Using fruits and vegetables to produce electricity

A. You may be young to really understand much of this, Thalia, but a fruit or vegetable doesn't exactly produce electricity, it just provides a conductive liquid that completes the circuit.

To take it one step at a time, look at the diagram. As it is shown -- fruit or no fruit -- the bulb will light because there is a battery and a complete circuit. The current flows from the battery, through the blue wire, through the bulb, through the brown wire, through the left electrode, through the red wire, through the right electrode, and through the green wire back to the battery.

Now, for the second step, if the orange juice is a conductive liquid (electrolyte), which it is, the red wire can be removed and the system will still theoretically work because electricity can flow from the left electrode through the juice to the right electrode, so we still have a complete circuit. I emphasize "theoretically" because liquids do not conduct electricity anywhere near as efficiently as metal, and real flashlight bulbs require a significant amount of electricity; while some very small amount of electricity will flow, unless you have very large electrodes and extremely close spacing it will not actually be enough to light a bulb; it might be able to light an LED (light emitting diode) and surely will be able to turn on an LCD (liquid crystal display).

Next, for the third step, you can remove the battery and connect the loose ends of the blue and green wires together. Now you have a complete circuit, but you don't have a battery anymore so the light goes out.

Finally, make the electrodes out of two different metals (say copper and zinc) and the lamp will light again (theoretically). This is because two different metals put into a conductive electrolyte actually comprise a battery. In fact, that's what any battery is: two different metals with a conductive solution between them. So it's not that the orange "produces" electricity, it is that copper plus zinc plus orange juice as the conductive electrolyte make a battery.

In this case, the acidic juice allows a small amount of zinc and copper to dissolve in solution. But the zinc has a stronger propensity to go into solution so it drives the copper back out of solution. The net result is that copper dissolves from the copper electrode and deposits on the zinc electrode. Copper metal thus moves from the copper electrode to the zinc electrode (eventually completely coating it). Copper, like all metals (and all materials) has electrons. When the copper dissolves into solution it leaves some of its electrons behind on the copper electrode and dissolves into solution as a positively charged ion, but those electrons that don't travel through the solution with the copper ions, must travel to the zinc electrode through the wire. This movement of electrons is electrical current.

As I said, some of this will be over your head, but that's as simple as I can make it, and you can absorb what you can from it. Good luck.

In real commercially-sold batteries, the surface area of both metals is very large and placed very close together to maximize the electricity. The actual amount of electricity you will produce is very small and you will probably not be able to find even a penlight bulb that you can light with this little bit of electricity, but light emitting diodes (LEDs) require less electricity, so you may be able to light a very small one. The liquid crystal displays on electronic calculators use even less electricity (almost none) so you will be able to operate that if you can find one and (with a parent or teacher's help) connect your "orange battery" in place of the built-in battery.

There is a great youtube video on lemon batteries at:
youtube.com/watch?v=AY9qcDCFeVI

A. You can get a "scientific toy" called a "potato clock". It comprises a small LED display and a few pieces of wire that are attached to a piece of copper and a piece of zinc. You connect the other ends of these wires to the LED and put the copper and zinc into to potato. Lo and behold, the clock starts to go and will go for a go few weeks until the potato shrivels up. the clock will work with most fruits, such as apples, oranges, peaches, grapefruits etc, but it is not very good with vegetables such as brussel sprouts or cabbage!

A. Remember that every explanation of things is our physical world is only a simplification, an idealization, a helpful way of viewing things.

So it is difficult for people to answer your questions when we don't know what grade you are in, Colton, because the explanation needs to be a simplification that you can understand at this point in your education, but not so simplified that you learn nothing new from it.

