June – Speaker: Gary Derman, CCAS. “What is electricity? What is magnetism? A discussion.”
Gary Derman graciously gave us a talk of heroic breadth – the entirety of electromagnetic theory. This is normally a two-semester course at whatever level it is taught – thus the “heroic” description. In discussing it, Gary followed the Caltech undergraduate course that is found in the second volume of the famous “Feynman Lectures on Physics”, the second of the three volume “red books.” These are brilliant lectures, with incredible physical insight. They are also hard – they pretty much left the very bright Caltech undergraduates in the dust, but were loved by the faculty! You can view some of them by Feynman himself on YouTube under “Feynman Lectures.” (The YouTube lecture on “The Character of Physical Law” is also brilliant, and much more generally accessible!)
The Feynman Lectures aside, Gary started with his own introduction into the world of electromagnetism (or E&M, as it is colloquially called) via electronics and ham radio. Many of us were introduced that way, and seeing the ARRL sign was a great reminisce. He then jumped to his topic – what are they? A very deep insight was provided quickly – they are just different aspects of the same thing – the electromagnetic field! You can generate one from the other! (That they were such took some genius physicists many years to figure out!)
Gary then took a look at what our (normal, everyday) matter is – atoms which are mostly empty space, and made up of positively charged protons, negatively charged electrons, and neutral neutrons. Chemistry and the Periodic Table are explained nicely by the relative numbers of these entities found in atoms.
One informative bit of discussion that emerged from Gary’s “Question Everything” slide was about the meaning of the word “theory.” Theory is a much abused word with two common meanings: one is a “hypothesis” or guess about how something works (“I have a theory about that …”) and the other is a well proven model of how the world works. (“Einstein’s General Relativity and Darwin’s Evolution are perhaps two of the best proven theories of nature.”) This is a good point to keep in mind when someone, either innocently or purposely, confuses them.
Gary then dove into the Bohr atom model of the atom, where the electromagnetic force keeps the electrons “orbiting” the nucleus. One of the interesting points here is that classical electromagnetism predicts that the electrons will quickly spiral into the nucleus, while continuously radiating energy…with disastrous results. This doesn’t happen (as we plainly see!), but the answer why is not to be found classically. Rather, it took Bohr’s insight that the energy transitions which radiate energy away must obey quantum rules, which say that these transition can only occur in discrete (quantum) leaps, and that there is a ground state beyond which the system can’t lose any more energy. This became “quantum electrodynamics”, which was the next step past the classical E&M Gary concentrated on.
Gary then mentioned the four states of matter (solid, liquid, gas, plasma) and how these differ in their electromagnetic character.
After a “quick course in physics”, Gary discussed the difference between electromagnetic forces and gravitational forces. Two prime differences immediately emerge: 1) gravity is about ten to the thirty ninth power times weaker than electromagnetism and 2) gravity is purely attractive, whereas E&M forces can be attractive or repulsive, depending on the charge. (There may actually be a repulsive gravity force, which we astronomers are familiar with, called “dark energy”, but that is another story!)
Gary then got into the “field” concept of E&M, with the familiar “lines of force.” These lines, a classic example of which we can see using iron filings and a magnet, denote the field strength and direction, or “the vector field.”
A “fun fact” about electric currents that Gary showed was that, despite the field moving around the circuit at the speed of light, the individual electrons that create a current only move very, very slowly…for a DC current, they take two minutes to go one inch!
Gary then turned to magnetism, which is a “dipole” type of phenomenon nowadays – a magnet has a north and a south pole. (In the early universe, there probably were “magnetic monopoles”, which take very complicated quantum field theory to describe. They are no longer around.) Gary mentioned the three varieties of magnetism – ferromagnetism, diamagnetism and paramagnetism. We are most familiar with ferromagnetism in the everyday world. The detailed physics of how such magnets work is the huge area called “solid state physics” or “condensed matter” physics.
Gary then went on to some of the other effects seen in E&M. These are the classical effects where: 1) time varying magnetic fields can create an electric field (Faradays Law of Induction) and 2) magnetic fields can be created by either currents or time varying electric fields (Ampere’s Law with Maxwell’s addition).
With these last two effects, Gary had marched through the four “Maxwell’s Equations” which succinctly describe classical E&M. These equations, which you can see on numerous T-shirts, come in integral, differential and tensor for (each being a bit more compact), and are some of the most beautiful laws of nature.
I think the CCAS should thank Gary for a Herculean effort to shed some light (electromagnetic waves, of course) on this beautiful subject and hope that the Electromagnetic Force () may be with him!!!!
2017 May – Stella Kafka, Director, AAVSO. An overview of the American Association of Variable Star Observers and its programs.