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What would theoretically lead to a greater burn if both materials are kept at a constant temperature when applied to the skin? Water or Brass?

This is a question that would help with the understanding of specific heat and conductivity and which has a greater effect when it comes to transfer of energy between objects.

I wrote this question thinking it would start a discussion on the difference between thermal conductivity in solids vs liquids and to discuss the reasoning on why one would burn worse than the other if the two are kept at the same temperature. This is a discussion question rather than a homework question. Any explanation would be helpful. Thanks

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    $\begingroup$ Stack Exchange is not a discussion forum. $\endgroup$ – AccidentalFourierTransform Jul 11 at 15:31
  • $\begingroup$ Well it got put on hold because it was considered a bad homework question although it met all the criteria for an allowed homework question. I specified what I wanted explained and showed that I did some research into it and it still got put on hold. $\endgroup$ – e1f234 Jul 11 at 15:37
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    $\begingroup$ There is no criteria for allowed homework questions. HW questions are off-topic, no exceptions (even if some users sometimes look the other way). Either you want an answer for your problem (in which case it is HW-type) or you want a discussion (in which case it is off-topic, because this site is not meant for discussions). $\endgroup$ – AccidentalFourierTransform Jul 11 at 15:40
  • $\begingroup$ How can I edit the question for it to be allowed? I didn't even understand why it was considered a homework question in the first place and was looking forward to an answer. $\endgroup$ – e1f234 Jul 11 at 15:42
  • $\begingroup$ @AccidentalFourierTransform wait, if all HW questions are off-topic then why do we have a HW tag? $\endgroup$ – Gyromagnetic Jul 11 at 15:50
  • $\begingroup$ @Gyromagnetic Purpose of homework tag?. $\endgroup$ – AccidentalFourierTransform Jul 11 at 15:56
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    $\begingroup$ @Gyromagnetic Questions in the vein of "do my homework for me" are off-topic. Conceptual questions that arise as part of a homework problem can be on-topic, but use the "homework-and-exercises" tag. $\endgroup$ – Chris Jul 11 at 15:57
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    $\begingroup$ Exactly. I believe this is a conceptual question. I already know that water burns cause more severe burns and was hoping for a conceptual explanation. $\endgroup$ – e1f234 Jul 11 at 16:08
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    $\begingroup$ @AccidentalFourierTransform There's no blanket ban on "homework" questions. The guidelines on homework actually say "Can I ask a homework question here? Yes, but there are a couple of things you need to make sure of first." So saying that "there's no criteria for allowed homework questions" goes against the policy posted to meta. $\endgroup$ – JMac Jul 11 at 16:16
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    $\begingroup$ the original version was poorly presented, indeed presented as a homework-like question, It’s been revised and has collected reopen votes (3 right now). That’s how the system works. Nothing more to say. $\endgroup$ – ZeroTheHero Jul 12 at 20:25
  • $\begingroup$ If you believe some users on this site, any questions that apply physics to the real world are "homework" and must be closed. The best questions are QFT, string theory, CFT, etc. questions that have been carefully drained of any real-world content. That is, to some, the only "real" physics. $\endgroup$ – knzhou Jul 14 at 8:47
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    $\begingroup$ @AccidentalFourierTransform I must say I don’t understand your reasoning. What is off-topic here is defined by the users here. It doesn’t make sense to make such absolute statements about what users believe is off-topic, when you directly admit there isn’t consensus on this. This just seems like a way of characterizing your opinion as the only permissible one without doing the work of arguing why. $\endgroup$ – knzhou Jul 14 at 9:11
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    $\begingroup$ @knzhou Lets not fool ourselves (and the OP). Every post that ask the solution to their hw is, at least in principle, closed. Some users sometimes pretend they didnt see the hw question, or that they saw some conceptual question somewhere in there, and they let it be. But that is the exception. Whether there is an explicit consensus or just a tacit agreement, that is how things work here: hw questions are off-topic, always. Saying othrwise is just misleading the OP, who will rightfully complain when their questions are closed, specially when other similar hw questions, by other users, were not $\endgroup$ – AccidentalFourierTransform Jul 14 at 10:55
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    $\begingroup$ @knzhou It is not whether it actually comes from a school assignment or not; it is about the question itself, and how it is formulated. Some teachers assign very good questions, which look nothing like standard hw, and that is perfectly ok here (although if we knew it was actual hw, we probably should not answer until after the deadline). Conversely, some people naturally come up by themselves with questions that are 100% hw-like, and we do close those. So some actual hw questions are on-topic, and some non-hw questions are not; it's not about the actual origin, but the question itself. $\endgroup$ – AccidentalFourierTransform Jul 14 at 11:04
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    $\begingroup$ @knzhou There is a very clear de facto agreement that hw-like questions should be closed. A minority complains from time to time, but these questions keep getting closed. So I disagree there is a disagreement. These questions always end up closed, unless they go unseen for some reason (posted very late at night, with a single uncommon tag, etc.). Again: saying otherwise is misleading to the OP, and everyone who hasn't spent as much time as you and me here. HW-like questions will end up closed, so we better be upfront about it. $\endgroup$ – AccidentalFourierTransform Jul 14 at 11:14
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This is a question that would help with the understanding of specific heat and conductivity and which has a greater effect when it comes to transfer of energy between objects.

