Cells and Batteries Lab

ELECTROCHEMICAL CELLS AND BATTERIES

UM Physics Demo Lab 07/2013

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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

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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

Property of LS&A Physics Department Demonstration Lab

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.

Set the multimeter switch to V to read the DC voltage of the setup. What do you find?

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.

Predict what will happen to the voltage if you add more coke to the beaker.

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.

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 / Voltage (Predicted) / Voltage (Measured)
1 & 2
2 & 3
1 & 3

Does this agree with your prediction? Explain how adding a second cell affects the battery’s voltage.

Challenge Work:

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.

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.

Everyday Applications

Batteries are used everywhere. Electricity stored in batteries powers all our mobile devices and soon may power a significant number of automobiles.

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?

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 cellbattery. 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.

Summary:

Electrochemical cells are very simple devices constructed from an electrolyte solution, a cathode metal (positive terminal), and an anode metal (negative terminal).
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.
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 differencein electrical potential or voltage between the anode and cathode.
Electrons flow from the negative anode to the positive cathode when a cell or battery is connected to an external circuit.
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.
Cells may be connected together to form batteries.
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.
Cells connected in parallel can provide more current at the same potential difference as a single cell.

Final Clean-up

Please dispose of the paperdiscs 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, (accessed May 25, 2006).

Energizer, "How Batteries Work," Learning Center, (accessed May 25, 2006).

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.

Traveling charges are called a current. The current is determined by two things.

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.
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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