Magnetism is a physical property that certain materials posses that permits them
to attract or repel similar or other materials that exhibit magnetism. Magnetism
and electricity are intertwined. Electricity can generate or alter magnetism,
magnetism can generate or alter magnetism. Hence the term “electromagnetism” is
used to describe the conjoined phenomena.
Magnetism itself cannot be seen, but the effects of it are readily apparent.
Sprinkling iron filings on a page of paper that has a magnet placed beneath the
paper shows the magnetic field near the magnet can align and attract the iron
filings on the face of the paper. Holding a magnet near another magnet or a
piece of magnetic material such as a small iron plate, will show that an
attraction or repulsion between the two items may begin a bit of a ways from
each other without the items actually touching. The effect reveals that there is
an invisible magnetic field around the magnet. Passing other “magnetic
materials” through that field can result in the generation of electricity.
Similarly, winding a length of wire into a coil around an iron nail and
connecting the ends of the wire to the terminals of a current source such as a
battery will produce a magnet out of the nail allowing it to attract other
nails. These properties of electromagnetism are found in thousands of electronic
devices that are used to improve our lives. Using your LEFT hand, point your
thumb straight out to indicate the direction of the flow of a current. The
fingers wrapped around then indicate the direction of the magnetic field lines
of force. This is known as the left hand rule of magnetism.
Several characteristics of magnetism need to be described at this point that
will later be necessary to understand the purpose and performance of certain
electronics devices used in the circuitry of electronics and radio. The lines of
magnetic force (or magnetic field) is designated as ø. If 100 lines of force are
produced in a coil, ø=100. If the cross sectional area of the coil is 2 in2 then
the flux density, designated by the symbol B, is 100/2 = 50 (B =ø/d). The
intensity of the magnetic field is termed magnetomotive force or MMF. MMF may
be computed by multiplying the number of turns in the coil by the current
flowing through the wire, F = NI. Either increasing the number of coil turns or
increasing the current in the coil or both will increase the strength of the
magnetic field produced. The magnetization vector is the distribution of
magnetic moments in the region. The symbol dm is the magnetic moment, dv is the
volume element.
Magnetic field intensity is represented by the symbol F. If a 3 inch coil with
60 amp-turns (2 amps x 30 turns in the coil) has a field intensity of 60/3 = 20.
The field flowing from a wire or coil of wire are pushed back by air but when a
piece of iron is introduced into the magnet field, the density of lines of force
(or flux) will increase. The iron expands the flux from the wire outward and
concentrates the flux. In other words, the magnetic flux density increases. If a
wire with a current of 1 amp flowing is turned into a 1 inch diameter coil
having 20 turns, a flux density of 20 lines/in2 will be created. If, however, we
wrap that wire coil of 20 turns around a nail, a flux density of perhaps 200,000
lines/in2 may be created.
Finally, permeability (a variant of the word permeate or saturate, not the word
permanent) represented by the symbol µ, is the ability of as core material to be
permeated by lines of force. The permeability of air may be assumed to be 1.
Materials such as iron, nickel, and cobalt are highly permeable and have values
of hundreds to thousands of times that of air.
These are the equations you need to be familiar with from this section:
ø = Bd for magnetic flux measured in Webers
B = ø/d for magnetic flux density measured in Tesla
M = dm/dv magnetization vector measured in Amperes per meter A/m
H = 1/µB-M magnetic field intensity measured in Ampere per meter A/m
F= NI for MMF intensity measured in Amperes
µ = B/H or µ(H+M) for permeability measured in Henrys per meter h/m
Magnetism has been known of for thousands of years since discovering that
certain rocks (lodestone) could attract iron. Compass needles were originally
magnetized by rubbing them on lodestone then floating the needle in water to
permit it to point north thereby allowing marine navigation out of the sight of
land references. Study of magnetism progressed from the ancient world up until
1819 when Hans Christian Orsted discovered that a compass needle would respond
when an electric current passing through a closed loop of wire was passed near
the needle. Soon, in 1831, Michael Faraday discovered that when passing a loop
of wire through a magnetic field, an electric current was produced. James C.
Maxwell then describe this phenomenon and tied it together with electricity,
magnetism, and optics (he proposed that light is an electromagnetic phenomena).
Maxwell’s equations together define the speed of light (as “c”, a constant speed
in a vacuum at 299,792,458 meters per second or 186,263.7559 miles per second).
The speed of propagation in a vacuum is the fastest that electricity can
travel, that speed is somewhat less when traveling in wire. Some wire,
especially coax, is specified with a velocity factor indicating how much more
slowly that electricity will travel. Velocity factor becomes important when
computing the length of wire to use in resonant or tuned circuits for radio. In
1905, Albert Einstein used Maxwell’s equations to describe the special theory of
relativity. When electromagnetic phenomena are propagated through wires, air, or
empty space, it is called radiation. As waves of electromagnetic radiation they
occur at various wavelengths to produce a spectrum from radio waves to gamma
rays.
Faraday’s discovery of the ability to create electricity by passing a wire
through a magnetic field resulted in the first hydroelectric power generators
that allowed large scale electricity production by attaching rotating coils of
wire attached to a shaft placed in a stationary magnetic field together at
hydroelectric dams, coal fired steam generators, and eventually nuclear powered
generators to feed a grid of wire distributing electricity to all of civilized
society. Conversely, when electricity is applied to the coil with rotating
magnets attached to a shaft, we have a motor. Generators can drive motors,
motors can drive generators. We use both now every day.
Electromagnetism is used in many aspects of modern electronics. A relay uses an
electromagnet to close a switch to control another device or signal the state of
another device. A variant of a relay is a buzzer used for audible signaling.
Still another variant called a solenoid causes movement in an arm that can open
and close doors, valves, and other things. A speaker converts varying electric
currents to sound by using an electromagnet to move a cone of paper or plastic
to create a sound wave. A microphone converts sound to a varying electric
signal by the same approach. A device called an inductor or transformer (a coil
of wire sometimes wrapped around a magnetic core) to slightly delay a varying
electric signal in time. That delay only occurs with varying electric signals,
not direct current signals. That characteristic, combined with a similar varying
electric signal delay caused by devices called capacitors results in
combinations of those two devices to create tuned circuits that can be used to
select only certain signals meeting desired ranges of frequency to be passed
through the circuit (more on tuned circuits later).
73… W3SEH