If the question is about that, then it should be clear that this is what it's about. In other words, the question could be something like:

"How do the thermal conductivity and specific heat influence the amount of heat transferred between two objects?"

And the body of the question would contain what you think the influence is, why you think that, and what part of that line of thinking makes you unsure of the answer. You could then give an example -- water causes worse burns than metal at the same temperature, and they have X and Y for the thermal conductivity and specific heat capacity, but you don't understand why it behaves that way.

That would be an entirely answerable conceptual question. This is of course just a rough idea of what the final question could look like, but hopefully it explains the general things that go into making a good, clear conceptual question.


And note, the specifics of the setup -- that you're interested in burning skin, the temperatures of the objects, that it's only held by gravity, etc. are all not required for the conceptual question. It could be about transferring heat between any two, different objects with different properties. That all makes it general and useful to other people down the road, rather than being tied to a specific instance with a specific setup.

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  • $\begingroup$ This would defeat the point of the question. If somebody asks whether copper or water burns skin more, the answer will be worded in terms of specific heats and conductivities. The OP presumably doesn't know that already, or else they wouldn't have asked the question! $\endgroup$ – knzhou Jul 14 at 8:50
  • $\begingroup$ This is like closing a question about why the sky is blue, by saying that it should have been titled "Why is the wavelength dependence of Rayleigh scattering a fourth power?" That is the answer, not the question. $\endgroup$ – knzhou Jul 14 at 8:52
  • $\begingroup$ A good answer extracts the conceptual content of the question, without requiring the question to be already perfectly textbook-worded. Your answer here is a good example. The question is explicitly about how to plot two things in MATLAB. But instead of closing it instantly, you thought about what OP actually needed, generalized the question in your head, and answered that true question. $\endgroup$ – knzhou Jul 14 at 8:54
  • $\begingroup$ @knzhou ...and that's why we require prior research. That's how it's always been; that's how the community wants it to be. Perhaps you enjoy answering naive showerthought questions, and you get some satisfaction when walking OP through, but that is not how it is meant to be. If someone wants to know why the sky is blue, the first step is to google that question, which is likely to land them on some explanation of wavelengths and Rayleigh scattering. So yeah, the question we want is "Why is the wavelength dependence of Rayleigh scattering a fourth power?", not "why is the sky blue?". $\endgroup$ – AccidentalFourierTransform Jul 14 at 10:59
  • $\begingroup$ @AccidentalFourierTransform Would you also say that the question answered by tpg2114 that I linked should be closed for the same reason? If no, what's the difference? If yes, doesn't that show that making such strong statements about "how the community wants it to be" is baseless? $\endgroup$ – knzhou Jul 14 at 11:02
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    $\begingroup$ @knzhou Sorry, but like 80% of the posts here, that one is just boring to me, and I don't really want to read it: I would not vote to close it, but would not upvote it (or any of its answers) either. That's how it is for me with the majority of questions here: if I don't find them interesting enough, I will probably not interact with them at all. (And I hope most people do this too: if the post is not in your general area of interest, perhaps you're not the best to judge its quality). $\endgroup$ – AccidentalFourierTransform Jul 14 at 11:10
  • $\begingroup$ @AccidentalFourierTransform That is a decent principle, but it should be applied consistently. It's already applied to all QFT questions, which is why they are never closed, but it cuts both ways. If you are not interested in mechanics, electromagnetism, or any of the rest of that messy "real world" physics (and I'm pretty confident of this, since I've never seen you ask or answer any question involving a real object that one can pick up and touch), then why are you making such strong statements above about closing such questions? $\endgroup$ – knzhou Jul 14 at 11:13
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    $\begingroup$ @knzhou I haven't done much closing (in the review queue) in the last year, so perhaps things have changed a bit, but what I recall is that most hw-like questions did get closed. So I am merely describing the situation rather than explaining how I would like it to be. "Real world" physics were always required to show some prior research, perhaps even more than other high-level areas. $\endgroup$ – AccidentalFourierTransform Jul 14 at 11:22
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The OP's comment on the question shows clearly that he/she doesn't understand why the question is so poor as to be unanswerable.

If you "maintain the material at constant temperature" (and let's ignore if that is practically possible) the amount of "burn" has nothing to do with either specific heat or conductivity. It depends entirely on the heat transfer process between the water/brass and skin.

The answer is more about biochemistry as physics, and the situation as specified in the question is so poorly controlled that the answer could be anything.

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  • $\begingroup$ This is an interesting answer: you are criticizing the question as reflecting such a bad understanding of thermal physics as to be unanswerable. But the original version of the question didn't ask in terms of specific heat or conductivity at all. The OP was told to put that in by a mod, through the highly upvoted answer above, because the absence of this was reason for closure. Do you have any comment on that? $\endgroup$ – knzhou Jul 15 at 10:19
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    $\begingroup$ @knzhou If you check the timelines, OP edited the original question to ask about specific heat capacity and thermal conductivity a full 3 days before asking this question here on meta. Nobody told them to put anything in -- OP clarified that they were interested in those two things and how it impacts burns, to which I provided guidance on what would make that question on topic. OP then asked it as a new question, which is (as of now) open. $\endgroup$ – tpg2114 Jul 16 at 0:44
  • $\begingroup$ @tpg2114 Ah, you're totally right, my mistake! $\endgroup$ – knzhou Jul 16 at 0:52

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