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Name: Bill F.
Status: other
Grade: other
Location: WI
Country: N/A
Date: 7/22/2005


Question:
On occasion, I have been holding a large electrolytic capacitor that has been discharged for a while. If it is then rapidly charged, the capacitor feels cooler. I could theorize that the random motion of electrons is constrained, but this would be going past my expertise.

I have never seen this (apparent) phenomenon explained, or confirmed. Any ideas?


Replies:
Wow, really? how fast does this cold reach the outside? How long does it take to fade away? Please tape a thermocouple to the outside and plot the temperature vs. time. Note the manufacturer, model/series or part-number, microfarad and volt age ratings.

I cannot think of any reason the electric field could constrain electron motion. In metal parts they are free and random regardless of field, and in the dielectric (insulators) they are bound, trapped in place, also regardless. It might very plausibly constrain ions, dangling polar groups like -OH's, water, crystal-lattice vibrations, and the like. I imagine a little of this effect might be found in hi-K ceramic capacitors if you looked.

Thinking about it, I think you may have it backwards. If motion is constrained, entropy is decreased. That usually heats things up, not cools them down. When certain free motions cease to be allowed, the kinetic energy that was in those free motions must be transferred away and added to the kinetic energy already inhabiting all the other free motions in the substance. The average energy per degree-of-freedom increases, and that is a higher temperature. Conversely, when new motions are suddenly allowed (entropy increased), they start at absolute zero, kinetic energy flows into them from elsewhere, and all those other places are cooled down. The new "place" or motion only warms up to equilibrium with them, so it has the same (relatively cool) temperature. There are special cryogenic refrigeration methods based on this kind of thing. They use magnetic fields instead of electric.

I just thought of a simpler explanation that would result in quick cooling of the outside shell. Water is very polar, has a high dielectric constant of >100. So if there is an intense electric field nearby, the water would rather be in it, just like iron filings would rather be in a magnetic field. While at zero charge, water from the electrolytic-capacitor's electrolyte may have wetted the shell, especially if the last room-temperature change was a substantial cooling down from hot. Then when the electric field is applied, some of that water evaporates from the shell to condense in/on/near the porous aluminum oxide dielectric. Perhaps this evaporation makes the cooling you perceived. Even the fact that electric field pulls metal sheets closer together could attract water by causing increased capillary attraction. If this theory is right, perhaps the cold is generated only at the shell, and will quickly be warmed back up from the inside bulk, which did not cool, as well as by the outside environment.

Jim Swenson


I am guessing here myself. I know many electrolytic capacitors use oxidation on one of the two internal plates as the insulation. (really thin, thus adding to capacitance) One might theorize that having spent some time in a discharged state, the oxidation has worn down a little, with the oxygen dissolving in the electrolyte. Then a recharging cooling effect might occur, if the re-oxidation of the plate is an endothermic reaction.

Unfortunately, this theory quickly runs into a chemistry question, and is beyond my own knowledge. Chemists?

Ryan Belscamper



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