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Electric Current
We need to look at several aspects of the nature of electric current
because it varies depending on the material in which it is flowing. In this lesson we will
consider electric current in metals, solutions and at electrodes.
As you will see, something different happens with electric current in each of these
materials.
In Metals
| Let's review our old model of metallic bonding. Metals consist of closely
packed atoms which have loosely held electrons which are relatively free to move from one
atom to another. |
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| If we connect something negatively charged (a negative voltage - black
wire) to one end and something positively charged (a positive voltage - red wire) to the
other end, the loosely held electrons in the wire will flow from one end toward the other.
This flow of electrons is related to the electric current in a wire. |
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Unfortunately, when electric current was first created and studied, it was presumed
that something positive was moving from the positive end toward the
negative end.
When you hear someone talking about electric current flowing from positive to negative,
they are using this original concept of electric current.
Much later, electrons were discovered. They were found to have a negative charge and it
was determined that they were what was moving in metal wires when electric current flowed.
To avoid confusion with terminology and to keep the concepts separate, I will usually
refer to electron flow, rather than current.
In Solutions
| In solutions things are different. Free electrons are not
available to carry the current. Instead, charged ions are present in the solution
and are free to move around. |
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| If we "connect" something negatively charged (a negative voltage
- black wire) to one side and something positively charged (a positive voltage - red wire)
to the other side, the positively charged cations move one way (toward
the negative) and the negatively charged anions move the other way
(toward the positive). |
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At Electrodes
Chemistry comes into play at the electrodes. Electrons are transferred into or out of
the solution at the electrodes.
| The electrode where electrons are transferred into the
solution is called the cathode. The chemical process (or reaction) that
occurs at the cathode is called reduction because the negatively charged
electrons will reduce the oxidation state or charge of the chemical they
attach to. |
Cathode
Reduction |
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| An example of the kind of thing that can happen is that a sodium ion takes
on an electron from the cathode and becomes a sodium atom. Again, this is called reduction
because the charge is reduced from +1 to 0. |
Na+ + e-
 Na |
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Note: The intent here is to build a
concept of what can happen at a cathode. For reasons to be explained later, under the
conditions shown here, a different chemical reaction would occur. |
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| The electrode where electrons are transferred out of the
solution is called the anode. The chemical process (or reaction) that
occurs at the anode is called oxidation because removing negatively
charged electrons from a chemical will increase the oxidation state or
charge of that chemical. |
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Anode Oxidation |
| An example of this might be removing an electron from a chloride ion. The
removed electron goes from the chloride ion to the anode. The chloride ion becomes a
chlorine atom. Again, this is called oxidation because the charge or oxidation state is
increased from -1 to 0. |
Cl-
Cl + e- |
Note: This equation is an
oversimplification and will need to be modified later in this lesson. |
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Note: Again, the intent here is to
build a concept of what can happen at an anode. For reasons to be explained later, under
the conditions shown here, a different chemical reaction might occur. |
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E-mail instructor:
Eden Francis
Physical Science
19600 South Molalla Avenue
Oregon City, OR 97045
(503) 594-3352
FAX (503)-650-6657TDD (503) 650-6649 |
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Clackamas Community College
©1998, 1999 Clackamas Community College, Hal Bender |
Na+
Cl-