Teaching Tips

Teaching Tips: The Potato Experiment

Synopsis:
In this module students use a simulation to investigate the components of an electrochemical cell. Students create an electrochemical cell using a potato, wires, and two dissimilar metals to produce a voltage read by a voltmeter.

Big Ideas:
a. A complete path is required to conduct a charge.
b. An acidic solution and dissimilar metals are required to create an electrochemical cell.
c. Potatoes and lemons are acidic.
d. As the surface area of an electrode increases, the voltage produced by the electrochemical cell also increases.

Misconceptions Addressed:
a. Complete circuits are not required to conduct charges.
b. The size (amount) of the acidic solution changes the voltage produced by an electrochemical cell.
c. An electrochemical cell with electrodes made from the same material will create a voltage.
d. The surface area of an electrode does not impact the amount of voltage produced by the cell.

NSES Grades 5 – 8 Content Standards Addressed:
a. Physical Science
i. Properties and changes of properties in matter
ii. Transfer of energy

The Missing Setup

This experiment allows the student to test a number of configurations: 1 potato with a zinc nail and a copper penny, 2 potatoes with pennies and/or nails, a lemon with a nail or nails and penny(s), two lemmons, a potato and a lemon, etc. When students use different configurations, they obtain different readings on the meter. One configuration that students may wonder about, however, is not possible. Students may wonder if it is the metals causing the flow of electricity or the potatoes and lemons. A logical question, and one you may bring up in class, is "What if the apparatus could be setup without any potatoes or lemons?" Like this:

Have the students predict what the screen on the meter will read (there will be no current flow, so the meter will read zero). Try to promote a debate if there is a difference of opinion. Ultimately, this presents an opportunity to reinforce definitions and conceptual understandings. What function does the potato serve (it provides an electrolytic solution that actually dissolves the metal and is a conductor of electrons). Why is that function necessary (without electons being exchanged, there would be no current flow)?

What effect would a bigger potato have on the experiment? (no change in voltage, but lasts longer). How about a bigger lemon? (no change in voltage, but lasts longer).

Note: When discussing electricity, it is important to not to develop misconceptions in the minds of the learners. Be careful with analogies, and when you use them, take a minute to remind the students that analogies can be misleading, while one aspect of the analogy may be correct, another may not apply. Also, be careful with explanations, for example, you should not say that voltage flows through a circuit, it does not. Voltage doesn't go anywhere, it's the electric charge that's moved. Voltage pushes the charge through the circuit.

Potatoes as a unit of time?

The ancient Inca Indians valued the potato not only as a food, but as a measure of time. Units of time were correlated to how long it took a potato to cook. This is interesting, because the Incas lived in a mountainous regions. Altitude may have something to do with how long it takes a potato to cook....hmmm.

Strange tales about potatoes

Europeans didn't trust the South American vegetable, that blamed Leprosy, Rickets, and Tuberculosis on potatoes.
King Frederick II believed potatoes were very healthy and threatened to cut off noses and ears of his subjects who wouldn't grow them.
Marie Antoinette wore potato blossoms in her hair. Some believe that this popularized potatoes in Europe.
Peru has many, many varieties of potato, including blue ones.
In addition to the acid needd in our experiment, potatoes contain many nutrients: potassium, iron, calcium, zinc, phosphorus,Vitamin B and Vitamin C (45% of our daily body requirement in one medium potato).

Battery made of paper?

Recently, Dr. Kee Bang Lee and other scientists in Singapore have developed a paper battery that is powered by urine! "The battery is composed of paper, soaked in copper chloride, sandwiched between layers of magnesium and copper. The whole thing, once laminated in plastic, is just a millimetre thick, and 6cm by 3cm in size. The researchers report that with just 0.2 millilitres of urine the battery will provide around 1.5 volts, with a maximum power output of 1.5 milli-Watts. The performance varies according to the geometry of the battery, and the materials used. (See The Register, August 15, 2005, http://www.theregister.co.uk/2005/08/15/pee-powered_battery/)