Lesson Overview: The Potato Battery Controlled Experiment

Goal: In this acivity students work in teams to construct simple potato batteries and design a controlled experiment, which investigates how a variable of their own choosing influences (if at all) the voltage produced by their battery. Student teams then share the results of their experiments in a short presentation format. Individually, students write laboratory reports documenting their understanding of the potato battery.

The following materials are provided:

- Multimeter
- Wires with alligator clips.
- Metal strips (copper, zinc, aluminum, iron, tin, magnesium, silver, etc…)
- Potatoes.
- Additional fruits/vegetables (lemon, apple, etc…)

Objectives:

1) Students will gain experience working in collaborative teams by designing and performing an open ended controlled experiment.

2) Students will be able to define and identify the experimental variable, dependent variable, and at least three control variables.*

- Students will rank variables in terms of the effect on voltage. (no effect, small effect, large effect).

*Advanced students may also investiate how their chosen experimental variable affects current. In essence, performing two controlled experiments concurrently. However, if this is their first experience designing a controlled experiment, it may be advisable to have students focus on one dependent variable (voltage) to avoid confusion regarding the elements of experimental design. The electrical current tends to be vary small and highly variable, even under optimal circumstances, so I recommend focusing on voltage.

Rationale:

Avoid over-teaching!! I’ve made this mistake myself too many times. This activity provides students an excellent opportunity to teach themselves and learn from one another. Teacher’s should wait unit the final closure of the lesson series to provide deeper direct instruction.

I’ve used this lesson to teach experimental design principals in my general chemistry classes, and as an introduction to the more complex Galvanic cells covered by my advanced (honors) course, as well as AP Chemistry. It would also work well in a general science course, introductory physics, or a science elective (such as my Sustainable Energy course). While the potato battery experiment provides an opportunity to reinforce chemistry concepts such as the metal activity series (EMF series), electrolytes, and oxidation-reduction (Red-ox), prior knowledge of basic chemistry is by no means a requirement. I would even argue that this mini-unit is most successful, in terms of student engagement, when presented very early in the learning sequence. The experiential (“hands-on”) context provides a foundation for students, upon which more abstract concepts may scaffolded later on.

At minimum, however, students should have some prior knowledge of voltage (electromotive force), current, and resistance. Relating these variables mathematically, using Ohm’s law (V = I x R), is optional. What follows are some useful qualitative definitions of terms. Depending on where your students are in their learning progression, you may choose to address each of these in greater detail (and more quantitatively), than I do here:

Prior Knowledge:

I provide a handout that explains how to design a simple controlled experiment: How does sunlight affect plant growth? Since most middle and high school students already know the outcome, this simple question serves as a good model, allowing students to focus on understanding terms like experimental variable, dependent variable, and the need for control variables.

Key terms:

__Voltage__ (electrical potential, electromotive force) is the stored potential energy that exists whenever there is a separation between charged particles. Sometimes described as electrical pressure, voltage is the “push” that may set electrons in motion through an external conducting circuit (that is, if we choose to view electrons themselves as particles). The unit of measure is derived from units of energy divided by units of electrical charge (Joules/coulomb). It is analogous (similar in some ways, but not equivalent) to the gravitational potential energy that exists between particles that have mass.

__Current__ is the flow of electrons past a given point in a conducting circuit. The unit of measure is derived from units of electrical charge divided by units of time (Coulombs/second). The term “Ampere” or “Amp” is frequently used in its place (1 Amp = 1 C/sec). It is analogous to the flow of water through a hose (liters/second, for example). The coulomb itself needs defining: I’ve found that the simplest way is to think of the coulomb as *a count*, analogous to the *dozen*, but a much, much larger count. One coulomb is equal to 6.2 x 10^{18} electrons. Therefore, a current of 1 Ampere is equal to a flow rate of 6.2 x 10^{18} electrons/second.

__Resistance__ is just that. It is the resistance to the flow of electrons, analogous to friction. Even good conducting materials, such as the copper metal in a wire, impose some resistance against the flow of electrons in a circuit. Appliances (“loads”) such as light bulbs and computers, impose greater resistance.

Taken together, every electrical circuit (a loop of wires + loads) requires a minimum voltage (push) to overcome the resistance (friction), thus producing a current (electron flow). This is a qualitative statement of Ohm’s Law.

Pre-activity:

Once students have a handle on terminology, I instruct them in how to use a multimeter to measure DC voltage. Next, I show them a schematic diagram of a potato battery (the identity of the metals *is not indicated)*. I then ask students, working in teams, to brainstorm variables that may have an effect upon the voltage produced. (ex: type of fruit/vegetable is always popular, mass of potatoes, condition of potatoes—peeled, juiced, etc.., guage of wires, types of metals—pairs of the same metals, or combinations? Let the students investigate this—depth of insertion, distance apart, type of connection between two batteries—series vs. parallel, and so on…). Teams then choose one variable from the list as their experimental variable, and get to work designing their experiment. It’s important to ensure that different teams choose different variables—if possible—so teams can share their results at the end and learn from one another. Also, its essential that at least one team chooses to investigate how different metal pairs affect voltage. For all other teams, I provide a copper and zinc strip.

This project is a near guarantee of student engagement. Good luck!