fruits as conductors

[jleuckie]

My child is doing a science fair project on fruits as conductors. Her hypothesis is that a pear will be the better conductor because of its density. She came to that conclusion after some research on why a potato always seems to be the top produce for experiments on electricity. She found that the density of the potato makes it a good conductor. Since the inside of the potato is similar to the pear, she deduced that it would be the best fruit for conducting an electrical current.
We talked about how a conductor is an object that allows current to flow easily. However, we are running into a lot of things that say lemons conduct the best electricity. She used a multimeter to measure resistance in ohms, to find which fruit had the lowest resistance, which would mean electrical current would flow easiest. She found the mango to have the lowest resistance, making it a better conductor.

Our issue: Do scientists use conduct and generate to mean the same thing? Because to conduct is to lead where to generate is to make or create. She is not trying to generate electricity, only find out which fruit allows electrical current to flow with the least resistance. Maybe we just need help with wording...
Wishing I was science savvy at this time....

[rmarz]

jleuckie - The two terms you referred to - conduction and generation - are clearly very different. So far, your experiments have been pretty logical. Perhaps some of the confusion is in things you might have read about fruit and vegetable 'batteries'. Many science projects use fruits and vegetables, in combination with specific electrode elements to create an electro-chemical reaction in which the lemon juice or potato juice play an important part in reacting with these electrodes, thus 'generating' electricity. The electrodes are chosen to have very different electro-potentials to maximize generated voltage and current. Usually, common choices are copper and zinc, which are readily available.

Any of these 'juicy' fruits and vegetables also will exhibit properties of conductance (or have measurable resistance) which allows them to conduct electrical current when an external voltage, or electromotive force is introduced. This is a factor of the free electrons in the juice of the fruit or vegetable (F/V for short) you are using. Using a multimeter is a simple way to determine which fruits or vegetables would be better conductors. A few tips. Make sure the electrodes you place into the F/V are of the same material. If you use different metals, you will be introducing an electro-chemical 'battery' process which will create errors in your measurement. Once you place your electrodes in the F/V, try to be consistent with regards depth of the electrode into the F/V, and keep the spacing between electrodes the same. When you have the electrodes placed, make resistance measurements on the 200Ω, 2000Ω and 20KΩ ranges (or whatever ranges are on your meter) to see if you get consistent results. Pick one range to make all your comparisons. These procedures will minimize variables of your setup.

I have helped other experimenters in the past, but have been pretty much involved with only lemons and potatoes. Interesting the observation you made about the mango. Perhaps the F/V drawer in your refrigerator may yield other interesting results.

Rick Marz

[jleuckie]

She used copper for both sides of her fruit since she was looking for conductivity only, not generation of electricity. All measurements were taken from the same range, 2m, which my husband taught her about how the ranges are showed on the multimeter and interpreted. The mango average (we took 3 different readings) taken at the 2m range on the multimeter, which he showed her how that would convert into 6,667 ohms.

We purchased the fruits on the same day, let them sit on the counter until the next day so they would all be at room temperature so there wouldn't be temp. variations.
Pear - 17,000
Apple- 20,667
Lemon-30,667
Lime-39,000
Tomato-44,667
Orange-53,333
Kiwi- 61,000
Grapefruit-177,000
I was surprised to see where lemon fit in on these readings, but I continue to try to help her understand the this is only showing that the pear is the better conductor, not necessarily a better generator of electricity. After she completed the experiments she cut open all the fruits to examine the insides. She noted the "walls" in each section of the fruits, the amount of pith, or skin as she called it, and the "juice pockets" that all are self contained in thin "walls". She deduced that all of those things were obstacles within the fruits that limited the flow of current. It was harder for the current to travel through all those extra things. In the pear, mango, and apple she noted that the insides appear with simple observation to be of one solid form. You can't see differences in the meat. It all appears the same. She thought pears were more firm, tougher to eat than apple, so those would be more dense. She'd never tried a mango before, so she didn't have that background knowledge as a comparison.

[rmarz]

jleuckie - Your experiments seem to have been done with very good lab practice. I was off in my 'guestimate' as to what the actual resistance measurement you would see when I recommended the lower ranges. It did pique my interest, so I tried a lemon and lime from my refrigerator and indeed got measurements in the 25-40KΩ ranges, using the 200KΩ setting on the multimeter. I used copper electrodes inserted about 2 cm into the fruit, and spaced about 2 cm apart. I was correct in assuming there was a difference in reading between ranges. I got a very different reading using the 200KΩ versus 2000KΩ range, suggesting some error in measuring F/V items versus a fixed component resistor (say 39KΩ) that would give the same measurement in either range. I also noticed that in the 200KΩ range, using copper electrodes, the resistance would increase over a short time suggesting some other action was taking place, likely with the electrodes, electrolysis or some other phenomena. When switched to the 2000KΩ range, the initial resistance was higher, but started to drop over time. I think we have stumbled on to at least two more science projects once we define a hypothesis for what is going on. This is all somewhat beyond a K-5 student. You seem to have the makings of a good experiment, good luck with your analysis.

Rick Marz