Read the fifth chapter of Instant Physics (Rothman) and
write a one-page reaction to or summary of.
Due: Nov. 17
Monday:
We returned to our conversation on electricity and magnetism. It concerned charges with like
charges repelling and opposite charges attracting according to Coulomb's Law (named for
Charles-Augustin de Coulomb).
F = k q1 q2 / r2
Coulomb is also the name of the unit of charge. We noted that the same (energy) analysis that told
us that a body can "escape" the Earth's gravitational field tells us that an electron can escape the
electric field of the nucleus -- in a process known as ionization.
While electric fields are associated with electric charges (positive and negative), magnetic fields are
associated with magnetic poles (north and south). Incidentally the Earth is a big magnet. A big difference
between electricity and magnetism is that while we can isolate a sngle charge, we cannot isolate a single
magnetic pole. If we break a magnet with a north and south pole in two, each piece has a smaller north and
south pole. Said another way there are no magnetic monopoles.
If charges move, then we have a current. Current is measured in amps (or amperes) -- another name for
Coulombs/second. Voltage is another concept from electricity. Voltage and its unit volts are named for
Alessandro Volta who did early work in batteries. Some other work in electricity was Luigi Galvani's work on
twitching frog legs. Voltage and current are then related through the concept of resistance which is
measured in ohms (named for Georg Ohm). Ohm's law (V=IR) says that for certain materials the current is
proportional to the (applied) voltage. An ohm is volt/amp. A volt is a Joule/Coulomb. In the ohm we divided
volts by amps. If we instead we multiply a volt by an amp we get (Joule/Coulomb)*(Coulomb/Second) =
(Joule/Second) = Watt -- the unit of power named after James Watt.
Andre-Marie Ampere discovered that moving charges give rise to a magnetic field. With Ampere we begin
to see the connection between electricity and magnetism. If we add our concept of frame of
reference to this idea, we can say that if we have a simple (DC) current with its associated magnetic field,
then there is a reference frame that moves with the charge(s) and in which there is no magnetic
field. (Adding the concept of reference frame to electricity and magnetism will yield Relativity.)
Further connections between electricity and magnetism were studied by Michael Faraday in Britain and
Joseph Henry in the U.S. They discovered independently a phenomenon known as magnetic induction -- that
a changing magnetic field gives rise to an electric field. Henry has an unit of inductance named after
him and helped found the Smithsonian here in the U.S. Michael Faraday rose through the ranks in the
Royal Society. Faraday was a very mechanistic, model-based, visualizer type thinker. Faraday gets more
credit since he published first -- it didn't help that Henry was from the U.S. which was considered a
"backwater" at the time.
We end our discussion for the day talking about direct current (DC) in which the charges move in one
direction and alternating current (AC) in which the charges move back and forth. While the charges
move back and forth, the power travels down the wire -- it is a basic wave phenomenon -- just like the
water in the ocean moves up and down (horizontally in place) but the waves come to shore. Within technology
there was a battle between Thomas Edison who pushed for DC usage and Nikola Tesla who pushed for AC.
Edison used his power and influence to suppress Tesla's idea (which was better). Eventually Tesla
teamed with George Westinghouse to counteract Edison influence. Ac was used in the development of the electric
chair by some from Edison's team to give credence to the idea that Edison thought AC was
dangerous.
Wednesday:
We recalled the result of Ampere that an electric current leads to magnetic a field as well as
the results of Faraday and henry that a changing magnetic field induced an electric field.
We did a demonstration that had a large coil of wire attached to an ammeter (which measures current)
but there were no batteries in the circuit to cause current to be detected. We brought a large magnet
nearby. When the magnet was stationary the ammeter needle sat on zero, but when the magnet was
moving the needle was deflected.
James Clerk Maxwell (of maxwell's Demon fame) put all the pieces (Coulomb's, Ampere's, Faraday's and
Henery's, etc.) into a set of four succinct equations -- Maxwell's Equations. Maxwell also added to
Ampere's Law -- the so-called maxwell Displacement Current -- which is similar to Faraday/Henry in that
a changing electric field gives rise to a magnetic field.
We considered a scenario of putting a time-dependent (say a cosine shaped) current into an antenna.
That would yiled a (time-dependent) magnetic field (a la Ampere) in the surrounding space. That magnetic
field would give rise to a (time-dependent) electric field (a la faraday/Henry) in the "space surrounding
the space". That electric field would give rise to a (time-dependent) magnetic field (a la Maxwell). Etc.
Etc. In other words maxwell's Equations have a wave solution. A wave is a propagating disturbance -- in
this case the disturbacnce was electromagnetic. Maxwell found that the speed of the wave was c the
speed of light. Thuus Maxwell's equations represent a unification not only of electricity and magnetism
but also of optics/light. The experimental verification of this electromagnetic wave was done by
Heinrich Hertz.
The Hertz experiment generated what we would tend to call radio waves but it turns out there is an
entire spectrum of electromagnetic waves which differ by their frequencies. The frequency is the number of
cycles per second (another name for the unit cycles per second is Hertz). A related concept is the period --
the number of seconds per cycle. T=1/f
We mentioned a result from quantum mechanics E=hf that the energy (of a photon) is proportional to the
frequency. This is why the high frequency parts of the spectrum, ultraviolet and x-rays, are more dangerous.
We also brought up VHF (Very High Frequency) 30 MHz to 300 MHz and UHF (Ultra High Frequency) 300 MHz and 3 GHz.
We went back to the time of Newton who wrote about light in his book Opticks. Netwon though light was a particle.
His contemporary Christiaan Huygens thought light was a wave. Both could use their idea of the nature of
light to describe the known experimental results on light. Huygens developed a way of thinking about waves
known as Huygens' Principle which says that each point on a wavefront can be thought of as a new source of waves.
Later this discrepancy was settled (or was it?) by Thomas Young (who also did work in Hieroglyphics) and his
double slit experiment. If a wave impinges upon a surface with two slits, then those two slits act as
coherent sources (a la Huygens). A wave at its "crest" is at its most distrubed in the "positive"
direction while a wave at its "trough" is at its most disturbed in the "negative" direction. The waves
from the two slits are coherent -- have their crests and troughs synchronized. After emerging through the slits
there will be places where crest meets crest (and trough meets trough) leading to bigger crests and troughs.
These spots are said to have constructive interference and in the case of light appear as bright spots.
there will be places where crest meets trough tending to cancel each other out. These spots are said
to have destructive interference and in the case of light appear to be dark regions. This phenomenon is a
wave phenomenon and so when observed seemed to settle the issue about the nature of light in favor of
waves. But Einstein would come along later and with the concept of photons say that in certain circumstances
light does behave as a particle.
