Red hot iron or red hot lead?
Please explain.
Lead melts at 621 degrees F
Iron melts at 2795 F
Therefore, lead would melt long before iron even gets to the red-hot stage.
I can’t find any indication that the temperature of a red-hot metal (500-800[sup]o[/sup] C) depends on what kind of metal it is.
That said, at those temperatures, lead will be liquid while iron will still be solid, so if one were to touch the metals, more of the lead would stick to you and inflict even more burning damage.
Emissivity demonstration.
Wiki on Emissivity
Emissivity Coefficients (from CRC)
Lead unoxidized 0.05
Lead oxidized 0.63
Iron unoxidized 0.05
Iron oxidized 0.74
So at a given temperature, unoxidized iron or lead will be equally bright, but oxidized iron will be brighter than oxidized lead.
Correct me if Im wrong (I did physics but my focus was more on the EE side of things), but I’d think that Squink’s emmissivity is dependent upon the relative ambient temperature, it seems to be related to a delta-T instead of an absolute-T.
If the particular substance was at perfect equilibrium with the background (i.e. both at the same temperature) then it would have an effective emmissivity of 1, at which point it would behave as a black body spectrum. Think of the “Predator Effect” when Ahnold slathered himself with mud to become invible to the Predator’s heat vision.
So, equally “red” red hot lead and red hot iron would be at the same temperature assuming the ambient environment was also at the same temperature.
Does this have anything to do with a pound of feathers and a pound of lead - which is heavier?
Trick Q right?
Well, that’s the thread it’s from, but I was beginning to have my doubts…started wondering if it was instead a “stupid” question.
We’re all forgetting that heat and temperature are not synonymous. Squink’s figures refer to temperature while the OP asked which is hottter. The lead will be hotter despite being the same temperature.
Heat is the amount of kinetic energy contained within an area, temperature is just the average velcocity of the particles in an area. They are the same thing if the two areas contain identicle particles, but if there are different particles or numbers of particles they aren’t the same at all.
Lead is denser than iron. As a result the atoms can be travelling at the same average speed, and thus have the same temperature, but because they have much more mass/volume they also have more energy to impart, and are thus hotter.
This is the reason why a shower gets hotter when the pressure increases, despite the water being at precisely the same temperature. Higher flow rate = more particles/unit area = hotter despite temperature remaining stable. Similarly a cubic metre of air at 80oC will be just mildly unpleasant at worst, like air from a hair dryer. Immerse the same body part in a cubic metre of water at that temperature and you will scald yourself. Same temperature, totally different heat.
This all gets complicated because of relative thermal densities/ heat capacities and so forth, and it’s been so long since I studied this that I probably couldn’t remember how to calculate it even if I did have the figures. However I would guess that the bonding in lead is so similarly to that in iron that the thermal density is correlated pretty well to density. The only major complicating factor is that lead melts before it goes red, and the resulting free energy change probably means it’s even hotter at these temperatures than density alone predicts.
But this is the straight dope. I’m sure someone will be along to do the enthalpy calculations eventually.
OK, Blake, you got me. 
The question was asked in the spirit of us poets, who freely interchange the concept of heat & temperature (If I stuck a thermometer onto a red hot ball of lead and a red hot ball of iron, what would the reading be…and don’t explain that the thermometer would melt because it was made from glass!). Specific heat, phase change and overall energy absorption aren’t in the spirit of the OP, but that’s my fault for not specifying that both substances were experiencing the same atmospheric pressure and that the pressure would be high enough to keep the lead from slipping into its liquid state, neither sample was oxidized, etc.
But this is the straight dope. And I should know better. I am ashamed. 
Iron? Lead? Are you kidding? A better question would be Platinum or… that is it, I guess. Not to many metal themed babes.
Even if both substances were experiencing the same atmospheric pressure and that the pressure would be high enough to keep the lead from slipping into its liquid state, neither sample was oxidized, etc. the lead would still be hotter at the same temperaure because it’s denser.
I’ll let you get back to your intended topic now. 
AHA! That’s the wording I was looking for! Thanks dude!
And I realize this has come dangerously close to violating the “Don’t post questions you already know the answers to” rule, but I was beginning to doubt whether I really knew the answer to the question I meant to ask but so miserably failed to.
So, umm, where is this feathers thread?
Unless you were going for a “heat and temperature are different” thing, note that there’s nothing that precludes molten metal from being red-hot (or brighter).
Thanks for opening this thread, Inigo, I was puzzled by your comment in the other thread.
So I think I’ve got the lead/iron thing figured out. But why does the sun apparently glow yellow, but the hottest part of the flame of a match is blue? Obviously, the sun is much hotter, but it isn’t blue or white. If red hot iron and lead are the same temperature/have the same heat/whatever, do gasses give off different colors at different temperatures/heats?
The sun is white. The reason why it appears yellow is fairly complex, but the simplistic answer is that the shorter wavelengths get filtered out by the atmosphere on Earth, leading to a yellowish looking sunlight. Even more simplistic is to say that the blue light gets filtered out to make the blue sky, leaving more of the longer red and yellow wavelengths. But of course if the sun were really yellow then we would be unable to percieve blue objects in sunlight. This is not the case, we can see all colours in sunlight, thus proving that the sun is indeed glowing white hot.
Yep, they sure do, that’s the basis of ‘neon’ lights, which emit different coloured light when the gases they contain get excited. The original neon lights literally contained neon, hence the name, but other lights with different gases can produce any colour desiered.
I can’t actually answer your question, Ravenman, but there are a few things that I can see are confusing the question:
- There are two completely different processes going on here. The matchhead is undergoing combustion, and the sun is undergoing nuclear fusion.
- Also, you’re right next to the matchhead, whereas you’re looking at the sun through a couple miles of atmosphere, which screws things up quite a bit vision-wise, and through about ninety-three million miles of vacuum. Plus, your eyes ain’t exactly equipped to deal with the sun’s color.
I read the question as the two metals still being in a solid state, glowing red before they melted – an unfortunate assumption on my part.
No, it is not. The glow of neon lighting, regardless of the particular gas or gas mixture used in a given tube is caused not by temperature, but by electrical current. Simplistically speaking, the current flow gives the gas atoms or molecules some extra energy, which they then give up in the form of a photon. Although this may incidentally heat the gas, the heating plays no significant role on the operation of the lights under normal conditions.