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James Clerk Maxwell
E-field lines originate on positive charges and terminate on negative charges (Gauss’ Law)
B-field lines always form closed loops.
Changing B-field induces an emf, an E-field (Faraday’s Law).
B-fields are created by moving charges, currents (Ampere’s Law).
Maxwell Predicted…(1865)
Electric and Magnetic fields are interrelated.
Varying E-field with time yields B-field.
Solution of Maxwell’s Equations in vacuum yield propagating, fluctuating, electric and magnetic fields
Each varying field induces the other.
Experimentally validated Maxwell’s EM theories.
First to generate and detect EM waves in the laboratory.
Heinrich Hertz (1887)
Radio Wave Transmission
E-field wave: High-frequency reversal of the polarity of an
electric dipole creates a varying electric field (time and space).
E
(x,t)
Radio Wave Transmission
B-field Wave: Changing polarity of a dipole creates a current. Changing current induces a varying magnetic
field.
B
(x,t)
Reception of Radio Waves
E-wave Reception: Antenna E-wave supplies changing E-field at wire
Induces AC current
B-wave Reception: Loop Antenna B-wave sets a changing magnetic field
through the wire loop varying magnetic flux induces a varying EMF Induces AC current
Blackbody Radiation All bodies emit thermal emission due to
movement* of charges in matter. The emission spectrum is temperature dependent.
Stars, fire, incandescent light bulbs, lava, hot coals, etc. peak in the visible part of the spectrum.
Humans’ thermal emission peaks in the infrared, ~12 μm.
Polarization of EM Waves
EM waves from radio antenna are plane (linearly) polarized. Created by charges oscillating in a line.
Thermal emission sources are non-polarized Created by random thermal motion.
Polarization of EM Waves Non-polarized light can be polarized by use of
Polaroid sheets. Consist of plastic with needle-like crystals oriented
lengthwise Allow only one orientation of wave to pass
Sheet parallel to polarization passes light unimpeded. Sheet perpendicular to polarization reduces flux to 0.
Two perpendicular Polaroid sheets reduces EM flux to 0.
Polarized sunglasses 3D-movies!
H-ITT You observe two otherwise identical, thin
tungsten filaments. Filament 1 glows red. Filament 2 glows yellow/white. What can you say about the respective currents through the filaments?
A.) I1 = I2B.) I1 < I2C.) I1 > I2D.) There is no current.E.) Not a thing.