Bio Batteries!
Day _______________________________________
Learning Objective(s):
AZ DOE---Grade 6, Strand 4, Life Science, Concept 1:
- PO 2. Describe the structure of each of the following cell parts: cell wall, cell membrane
Grade 6, Strand 5, Physical Science, Concept 3:
- PO 2. Identify several ways in which energy may be stored.
- PO 3. Compare the following ways in which energy may be transformed: mechanical to
electrical
Materials:
- Digital multimeters
- Anode and cathode (zinc and copper strips, or galvanized nails and copper wire)
- Jumper wires with alligator clips to connect to anode and cathode
- Low power red LEDs with color-coded leads
- Low power piezo buzzer
- Pocket calculator with battery removed
- Assorted fruits and vegetables (lemons, limes, oranges, apples, pears, bananas, potatoes,
onions, etc)
- Precut lemon halves
- Pre-prepared saltwater solutions of varying concentration
- Cups to hold liquid
- Video from NCIS episode “Power Down”
During
- Play NCIS clip from 28:41 - 29:22. (Lemons power a boombox).
Snack
8 minutes
20
minutes
Engage
- Students are shown a regular battery powering an LED.
- Ask “What does a battery do?” “What is inside a battery to make electricity?”
- “Have you every had a battery run out? Why does that happen? Show two or
three lemons powering an LED.
Explore
- Students attempt to make their own battery using materials given (anode,
cathode, fruit, multimeter). Students can assemble their own battery, but should
work in groups to share multimeters and combine fruits later in the lesson.
- Choose a fruit or vegetable of their choice from a variety provided. Encourage
students to choose something other than lemons so a wide range of fruits are
tested.
- Fruit works best if it is rolled or gently squeezed before inserting zinc and copper.
(Let students know this before they pierce the fruit’s skin to avoid sticky messes!)
- Insert zinc and copper strips or wire and nails so they do not touch each other.
- (If the anode and cathode strips touch together, there will be no current. Students
should figure out the proper spacing using trial and error.)
10
minutes
10
minutes
- Test DC voltage and current with multimeter, record in lab book, write on board.
- (Disconnect multimeter before switching modes. Red connects to Cu, black to Zn.)
- They should try to diagram a battery schematic in their lab book. (See sample
below.)
- Students try to power an LED with a fruit, but fail. (They need at least two lemons.
Make sure they connect the positive lead to the red side of the LED and the
negative to the black! Connecting them backwards will destroy the LED.)
- Remind students that many things use more than one battery. Maybe this LED
bulb needs another lemon or two? Encourage them to hook the lemons together
(in series) to see how many it takes. (The LED might be dim, so look closely).
Explain
- How does a battery work? (See “Electricity and Batteries” below.)
- Talk about the different parts of a battery, diagram on board (this should include
the anode, cathode, electrolyte, and a completed circuit. Show how electrons
move around the circuit.)
- Focus most on electrolyte (the fruit in this case).
- Show a cut lemon half.
- Explain that the lemon has citric acid inside its cells that works like a battery. (Fun
fact: The juice vesicles inside a citrus fruit are actually modified hair cells.)
- When we soften the fruit, we break cells and let the acid mix better. Remind
students that fruit is made up of cells, which have membranes that keep the citric
acid contained. Breaking the membranes lets ions travel through the entire fruit
more easily.
- Call attention to different voltage and current numbers from various fruits.
- Explain voltage and current, write “Power (watts)=Volts • Current” on board.
- Calculate how many watts one lemon produces.
- “Which fruits make the best batteries?” Use values they recorded to decide as a
class.
- What makes these fruits better? (The stronger the acid, the better the battery.
Stronger acids dissociate into ions more readily, which lets this reaction take
place.)
- But batteries don’t always have to be acidic – saltwater works too.
Expand
- Based on their fruit results, students make predictions about what saltwater
concentrations will make the best batteries.
- Test voltage and current to see if their predictions were correct. If they were not,
students should come up with another explanation. (DI water will not act as a
battery, but low to high concentrations of salt should all be about .7 V. The more
salt, the longer the battery lasts. Voltage is constant.)
- When they are done, students can combine several batteries of their choice to
power a pocket calculator or piezo buzzer. They should estimate how much
power/voltage/current the devices require based on how many lemons they need.
- They can then use the calculator to answer the question in Evaluate.
Evaluate
Written questions: You are a new employee working in the NCIS as Abby’s
assistant. The power goes out and she asks you to go buy lemons for her.
In the tv show, Abby uses 8 lemons to power her boombox. Based on what you
know about lemons, is this realistic? Should you agree to buy Abby lemons?
