Kids, this activity uses a common potato and two different metals to make enough electricity to run a small digital clock. Try this activity then attempt to expand on it to make a science fair project. You'll need a large raw potato, 2 pennies, 2 large galvanized nails, 3 pieces of 6" long wire, and a small digital clock (such as a Radio Shack Stick-on Timer for $4.99). The digital clock can be taken from an inexpensive alarm clock or it can be purchased from an electronics store.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
First cut the potato in half and place the halves next to each other flat face down on a plate. Strip off about 2 inches of insulation from both ends of each wire. Wrap one end of one wire around one of the nails. Press the nail into one of the potato halves. Wrap one end of another wire around one of the pennies. (Do this by laying the penny across the exposed wire, positioned so that it is centered on the wire and almost touching where the insulation begins. Fold the end of the exposed wire over the top of the penny. Pinch the penny and wire between your index finger and thumb on one hand and pinch the overlapping wire with the other hand. Twist the penny until the wire tightens around the penny). Press the edge of the penny about half way into the other half of the potato. Attach one end of the third wire to the second nail and the other end to the second penny as before. Insert this nail into the potato that already has the penny stuck into it then stick the penny into the potato that already has the nail stuck into it. Pop the back off the timer/clock and remove the button battery. Connect the two wires coming from the potato battery to the contact on the battery holder. If the clock does not light then the polarity (+ / -) might be incorrect. Just touch the wires to the opposite contacts on the timer's battery holder in that case.
How does it work? Here's the chemistry - the potato contains phosphoric acid. This acid causes chemical reactions to occur at each of the electrodes (galvanized nail and copper penny). The reaction at the copper electrode strips electrons from the copper metal. The galvanized nail contains the zinc needed for the other reaction. The phosphoric acid dissolves the zinc in the nail, which also strips electrons from the zinc. The resulting zinc ions (Zn++) migrate into the acidic juices of the potato. This results in an excess of electrons on the zinc electrode. When a wire is connected from the zinc nail to the copper penny, electrons will flow. This flow of electrons is the electrical current that makes the digital clock function. After this, try hooking the potato battery to an oscilloscope to measure a voltage (about 0.5V); several potato batteries could be connected in series to increase the voltage.
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
September 2002
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Reference: www.quantumscientific.com/pclock.html
