We noticed that in our experiment that painting a metal increases its thermal conductivity; is this true? If so, can you guys send me a link to a research paper to support this claim.
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1$\begingroup$ What was the paint made of? Can you find more information about it? $\endgroup$– GuySoftCommented Jun 28, 2015 at 21:53
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$\begingroup$ Welcome to Physics Stack Exchange. Please note that proper punctuation is important as it helps the people who want to answer your questions understand what you're asking. I made two minor edits to fix up the punctuation. $\endgroup$– DanielSankCommented Jun 28, 2015 at 23:20
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$\begingroup$ Tell us more about your experiment. How did you arrange the samples, and how did you measure the thermal conductivity? $\endgroup$– The PhotonCommented Jun 29, 2015 at 2:41
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5$\begingroup$ Most likely you simply increased the radiation losses. Blank metal surfaces radiate very little, most paints, on the other hand, are probably pretty good black body radiators in the IR. $\endgroup$– CuriousOneCommented Jun 29, 2015 at 3:55
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$\begingroup$ I agree with The Photon. Experimental details would make it MUCH easier to properly answer the question. $\endgroup$– David WhiteCommented Aug 10, 2016 at 17:15
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4 Answers
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painting a metal increases its thermal conductivity; is this true?
No it is not true.
Thermal conductivity is a bulk property of the material. It expresses how well the metal conducts thermal energy through the bulk of an object made of that metal.
As CuriousOne commented, surface treatments do not affect how heat is conducted in the interior of the object - they can affect emissivity.
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In regard to the color of the paint or just the paint itself? Because if you are referring to the color applied to a metal then you don't really need a paper to prove that anymore. Its a old idea. Black or darker colors have a higher thermal conductivity because more colors are being absorbed. Lighter colors (like white) have more colors that they are emitting so they have less thermal conductivity.
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$\begingroup$ Welcome to Physics Stack Exchange! This doesn't answer the question. Comparing one paint to another doesn't explain why paint changes the conductivity of the metal in the first place. $\endgroup$ Commented Jun 28, 2015 at 23:21
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$\begingroup$ Yes it does, indirectly. If the emissivity of the bare metal is less than that of metal plus paint $\endgroup$– user56903Commented Jun 29, 2015 at 8:16
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1$\begingroup$ But emissivity is not the same as thermal conductivity, which is a bulk property of a material. As pointed out, shiny aluminum or steel has low emissivity/absorptivity, so radiation losses are small. However, as you can easily test by putting your hand on a hot plate, the thermal conductivity is quite high. $\endgroup$ Commented Jun 29, 2015 at 14:01
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$\begingroup$ Don't take the OPs word that what he's seeing is a change in thermal conductivity. Until he describes exactly what he's doing, there's no way to know what he's seeing. CuriousOne is very likely right. $\endgroup$ Commented Sep 22, 2015 at 2:08
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$\begingroup$ >However, as you can easily test by putting your hand on a hot plate :D Good analogy @CarlWitthoft $\endgroup$– dsignrCommented Nov 22, 2016 at 7:23
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It is trivially true because you are adding more material. So you have the original conductivity of the metal plus the conductivity of the paint. Additionally, the paint will reduce heat transfer away from the metal by convection etc.
Of course, this only applies if you are measuring conductivity from one part of the metal to another, and not measuring between metal and the other side of the paint.
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In my profession we look at heart conductivity of a system in terms of thermal resistance. You can look at thermal resistance in terms of Ohm's law; hence adding paint in terms of this transformational analysis results in the thermal resistance of the metal in series with the thermal resistance of the paint. If you only consider the thermal differential between the unpainted side of the metal and the other side where the paint is exposed, the total thermal between those 2 surfaces is the summation of the two thermal resistance. Higher thermal resistance will result in lower thermal conductivity from one side to the other.
