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I have been putting preserved plums, on a rack, to sun and dry on my balcony. When I take them in at dusk, the plums are noticeably hot to the touch. They feel warmer than the bamboo and metal racks they are on, the cardboard box I put the racks on, the netting I put over the lot, and the air outside. (Note that ambient air temperature doesn't start dropping until well after I have the plums indoors.) The balcony itself, made out of a light-colored concrete-like composite, and the metal railing also feel warm, but not as much as the plums do.

I recall some relevant concepts from physics classes, but I can't tell if I'm taking into account everything at play. Here's what I have so far:

  1. Plums are mostly water, which has a high specific heat (~4 kJ/kg/K) relative to air (~1 kJ/kg/K) and probably the other objects. I'm guessing the balcony also has a higher specific heat than air. Higher specific heat means that by the end of the day, the plums have stored more thermal energy than the cardboard box.

  2. Water and metal are good thermal conductors, so they will feel warmer to my hands than the other objects even if they contain the same energy per unit.

Is there something else in here about the plums converting radiant energy to thermal that the other objects don't, or something about air flow? Is it a sign (which I suppose is not for Physics.SE) of fermentation?

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    $\begingroup$ A pseudoscientist might say that fruit evolved with the purpose of taking in energy from the sun, therefore it is no surprise that the plum is hotter ;P $\endgroup$
    – user45266
    Aug 25 at 8:41
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    $\begingroup$ You clearly have not worked with hand tools out in the sun, and picked up that shiny wrench in your bare hands, only to drop it rapidly since it is really really hot. I won't even mention nice black-anodized tools... $\endgroup$
    – Jon Custer
    Aug 25 at 14:48
  • $\begingroup$ Thermal conductance is why the 70-degree water in your swimming pool feels cooler than the 70-degree air around the pool, and why 120-degree water scalds your skin while 120-degree air is just (extremely) unpleasant: the water conducts heat (in either direction) more quickly than air. $\endgroup$
    – chepner
    Aug 25 at 16:32
  • $\begingroup$ @chepner 120° water? Are trying to go into a pressure cooker? $\endgroup$
    – Kami Kaze
    Aug 26 at 7:24
  • $\begingroup$ @KamiKaze chepner meant degrees Fahrenheit. $\endgroup$
    – nanoman
    Nov 7 at 14:12
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You were on track...and then missed the mark.

Higher specific heat means that by the end of the day, the plums have stored more thermal energy than the cardboard box."

Correct. You're on track...

Water and metal are good thermal conductors, so they will feel warmer to my hands than the other objects even if they contain the same energy per unit.

But incorrect. You just veered and missed the mark.

You don't feel thermal energy stored in your finger tips; You don't even feel the temperature of the material. You feel the temperature of your fingertips.

This in turn is influenced by the specific heat capacity, thermal conductivity, and actual thermal energy stored in the material.

Of key importance in your scenario is that you are feeling the temperature after the heat source has been removed and things have been given time to cool down. Specific heat capacity does affect how quickly that happens since more energy must be drawn from the material for the same decrease in temperature.

The role thermal conductivity plays is that it determines how quickly your finger tips match the temperature of the material. What this means is that a piece of aluminum (good thermal conductivity) will initially feel hotter than a piece of plastic (bad thermal conductivity) at the same temperature upon initially touching it because it is bringing the temperature of your fingers to match its own faster. But hold your fingers on either long enough, and it will feel the same because your finger has reached the same temperature in either case. This is all assuming the act of touching it does not change the temperature of the object itself since it is transferring heat to or from your fingers after all (see next paragraph).

Specific heat capacity also determines how much an object's temperature changes due to you touching it as heat is transferred to or from your fingers. If an object is small enough it transfers so much of its own thermal energy to your fingers that it drop significantly in temperature while not raising your own hand temperature that much and doesn't burn you. This is the exact same mechanism that enables an object with higher specific heat capacity (for the the same mass) to take longer to cool at dusk since both metal and plum are exposed to the same cooling conditions.

