Stephen A. Lawrence wrote: > > Stephen A. Lawrence wrote: >> Rick Monteverde wrote: >>> The hot (1800+ degF) and warm (1450+ degF) glass I've worked with always >>> stays clear. Glass from a furnace is extremely clear, you can look at the >>> bottom of the pot and it looks like there's nothing in there. >> In this case it's presumably also not glowing, or at least not much, and >> that would seem to fit with the claim that it absorbs just as it radiates. > > I don't think I explained the reasoning here, and perhaps I should. > > Suppose you took a lump of glass and placed it in an (evacuated) oven. > Suppose further that the walls of the oven are dead black, absorbing > (nearly) all radiation which falls on them, and assume that they radiate > about as you'd expect a blackbody to radiate. > > Suppose further that the oven and the lump of glass are at the same > orange-hot temperature (and let's ignore the fact that the glass has > melted all over the bottom of the oven because that adds unnecessary > complexity to the experiment -- maybe we put the whole thing in > free-fall, or whatever). > > Now the walls of the oven are giving off a cheery orange glow. Assume > the glass is glowing orange, too, and assume further that it's glowing > just as brightly as the walls of the oven. (This is an assumption; we > know glass glows *some* but we haven't confirmed that glass glows as > brightly as something which starts out black.) > > If the glass is still FULLY TRANSPARENT, so that the radiation from the > walls is passing through the glass without being absorbed, then the > glass must also be COOLING OFF, because it's radiating more than it's > absorbing, and the oven walls must be WARMING UP, because they're > receiving (and absorbing) all their own radiation *plus* the glow from > the glass. > > Now, let's run heat pipes to the walls of the oven and to the lump of > hot glass, and lead the other ends of the pipes to a Stirling motor. As > the glass cools, the Stirling motor finds itself with a warm source and > a cool source and so it runs, and heat flows from the oven walls through > the Stirling motor and on to the lump of glass through the heat pipes, > thus keeping the glass hot enough to continue to radiate. > > Now we enclose the whole rig in a perfectly mirrored box so no radiation > gets in or out, and we run the shaft of the Stirling motor out through a > hole in the box (with careful friction-free seals around the shaft). > > Voila, we have a permanent energy source, which consumes nothing and > produces mechanical energy until the motor wears out.
Or ... err ... until the whole thing cools off so much it stops running... You can't use this arrangement to violate conservation of energy; I'm clearly wrong about that. Mechanical energy extracted from the system will show up as a loss of total heat inside the box. Anyhow the second law says you can't do that, either. > > That's what the second law of thermodynamics says you can't do. > Indeed.
