ELECTROCHEMICAL
CELLS AND BATTERIES
UM Physics Demo Lab 07/2013
Pre-Lab Question
What type of physical process makes it possible for a voltaic cell to separate
electrical charge and thereby drive an electrical current through a circuit?
EXPLORATION
Exploration Materials
2 plastic beakers
2 single metal bars (1 Cu, 1 Zn)
1 Cu-Zn U-bar
Cup of Coke (battery acid)
2 plastic spacers
1 Green multimeter
1 special calculator
2 banana leads with alligator clips
1 blue plastic tray
Paper towels (for messes)
Sandpaper
1. Build a voltaic (electrochemical) cell. Place one copper (Cu) and one Zinc (Zn) bar
in a beaker separated by a plastic spacer. Connect one lead from the multimeter to
the Cu bar, and the other to the Zn bar as shown below. To make a good
connection you can scratch the surface of the Cu and Zn with the test leads to get
through the oxidation on the surface.
Figure 1: Cu and Zn bars in beaker connected to a multimeter.
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2. Set the multimeter switch to V to read the DC voltage of the setup. What do you
find?
3. Add a small amount of coke to the beaker (up to the first line on the beaker). Read
the voltage of the system with the multimeter and record it below. Describe what
the coke does to the system.
4. Predict what will happen to the voltage if you add more coke to the beaker.
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5. Double the amount of coke in the beaker (up to the second line this time). Read
the voltage with the voltmeter. Does this agree with your prediction? Explain how
the quantity of coke affects the cell’s voltage.
6.
Now use the U-bar with the loose bars of Cu & Zn. Take the second beaker, and
alternate the bars as shown below.
Figure 2: Two beakers connected in a row with alternating bars.
Predict what the voltage will be across two cell beakers at certain test points. Present
your prediction to the GSI or instructor before measuring. After discussing you prediction,
add at least an inch of coke to the new beaker. Read the voltage with the voltmeter at the
different test points.
Test Points
1&2
Voltage (Predicted)
Voltage (Measured)
2&3
1&3
Does this agree with your prediction? Explain how adding a second cell affects the
battery’s voltage.
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Challenge Work:
1. You found different voltages with your tests in step 6. See what voltage the
calculator needs to operate. You need to make an electrical connection between
the calculator and the battery; use the alligator leads to do this. They clip onto
both devices and conduct electricity. Observe polarity by connecting the red lead to
copper (positive charge) and the black lead to zinc (negative charge). Draw a
diagram showing the connection points and voltage you used to power the
calculator.
2. Replace one of the electrodes (Cu or Zn) with a piece of Aluminum foil (folded up a
few times to make it stiffer and easier to handle). Record the voltage for a single
cell containing this Aluminum electrode. Be sure to indicate which electrode you
replaced.
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Everyday Applications

Batteries are used everywhere. Electricity stored in batteries powers all our mobile
devices and soon may power a significant number of automobiles.
1. The battery in your car won’t quite turn your engine over with enough vigor
to start the engine. Explain how to connect a second battery to your car so
that it can be “jump started”. Specifically, draw a diagram below showing
the positive and negative terminals of each battery and how they are
connected to each other to increase the current available to start your car.
Is this a series or parallel connection?
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APPLICATION
Materials
Stack of Cu & Al 2” diameter discs
Coke or other soft drink (battery acid)
1 green multimeter
Stack of paper discs
Paper towels (for spills)
1. Soak the paper discs with soft drink, and stack the Cu & Al correctly to create a
multiple cell battery. Start with a 1-cell, then add a 2nd cell and build your way up.
Measure the voltage with the multimeter. Produce a battery with a total electrical potential
of at least three volts. How many cells did you need to connect to achieve this? Draw
a diagram showing your final battery, including all the copper, aluminum and paper disks
(you can label them “C”, “A” and “P” in your diagram). Be sure to label the positive and
negative terminals for your battery and indicate the electrical potential of your assembled
battery in volts.