10
minutes
Abby wants proof that you know what you are talking about. If her boombox needs
16 watts of power, and 1 lemon provides about 1 volt and .0002 amps, how many
lemons does she need?
Use the equation Watts = Volts • Amps.
(1 V • 0.0002 A = 0.0002 W
16 / 0.0002 = 80,000 lemons!)
Lesson Summary:
Batteries are a common part of everyday life, but we don’t often think about them. How do all of our
electronic devices run? What’s so special about batteries, and how do they produce electricity?
Students will use fruit to explore the parts of a battery. Batteries contain three main parts: the anode,
cathode, and electrolyte. (see how a battery works below). Fruits (especially citrus fruits) contain acid
and dissolved particles that can act as electrolytes. We will use zinc and copper strips to act as the
anode and cathode. Students can test different fruit batteries to determine which ones work best and
think about why that might be. This lesson combines the concept of cells and the chemicals dissolved
in them with physics and energy.
Supplemental Terms, Definitions, and Explanations:
Battery: Something that converts energy from chemicals inside it to electrical energy.
Electricity: Energy caused by the flow of electric charge. If students know what an atom is, explain
that electricity is carried by electrons through metals and other materials.
Energy: The ability to do work. In our case, electricity is energy that can turn a fan or power a light
bulb.
Electrodes: These are the two ends of a battery. In this experiment, the copper and zinc strips are
electrodes.
Anode: Electrons (and electricity) flow through a circuit. Electrons leave the anode (zinc) and head
through the wires to the other metal strip.
Cathode: The cathode is the part that receives the extra electrons, mode of copper in our experiment.
Electrolyte: This is the fluid inside a battery. If students have seen a leaking battery, they might know
that batteries contain liquid, just like our fruit does. In our experiment, the fruit’s acid acts as the
electrolyte. The electrolyte carries charged particles from the cathode back to the anode to complete
the circuit.
Citric Acid: This is what gives certain fruits like lemons and limes their sour taste. It’s also good at
conducting electricity as an electrolyte and can be used to create batteries. Apples and pears instead
have malic acid, and potatoes have phosphoric acid.
Voltage: We measure this to see how powerful our battery is. Simply put, voltage measures how
much charge the battery puts out, which tells us what the battery can run. Measured in Volts.
Current: This is how much electricity flows through our circuit. Lemons put out a decent amount of
voltage but very little current. Measured in Amps. A good way to relate current and voltage is to think
of a water pipe. Voltage is similar to the water pressure; current is the amount of water coming out of
the pipe.
Power: This tells you how much work you can do with the electricity from the
battery. You get power by multiplying Volts • Amps. Power is given in
watts.
How a battery works:
Electricity is caused by electrons (charge) moving around a circuit. The
anode (zinc) loses electrons, while the cathode (copper) gains them. In
the middle, the electrolyte carries charge from the cathode back to the
anode with charged ions in the liquid. This converts chemical energy to
electrical energy.
(Image from:
http://en.wikipedia.org/wiki/Lem
on_battery#Chemical_reactions)
Details for inquisitive (grad) students:
In lemons, this process can be described as follows:
Anode: Zn -> Zn2+(into solution) + 2e-(stays in metal)
Cathode: 2H+(from solution) + 2e-(from copper) -> H2 (bubbles off)
The zinc slowly dissolves into the electrolyte, leaving 2 free electrons. At the same time, dissociated
hydrogen ions in the electrolyte take 2 electrons from the copper to form hydrogen gas. The 2 extra
electrons on the zinc replace those lost from copper. The same thing happens in saltwater, except H+
is replaced with Na+.
Sample fruit values:
These are taken from my own tests, and will vary depending on the individual fruits.
Fruit:
Lemon
Orange
Lime
Apple
Pear
Potato
Banana
Kiwi
Avocado
Voltage (V):
Current (mA):
.96
.14
.906
.067
.905
.071
.971
.057
.979
.13
.888
.21
.891
.09
.904
.12
.856
.197
Lemons produce a fairly high voltage, but very little current. This means that they cannot power
things that require much wattage (like an incandescent light bulb). Power(watts) = Voltage(volts) •
Current(amps). You can boost the voltage by connecting lemons in series and boost the current by
connecting them in parallel. To light one red LED, you will need at least 2 lemons connected in series.
For comparison, a AA battery has a voltage of 1.5 V, but current off the scale of my multimeter.
(Image from:
http://van.physics.illinois
.edu/qa/listing.php?id=2
392)
A simple battery schematic:
Students should be able to draw this (or something similar)
by the end of the lesson.