The salt in the ocean never settles out, not because the ocean keeps moving, but because the water can dissolve that much salt. Similarly, if you put a small pinch of sugar in your tea, it will dissolve. Even after you stop stirring and let it sit for hours or days, that sugar will never ever settle out. But if you put several spoonfulls in, the 'excess' is not really dissolved, it is just swirling around and it will slowly settle. There is an equilibrium point between the solid sugar settled at the bottom, and the dissolved sugar. As a molecule of sugar comes out of solution, another one dissolves into solution.

It is similar with metals and acids: a small amount of metal dissolves into solution and then there is an equilibrium. But if you put two different metals into solution, what happens is the more active metal (zinc) bumps the more noble metal (copper) out of solution. Eventually, the zinc rod will become completely covered with copper and there is no more zinc exposed and the 'battery' will go dead. But until then, as the positively charged Cu++ ions move through the juice from the copper rod to the zinc rod, electrons are forced to move through a wire from the copper rod to the zinc rod to balance that movement of charge.

A. Jamie: Pick some fruits and vegetables; lemons, limes, oranges, potatoes, and carrots have already been suggested; you could also try grapefruit, pineapple, and cucumber. Do the experiment and tell us your findings before you ask that they be explained.

Missy: you just put one lead of the meter on the one type of metal and the other lead on the other metal, and don't let the two metals touch.

Kasturi: A multimeter, sometimes called a voltmeter or a VOM ( ⇦ this on eBay or Amazon [affil links] ), will easily measure the resistance of the fruit or vegetable in question and the current flowing through it. It will also measure the voltage produced. They are available for $10-$15 from electronics stores or on-line. Good luck with the project.

A. In the USA pennies made after 1982 are copper plated zinc, not solid copper, and they would not be a really dependable electrode because they could lose their plating. If your penny is solid copper (before 1982) it should be better. But a piece of copper wire would be best because copper wire is made of very very pure copper.

A. Patrick. Did you wire it per my drawing above? Did it light when you put the red wire back in place? Remember that a conductive liquid cannot carry nearly as much electricity as a wire, and may not carry enough to light that bulb. But try again with the electrodes as large as practical, and nearly touching.

A. Scott, I am presently assisting my Daughter a 7th grader regarding her lime fruits project. We were able to utilize the carbon and the zinc rod from a discarded size "D" battery and insert it in the lime far apart. Our reading was 1.1 volt using a small digital multimeter, the resistance reading was 2000 ohms taken from analog voltmeter. Finally, we connected two limes in series and we got a reading of 2.2 volts meaning that you can increase the voltage indefinitely and the possibility that it can produce enough current to activate a light emitting diode. P.S. you can purchase LED diode from Radio Shack or from unused electronic toys.

I like your approach of using the carbon and zinc from a battery, Raymond. It helps the kids understand batteries and electricity without having to introduce the new concept of using copper instead of carbon while they're still trying to learn the first concept.

A. Paul, what you are no doubt supposed to do is find out the answer by doing an experiment. Watch the youtube video on lemon batteries and do the same stuff with the other three fruits. But you must use words very carefully in science: are you sure that you mean "energy"? Good luck!

A. Kimberly. current is flowing, but not enough. Get a bulb from a 1-1/2 volt penlite. Although you weren't able to light the 3v bulb, you may be able to light the smaller one. If not, try more batteries, or a bigger surface area for your electrodes (by crunching up a bunch of wire instead of exposing just a quarter inch of it. Good luck.

Zinc Anodes for Boats

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(affil links)

A. 1. You can get solid "zinc anodes" from a boating store, Jim. You can also get zinc from a carbon-zinc dry cell battery with some help from the science teacher. The easiest thing to find might be galvanized roofing nails; they are not solid zinc, they are only zinc coated, but for this experiment they will be good enough.

2. Any fruit or vegetable that is too dry or is not acidic would probably be a poor choice. So would a fruit or vegetable like brussel sprouts, cabbage, or broccoli that isn't solid. Lemon, limes, and grapefruit are probably the best.

3. The currents are very small, so a tiny LED would be good. Even better, a liquid crystal display. Any copper wire would be fine.