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Summary:
1. Electrochemical cells are very simple devices constructed from an electrolyte
solution, a cathode metal (positive terminal), and an anode metal (negative
terminal).
2. Cells work by electrochemically separating charge and making it energetically
favorable for electrons to flow from higher electrical potential energy to lower
electrical potential energy, that is, from the negative anode to the positive
cathode.
3. Traveling electrons are a current; the path they take is the resistance of the
external circuit. The electric potential energy available per electron is the
difference in electrical potential or voltage between the anode and cathode.
4. Electrons flow from the negative anode to the positive cathode when a cell or
battery is connected to an external circuit.
5. Conventional current is defined as the flow of positive charges from the
positive cathode terminal of a cell or battery to the negative anode terminal.
This is a commonly used engineering convention and is completely equivalent
to the actual flow of negative electrons from the negative anode to the positive
cathode which actually takes place in any real circuit.
6. Cells may be connected together to form batteries.
7. Cells connected in series provide a difference in electrical potential which is the
sum of the potential differences for the individual cells. Cells connected in
series provide the same current as a single cell.
8. Cells connected in parallel can provide more current at the same potential
difference as a single cell.
Final Clean-up
Please dispose of the paper discs and soft drink. Rinse the metal bars and discs in water,
and then allow them to dry on paper towels. Replace the other equipment to the carts.
Bibliography and recommendations for further reading:
Brain, Marshall, "How Batteries Work," How Stuff Works,
http://electronics.howstuffworks.com/battery.htm/printable (accessed May 25, 2006).
Energizer, "How Batteries Work," Learning Center,
http://www.energizer.com/learning/howbatterieswork.asp (accessed May 25, 2006).
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ELECTROCHEMICAL CELLS AND BATTERIES
In the static electricity lab we saw that separated charges travel to recombine to a state of
charge neutrality. The charges were separated by rubbing two materials together
(triboelecticity) or by polarizing a conductor. Another way to separate charges is
electrochemically, and this is the principal that allows us to chemically generate
electrical energy in a non-rechargeable electrochemical cell and to store and retrieve
electrical energy in a rechargeable electrochemical cell.
An electrochemical cell is composed of three components: a cathode, an anode, and an
electrolyte solution. See figure below.
Figure 3: Schematic of a commercial cell
A collection of cells connected together is called a battery. Usually the cells are
connected in series to increase the available electrical potential (voltage) compared to a
single cell. Sometimes cells are connected in parallel to increase the capacity (ability to
deliver current) of the battery when compared to a single cell.
Electrolytes: Battery Acid
An essential feature of a battery is the electrolyte solution that connects two electrodes.
Coke, lemons, and potatoes are fun materials that can be used, but powerful acids or
bases are more commonly used for commercial purposes because they produces cells with
a higher capacity to deliver current.
When the anode and cathode are immersed in the electrolyte, a chemical reaction takes
place. The result of that reaction is a shortage of electrons on the cathode and a buildup
of electrons on the anode. This charge imbalance makes the electrons want to travel from
the anode to the cathode.
The elegance of an electrochemical cell is that the chemistry of the electrolytic solution
has a preferential current direction for a given voltage, and won’t allow the electrons to
flow back to the cathode within the cell. For the charge imbalance to neutralize, the
current must travel through an external circuit. The traveling electrons are what power
your electronic device.
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Traveling charges are called a current. The current is determined by two things.
1. The cell has a maximum electric potential difference or voltage (defined as the
potential energy available per unit of charge) that depends on how it is built and
how much of the chemical reactants have been consumed. Electric potential is also
sometimes referred to as “Electromotive Force” or EMF.
2. The resistance in the external circuit also determines the current.
Electrical current is much like a current of water. A pump gives elevation to the water as a
cell gives electric potential (voltage) to electrons at the anode. The flow of water to lower
elevation is analogous to the current flowing in the circuit from higher electrical potential
energy to lower electrical potential energy and the constrictions in the water’s path are
analogous to the circuit’s resistance.
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