothermal energy, which also comes from nuclear reactions inside Earth).
I think that this issue is addressed by footnote 2
> And, more specifically, everything [lenses and mirrors] do is fully reversible—which means you can add them in without increasing the entropy of the system.
Presumably we can't say the same of the electrical devices that we use to collect, store, and transmit sunlight, or to create high temperatures from this stored energy. For example, if you use a "lunar panel" to store energy in a battery and then heat something on an electric stove, many of the components in this process will not be reversible, differently from mirrors and lenses. (Putting a hot object on top of the stove won't cause the lunar panel to emit light back in the direction of the moon!)
So I think footnote 2 is actually very important, because it's not that we can never use any energy source to create something hotter than that source, it's that we can never do so using only reversible processes, including purely passive optics.
- DenisM 8y agoI 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?
- whatshisface 8y agoCovering the lens with dust would let you light a fire. The dust will warm slightly above ambient temperature in the lens' shadow, so you can use it to power a Carnot engine that winds up a spring that is then released to rub a twig against a log.
- AlexCoventry 8y agoThis is genius.
- schoen 8y agoI 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 a fusion reaction that does get hotter than the sun.
- DenisM 8y ago>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 https://hypertextbook.com/facts/2003/EstherDorzin.shtml
- Dylan16807 8y agoOkay, 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.
- Isobit 8y agoWouldn't this layer of dust also stop the lense from working normally? If I understand the other commenters here, it's not just about reversibility, but "useful energy" loss - with lenses you are not paying the needed "efficiency tax" to achieve higher temperatures than the source.