lemon voltage

[ericb1]

Any help is greatly appreciated. I am helping my son in 2nd grade with his science fair project, and we are creating batteries from lemons. We eventually were able to get an LED to light (barely) after using 6 lemons and grapefruit wired together.

When I look online, I see several experiements (such as this one: http://www.youtube.com/watch?v=rB11w2c4RnM) getting 0.99 volts from one lemon? My lemon is getting half that, approx. 0.45 volts. I'm using 3 strands of 18 gauge copper wire twisted together for my copper, and 2 zinc coated nails for my negative end. And I've also mushed up the lemon so they're juicy inside.

Is there something else I'm missing here? Or are these people just buying super lemons somewhere?

Any help is greatly appreciated!

[rmarz]

ericb1 - I've always been a bit curious about this experiment. I understood the electro-chemistry behind it, but frankly never experimented with it. When I read your note I decided to go out to my back yard and pick a lemon off my tree and proceeded to the garage to get a piece of copper and a galvanized nail. Nothing like the empirical approach. I used a piece of copper strip about 1 sq inch in area. I took a 20d galvanized nail and plunged it into the lemon about 2 inches. It produced about 0.85 volts. The current produced, however was only about 0.25 mA (milliamps). I added a little more area and got the current up to about 0.5mA but then ran out of room in the lemon (it was pretty small).

I think there are several issues at hand. First, the most sensitive LED's will probably require about 5 mA to show much visible light. The forward voltage required for a red LED to conduct that current is probably in the neighborhood of 1.2 volts minimum. My nail had a surface area of about 1 sq inch, similar to the copper, so I would likely have to multiply the surface area 10 times what I had to product the necessary current, and probably have to use two lemons in series to get enough voltage to reach the Vf threshold voltage of the LED so it could be readily visible.

I think your electrode surface areas are too small and you will likely need at least 1.5 volts generated. Certainly 3 strands of #18 wire won't have much surface area. In my lemon, your copper wire would produce a few microamps of current. The wire gauge does not explain your low voltage readings, however. If your multimeter has some low current ranges, see what current is produced in your setup. Try a copper penny (most pennies are not copper these days, so find an old one) that will give you more area. I think there are several interesting experiments from this basic project that could visit the issue of electrode surface area vs current, the ratio of copper to zinc area in producing current etc., but they are probably lost on a 2nd grader. The ability to light an LED with lemons is probably the 'wow' factor for him. Hope this helps. Have fun.

Rick Marz

[ericb1]

thanks Rick, I really appreciate it. I stopped off at Lowes and got some 1/4" copper tubing, and cut a length of that and got some thick zinc screws as well, and viola! the volts jump to about 0.99 for each lemon so the more surface area (and better zinc on the negative side) seems to make a world of difference.