[Physics] Why do fruits left to dry in the sun feel so much warmer to the touch than other objects outside

everyday-lifethermodynamics

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?

Best Answer

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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