MadSci Network: Physics

Re: Why do the volt measurements of my potato battery fluctuate?

Date: Thu Mar 30 19:18:06 2000
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Physics
ID: 953405728.Ph

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,

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