Electrochemical Cells
and Batteries
UM Physics Demo Lab 07/2013
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 1: 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.
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.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
1
Traveling charges are called a current. The current is determined by two things.
1. The cell has a maximum electric potential 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 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.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
2