Ch. 25:

EM Radiation

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)

E-M Radiation

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 = I2B.) I1 < I2C.) I1 > I2D.) There is no current.E.) Not a thing.

Fin.