Hands-on Science Carnival 2010 Activity Stations: Electricity & Magnetism

Electrochemistry: Fruit Batteries

[Shopping List: galvanized or zinc-coated nails; heavy gauge copper wire cut into short pieces (the same length as the nails; 35mm film canisters; various fruits such as oranges, apples, lemons, limes, strawberries, tomatoes, peaches, bananas, kiwis, potatoes, pears; vinegar; flexible hookup wires (6"-12" lengths) with alligator clips; small LED light bulbs]

1. Choose your fruit, you might want to squeeze it a bit to make sure it's nice and juicy
2. Insert one galvanized or zinc-coated nail and one piece of thick copper wire into the fruit, about 1" apart
3. Connect the alligator clip of one test wire to the zinc nail, and another test wire to the copper wire
4. Connect the other end of the copper test wire to the red multimeter test probe (which should be connected to the "Volts" terminal), then connect the other end of the zinc test wire to the black multimeter test probe (which should be connected to the "Common" terminal).
5. Read the voltage on the multimeter (should be ~ +0.8 to 1.0 Volts). Reverse the connections to the multimeter and see what happens.
6. Disconnect the multimeter, then connect the leads to a small LED light bulb (the zinc wire should be connected to the flat side of the LED); it probably won't light up yet.
7. Connect any two fruits together in series by using 3 alligator test clip wires and connecting the copper electrode in one fruit to the zinc electrode in the other fruit, then connect the remaining zinc electrode to the wire on the flat side of the LED bulb and the remaining copper electrode to the other wire on the LED bulb. Now the LED should light up (if not check that the LED is not wired backwards). If the bulb does light up, reverse the LED leads and it should not light up.
8. Measure the voltage of the two fruits in series.
9. Try connecting two fruits in series to the small LCD clock.
10. Try connecting two fruits in series to the sound chip; it should start "chirping", but not very loudly. To amplify the sound attach the small piezoelectric speaker to the bottom of a paper cup.

What's Happening: Whenever two different metals are connected together by a wire, electrons will flow from one metal to the other, but this flow stops very quickly. By placing the metal electrodes into a fruit, particularly an acidic fruit like citrus, chemical reactions take place that replenish the electrons available and they will continue to flow as long as the circuit is connected to some load, such as the LED, clock or sound chip. One metal gives up electrons more easily so it is labeled the negative electrode (also called the cathode), and in our case that is the zinc coated nail. The copper attracts electrons, so we label it the positive electrode (or anode). The LED is a special semiconductor device which will produce light when a high enough voltage is applied to its terminals, and in the correct direction. One fruit, or more accurately one zinc-copper electrode pair cannot produce enough voltage to light up most LED's, but when two fruits are linked in a series electrical circuit, their individual voltages add together, which should be enough to light the LED, as well as the clock and sound chip. By the way, even though 2 or 3 fruit batteries might produce enough voltage to light the incandescent bulb in a small flashlight, they can't produce enough electrical current (or loosely speaking enough electrons) to power this kind of light bulb (it might if the copper and zinc electrodes were much, much larger, and the fruits large enough to cover these large surfaces, but that would be even less practical than the simple battery made here). The LED is very special in that it can operate with only a very small current.

The reason that the sound chip is so quiet by itself is that it can only move a very small amount of air, and less air movement means smaller sound waves. When we attach it to the bottom of a paper cup, the sound chip can vibrate the cup bottom like a drumhead, which moves more air and creates louder sound waves in the air. The tapered shape of the cup's side walls also helps amplify the sound waves even a little more (think of the large horn attached to old phonograph players).

Variations: Try different fruits; it really shouldn't change the voltage since it is a function of the metal pair, but some fruits are a little better at producing the chemical reactions on the metal surfaces. You can also make a battery using a 35mm film canister by poking the zinc nail and copper wire through the lid, filling it with vinegar or fruit juice, putting the cap on and connecting the circuit. Try different liquids: water; milk; salt water; sugar water, etc. If you have other metals (aluminum, iron, nickel, etc.), try those in different combinations to see what voltages you get, and which metal is positive vs. negative. You can also make a battery using an alternating stack of pennies (copper) and nickels (nickel), separated by pieces of paper towel soaked in salt water (be sure the metals do not touch each other and short circuit).