Jeffrey Goett
Electromagnetism
From a series of observational experiments, scientists found that both a magnet
and a current can create a magnetic field, a hypothesis that Andre Marie Ampere tested.
Michael Faraday later found that a changing magnetic field creates an electric field.
From this, James Clerk Maxwell created a model of electromagnetic radiation and
hypothesized that light is a form of electromagnetic radiation. While the famous
Michaelson-Morley experiments disproved the hypothesis that electromagnetic waves
propagate through a medium, Henry Hertz ran a series of testing experiments that
supported Maxwell’s hypothesis that light is a type electromagnetic radiation. Finally, by
applying an atomic theory of bonding to electrolysis experiments, scientists calculated a
good approximation of the value of the fundamental electric charge.
Even thousands of years ago, people observed that lodestones would interact with
each other. Over one thousand years later, a Danish physicist, Hans Christian Oersted,
observed that a current running through a wire exerts a force on a magnet. The direction
of the force formed a circular path around the wire. From these observations, Andre
Ampere reasoned that if one magnetic field interacts with another and the magnetic field
of a magnet interacts with a current, then current, too, must produce a magnetic field. To
test this model, Ampere placed two current carrying wires in proximity to each other and
checked to see whether, as he predicted, the wires would interact with each other. When
the currents ran antiparallel to each other, the wires repelled each other and when the
currents ran parallel to each other, the wires attracted each other. These results added
confidence to Ampere's hypothesis that current produced magnetic fields.
Michael Faraday inadvertently found the reverse relationship between magnets
and currents. He reasoned that if a steady current produces a magnetic field, then, by
symmetry, a steady magnetic field should produce a current. To test this, he created a
magnetic field by passing a current through a wire wrapped around a metal rod. On the
other end of the rod he wrapped another wire, in which he predicted the steady magnetic
field would induce a current. To measure the current, he hooked a galvanometer to the
wire. During the experiment, though, he found that a changing magnetic field, not a
steady one, produced a current in the wire. To further test this hypothesis, he induced the
current with a regular magnet and also without the metal rod. Thus he showed nature
induces an electric field in a loop of wire when the magnetic field through the loop
changed.
Maxwell devised a model of space as composed of vortexes and charged particles
that interact with each other. A changing magnetic field manipulates these vortexes,
which affect the charged particles and create the observed electric field in a wire. From
this model, he predicted that a changing magnetic field also creates displacement currents
in space even where there are no wires. This displacement electric field could be
detected by another wire and could be a means to send electromagnetic energy through
space in the form of waves. He predicted that metallic objects would reflect these waves
and glass would bend it. He also predicted that they would travel at 3*108 m/s. Finally,
he predicted that these waves were transverse waves, just like light. From all of these
predicted properties of the waves, Maxwell hypothesized that light was just a type of
electromagnetic wave.
Henry Hertz tested this hypothesis. He predicted that if he created a spark
between 2 metal spheres at a frequency of 5*108 s-1, then these changing electric fields
should create a changing magnetic field, as Ampere had shown. But Maxwell had
predicted that changing magnetic fields would create another electric field, the
displacement electric field, even without a wire being present. Thus, a self-perpetuating
cycle would form, the essence of an electromagnetic wave. This wave would cause a
fluctuating current in a wire a distance away from the source. Hertz observed this to
occur when he performed this experiment.
He also tested other predictions made by Maxwell, finding that these waves
would reflect off of metal sheets, bend through optics as light does, polarize, interfere
with each other, and propagate at the speed of light. Thus, Hertz's experiments increased
scientist's confidence in Maxwell's hypothesis that light was another type of
electromagnetic radiation.
Maxwell's hypothesis also stated that these waves propagated in a medium, which
he referred to as ether. Two scientists, A.A. Michelson and E.W. Morley, predicted that
if light traveled in a medium, then when an observer is moving relative to the medium, he
or she would measure different values for the speed of light. Assuming the earth moved
through this ether, then scientists on earth would measure the velocity of light traveling
antiparallel to the earth to be different from the velocity of light traveling perpendicular
to the movement of the earth. Observations by these two scientists, though, did not find
this direction dependence when measuring the speed of light. Some scientists continued
to argue in favor of the ether, hypothesizing that the earth pulled the ether along with it,
resulting in the observations. Further experiments, though, disproved this hypothesis as
well.
In addition to discoveries about the nature of magnetic fields, electric fields, and
electromagnetic radiation, scientists were also able to calculate the charge of a single
electron. During experiments, scientists observed that compounds in solution could be
decomposed into their constituent elements by passing a current through the solution. To
release one mole of an element with a valence of one, scientists passed 96,500 Coulombs
of charge through the solution. For elements with a valence of two, scientists passed
twice this amount of charge through the solution to release one mole of the element. The
atomic model of compound formation states that when some atoms bind together, one
atom gains a given number of electrons given by its valence number and the other atom
loses a given number of electrons given by its valence number. By passing a current
through a solution of these ions, electrons are transferred to atoms that had lost electrons
and electrons are taken away from atoms that had gained electrons, causing the molecule
to decompose into its constituent elements. Thus, in order to release one mole of an
element of valence one, scientist had to supply one mole of electrons to the solution. The
scientists, though, knew the amount of this supplied charge, 96,500 C, and the number of
particles in a mole. By dividing, they calculated the charge per electron to be 1.603*10-19
C which is a very good approximation of the actual value.