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  • $\begingroup$ On my second point, rather than "good thermal conductor -> feels warmer" it's "good thermal conductor -> transfers heat faster to my skin"? On your last lines, are those a separate third point on how fast all the objects cool down? $\endgroup$ Aug 24 at 20:38
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    $\begingroup$ @Tranquilled Assuming the temperature of the object does not change when you touch it, a good thermal conductor will feel initially colder or hotter when you first touch it since it brings the temperature of your fingers to match its own faster. If you hold your finger on two different objects of the same temperature long enough it won't matter and they will feel the same as your finger reaches the same temperature in the end. $\endgroup$
    – DKNguyen
    Aug 24 at 20:40
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    $\begingroup$ "But hold your fingers on either long enough, and it will feel the same". Isn't this only true if you exclude the rest of the universe? From experience I know I won't get hurt if I hold a hot wooden spoon indefinetely. Likewise, I can hold a hot metal tray indefinitely if I'm wearing oven gloves. I reckon this is because heat is transferred in such a small amount that the rate at which my body and the hot object transfer energy to the universe is enough to account for the transfer between the object and I. $\endgroup$
    – LoremIpsum
    Aug 25 at 10:56
  • $\begingroup$ @LoremIpsum That's why I said "assuming..." in the very next sentence. But in this case when I say feel the same, I don't mean "feel hot". I mean feel the same as the other object. That could include having both objects to drop to ambient if the thermal conductivity is too low and it takes too long to bring your finger to temperature, but also includes if both objects are maintained at a constant temperature. $\endgroup$
    – DKNguyen
    Aug 25 at 13:47
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    $\begingroup$ Oh I think you were referring to something like in a ambient medium with higher thermal conductivity than the object. Yes $\endgroup$
    – DKNguyen
    Aug 25 at 13:57
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Here is another effect which has to be factored in:

An object sitting in the sun will be heated, but as it warms up it will begin to re-radiate the energy it has absorbed. While the energy absorption occurs over a range of light frequencies, the re-radiation occurs in the infrared range of frequencies.

If the material being warmed by sun exposure exhibits a difference in its emissivity at those different frequency ranges, then it is possible for the object to easily heat up and not so easily re-radiate the incident energy. That object will then get significantly hotter sitting in the sun than another object that has an easier time re-radiating the absorbed heat.

BTW materials which are good at absorbing energy and bad at re-radiating it make the best solar heat collector surfaces. One such example is a metal plating called black chrome. If you leave a piece of this in the sun it can get hot enough to burn your hand if you try to pick it up.

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  1. I suppose that your plums are darker than the bamboo or metal rack, so they will get warmer while out in the sun.

  2. Due to their water content plums will keep warm much longer after you bring them inside -- as you correctly guessed.

Thermal conductivity works against these, and I suppose the metal rack will feel the hottest just when you bring them in -- but due to its low heat capacity and high conductivity it will lose its advantage pretty fast.

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  • $\begingroup$ Very good point about the metal rack. $\endgroup$
    – Bill N
    Aug 25 at 12:48
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Because your plums are not in equilibrium, the previous explanations based on purely 'optical' grounds are totally correct IMO. But probably not the whole story. Trying to address this very relevant aspect/sub-question,

Is there something else in here about the plums converting radiant energy to thermal that the other objects don't, or something about air flow? Is it a sign (which I suppose is not for Physics.SE) of fermentation?

(My emphasis.)

The answer is yes, there is. Another phenomenon to be factored in --although this is chemistry, and chemistry boils down to physics. And you guessed it right. It's fermentation. Fruits in the situation you describe are undergoing alcoholic fermentation, which is exothermic:

https://chemistry.stackexchange.com/questions/40823/why-does-a-mixture-get-warm-in-an-exothermic-reaction So the fruit would heat up, however mildly.

In fact, organic material is known to be capable of causing spontaneous fires when left to decompose --the sun providing some activation energy. It's known to happen to hay and wood chips, when they're dry. I don't think plums would spontaneously ignite though, as they have high water content and fermentation is so much slower and less exothermic than oxidative phosphorilation, which is what goes on in the mitochondria.

Suggestion: Google for "can fermentation cause a fire?" You bet it can.

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  • $\begingroup$ The other Answers seem to treat all objects as somehow the same, which they're clearly not This boils down to Joigus's suggestion, Google for "can fermentation cause a fire?" You bet it can. Sticks and stones and much of the rest of the stuff in your garden neither ferment, nor otherwise react to sunlight as fruits do. $\endgroup$ Aug 25 at 20:24

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