Electromagnetism
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: electromagnetics.
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 again).
- REVIEW:
- 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.
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