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.
Heinrich Hertz (1887) • Experimentally validated Maxwell’s EM theories. • First to generate and detect EM waves in the laboratory.
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
The Electromagnetic Spectrum c = λf
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 = I2 B.) I1 < I2 C.) I1 > I2 D.) There is no current. E.) Not a thing.