The discovery of the relationship between magnetism and
electricity was, like so many other scientific discoveries, stumbled upon almost
by accident. The Danish physicist Hans Christian Oersted was lecturing one day
in 1820 on the possibility of electricity and magnetism being related to
one another, and in the process demonstrated it conclusively by experiment in
front of his whole class! By passing an electric current through a metal wire
suspended above a magnetic compass, Oersted was able to produce a definite
motion of the compass needle in response to the current. What began as
conjecture at the start of the class session was confirmed as fact at the end.
Needless to say, Oersted had to revise his lecture notes for future classes! His
serendipitous discovery paved the way for a whole new branch of science:
Detailed experiments showed that the magnetic field
produced by an electric current is always oriented perpendicular to the
direction of flow. A simple method of showing this relationship is called the left-hand
rule. Simply stated, the left-hand rule says that the magnetic flux lines
produced by a current-carrying wire will be oriented the same direction as the
curled fingers of a person's left hand (in the "hitchhiking"
position), with the thumb pointing in the direction of electron flow:
The magnetic field encircles this straight piece of
current-carrying wire, the magnetic flux lines having no definite
"north" or "south' poles.
While the magnetic field surrounding a current-carrying
wire is indeed interesting, it is quite weak for common amounts of current, able
to deflect a compass needle and not much more. To create a stronger magnetic
field force (and consequently, more field flux) with the same amount of electric
current, we can wrap the wire into a coil shape, where the circling magnetic
fields around the wire will join to create a larger field with a definite
magnetic (north and south) polarity:
The amount of magnetic field force generated by a coiled
wire is proportional to the current through the wire multiplied by the number of
"turns" or "wraps" of wire in the coil. This field force is
called magnetomotive force (mmf), and is very much analogous to
electromotive force (E) in an electric circuit.
An electromagnet is a piece of wire intended to
generate a magnetic field with the passage of electric current through it.
Though all current-carrying conductors produce magnetic fields, an electromagnet
is usually constructed in such a way as to maximize the strength of the magnetic
field it produces for a special purpose. Electromagnets find frequent
application in research, industry, medical, and consumer products.
As an electrically-controllable magnet, electromagnets
find application in a wide variety of "electromechanical" devices:
machines that effect mechanical force or motion through electrical power.
Perhaps the most obvious example of such a machine is the electric motor.
Another example is the relay, an
electrically-controlled switch. If a switch contact mechanism is built so that
it can be actuated (opened and closed) by the application of a magnetic field,
and an electromagnet coil is placed in the near vicinity to produce that
requisite field, it will be possible to open and close the switch by the
application of a current through the coil. In effect, this gives us a device
that enables elelctricity to control electricity:
Relays can be constructed to actuate multiple switch
contacts, or operate them in "reverse" (energizing the coil will open
the switch contact, and unpowering the coil will allow it to spring closed
- When electrons flow through a conductor, a magnetic
field will be produced around that conductor.
- The left-hand rule states that the magnetic flux lines
produced by a current-carrying wire will be oriented the same direction as
the curled fingers of a person's left hand (in the "hitchhiking"
position), with the thumb pointing in the direction of electron flow.
- The magnetic field force produced by a current-carrying
wire can be greatly increased by shaping the wire into a coil instead of a
straight line. If wound in a coil shape, the magnetic field will be oriented
along the axis of the coil's length.
- The magnetic field force produced by an electromagnet
(called the magnetomotive force, or mmf), is proportional to the
product (multiplication) of the current through the electromagnet and the
number of complete coil "turns" formed by the wire.
Lessons In Electric Circuits copyright (C)
2000-2011 Tony R. Kuphaldt, under the terms and conditions of the Design