|MadSci Network: Physics|
Interesting question, Alex! It sounds to me as if you are "draining your battery"! Your potato and two different metallic electrodes are interacting electrochemically. Ions in the potato (probably salt ions in water) are differentially attracted to your electrodes. One electrode becomes more positive, the other more negative, while the ions are actually moving in the direction of the electrode they electrically "prefer". If enough ions move into a particular region of your potato, say, the area immediately surrounding one of your electrodes, that region can become "saturated". Ions are attracted to the electrode because it is oppositely charged. Trouble is, there's a "crowd" of other ions pressing around the electrode already, and those guys are NOT attractive to the incoming ions. They're in the way! This is pretty much what happens in your car battery if you let it run down. (Reaction products build up, preventing electrons from moving as easily as they would in a "fresh" battery.) This is also what happens in some of the electroplating tanks my company uses to recover silver from used film developing chemicals. (Note: there's a difference between the two cases. In the "battery" case you may be using up nearly all of the ions. In the "plating" case, you are only using up all of the LOCALLY AVAILABLE ions. There may be more ions in the pipe, but the "plate" can't attract them because of the "crowd" surrounding it. We get around this by stirring the solution with the "plate". Your potato battery is probably more like this second case.) To reuse your potato battery you can probably just poke your electrodes into different spots on the potato, taking care not to let the previously saturated / depleted regions get between your new electrodes. There should be enough fresh areas in your potato to permit several more "runs". I don't know if a potato is "rechargeable". Maybe you could find out by discharging your potato battery then running a slightly larger current -opposite the normal polarity of your spud - without moving your electrodes? (Depends on the exact chemistry involved. Might work, but it probably won't be efficient.) At any rate, I'm guessing that's what you're seeing. Different electrode materials, different electrode positions, and different 'taters might help you verify this. Please let me know your results! Your MadSci, -Matt firstname.lastname@example.org
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