REED COLLEGE SCIENCE OUTREACH PROPERTIES OF MATTER

16
Building Fruit Batteries (30 minutes)
7. Tell students that today they will be working together in groups of 4 to build
batteries using fruit (either lemons, apples, oranges, or tomatoes). How is this even
possible? Somehow, electrons must be able to travel through the fruit. When this
happens, we say the fruit conducts electricity. It turns out that citric acid (found in tart
fruits) allows electrons to flow more easily through sour fruits than through regular
water.
8. Before you begin, have the students write down their hypotheses to the question,
“Which fruit will make the most effective battery?” in the indicated spots on their
worksheets.
9. Now distribute the supplies to each table (4 fruits, 4 copper nails, 8 zinc nails and 5
alligator clip connectors— save the LED for later). Tell the students that you will be
giving them step-by-step directions, so they need to be very good listeners. Let the
students know not to eat the fruits, as they may get contaminated during the activity.
Instruct each student to take 1 fruit, 1 of copper nail, 2 zinc nails and 1 alligator clip
connector. As you give the students instructions, you should also follow along on the
overhead, while your teammates help the students.
10. When the class is ready, have the students take their fruits into their hands and roll
the fruit between their palms while gently squeezing. Explain that this will soften up
the fruit and make it easier for current to flow through it. (Try to avoid rupturing the
skin of the fruit if you can.)
11. Now have students take their copper nails and push it gently about 1” into one side
of the fruit. Then have the students take one of the zinc nails and push it about 1” into
the other side of the fruit so that the tips of the nails are close, but not touching. Add
the second zinc nail right next to the first (Fig 1).
Fig. 1. How to insert nails into the fruit. The nails can be a bit closer than in the picture above,
but make sure they are not touching or the battery won’t work!
12. Once everyone has prepared his or her fruit with nails, remind the students that
these fruits are actually little batteries. Copper holds onto its electrons more forcefully
than zinc does, so the electrons want to flow from the zinc nail to the copper nail. (This
is like an electron tug-of-war between copper and zinc, and copper is winning!)
13. Tell students that scientists measure how strong a battery is by using a device called
a voltmeter. (Hold up a voltmeter for everyone to see.) We can use this device to
measure the voltage of our fruit batteries. Remind students what voltage is by having a
student define it for you. (Voltage is a measure of how strongly the battery is pushing
on the electrons to create a current.) Demonstrate how a voltmeter works on the
overhead by holding the two probes of the voltmeter against the two nails on the