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Monday, May 26, 2014

CONGRATULATIONS, Seniors! You will be missed!

Another year, another WONDERFUL group of young men and women who have completed ETHS and are moving on to college, gap years, and amazing lives!  You will be missed, but also I couldn't be happier for you as you enter the next major phase of your young lives.  Enjoy it all, college is one of the best times in life, and make the most of your opportunities and many gifts and talents.  Try to make the world just a bit better!

Wednesday, May 21, 2014

Check out an Interview with Prof. Melissa Franklin, first tenured female physics prof at Harvard

Here is an interview with Melissa Franklin, the first female professor of physics to receive tenure from Harvard back in 1992.  She was a colleague at Fermilab, back in the days of the top quark discovery. She is an interesting person!

Monday, May 12, 2014

New Report on West Antarctica Ice Shelf - Collapse "Unstoppable"

Scientists have long feared and warned the world about the collapse of enormous glaciers and ice shelves on the western side of Antarctica, and new satellite and radar data analyses confirm some worst fears - it is melting at rates that exceed predictions, and within a couple centuries (incredibly short in geologic time) oceans would rise several feet. The video below has a brief summary of what measurements are suggesting.

Sunday, May 4, 2014

Maxwell's Equations and EM Waves

This is the portion of the Caltech series, Mechanical Universe, that shows what the four Maxwell equations are!  It is very well done.

The KEY IDEAS you need to remember is that, in electromagnetism, when a magnetic field changes through an area, i.e. a change in magnetic flux, then an electric field is induced that is perpendicular to the magnetic field  (such as a circulation).  But with the Maxwell displacement current, when there is a changing electric field and flux, a magnetic field is induced!  These induction processes are what creates electromagnetic fields in space!!

So if we have AdB/dt, an E-field is induced.
If we have AdE/dt, a B-field is induced.

 

Maxwell's Displacement Current - How Capacitor Circuits Work

Many students have a difficult time understanding capacitors...I certainly did.  How in the world can a circuit with a cut wire, where the capacitor is, have current flow?  If you actually cut a wire in any other circuit the current dies!  What is different about a capacitor?

The answer lies in the fact that a capacitor has a large area where charge stores, and therefore has an electric field that flows across the gap.  You don't get this with the two ends of a cut wire.  So there is something special about that electric field.  Remember, there physically is no current flowing across the gap, only the electric field.

James Clerk Maxwell figured out the details back in the mid 1800s.  This video will explain the details of what Maxwell discovered. He showed that, since it takes some time for a capacitor to charge up (think of our RC circuit derivations) or discharge, then the E-field is time dependent - it changes.  This means there is a change in electric flux across the gap of a capacitor.  Now, he knew Faraday's law said that when there is a changing magnetic field, then a circulating electric field is induced.  Does it make sense that if an electric field changes a circulating magnetic field should be induced?  Where have we seen circulating magnetic fields before?  These are produced with electric currents in straight wires!  This is Ampere's law.  Could the changing electric flux of a capacitor act like a current in a wire?  We call this the displacement current.

Turns out, YES!  Check it out.