Ch 22 1

Chapter 22

Electromagnetic Waves

© 2006, B.J. LiebSome figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey

Giancoli, PHYSICS,6/E © 2004.

Ch 22 2

Maxwell’s Equations

1. Coulomb’s Law ( Charges produce electric fields )

2. Equation for Magnetic Field (Currents produce magnetic fields )

3. Induction – changing magnetic field produces an electric field

4. Changing electric field produces a magnetic field

# 4 was added by Maxwell

Maxwell combined these equations and the result was an equation similar to a wave equation and thus he predicted a new type of wave consisting of electric and magnetic fields. Furthermore the equation predicted the velocity of this “new” type of wave and it was the velocity of light. This suggested that light was an electromagnetic phenomena.

Ch 22 3

22.2 Production of Electromagnetic Waves

Oscillating charges will produce electromagnetic waves:

Ch 22 4

Reception of Radio Waves

Antenna

•Oscillating vertical electric fields cause oscillating currents in the antenna.

•Oscillating horizontal magnetic field can induce a oscillating current in a loop antenna

Ch 22 5

22.2 Production of Electromagnetic WavesThe electric and magnetic waves are perpendicular to each other, and to the direction of propagation.

Ch 22 6

Nature of Electromagnetic Radiation

The success of Maxwell’s Equation appeared to be clear proof that light was a wave phenomena, but we will see in Ch 27 that Einstein suggested that light had a dual nature- some experiments show wave properties and others show particle properties.

•For wave properties we use c = f where c is the speed of light, is the wavelength and f is the frequency.

•For the particle properties light is a stream of particles called photons which have energy but have zero “rest” mass. The energy of a photon is proportional to its frequency.

Ch 22 7

Electromagnetic Spectrum

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Ch 22 8

Electromagnetic Spectrum

You are expected to know the order of the following parts of the electromagnetic spectrum. These are listed in order of increasing frequency and therefore in order of increasing energy of the photon.

•Radio Waves

•Microwaves

•Infrared

•Visible

•Ultraviolet

•X-Rays

•Gamma Rays

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Ch 22 9

Ex 22-1: The antenna of a cell phone is often ¼ wavelength long. A particular cell phone has a 8.5 cm long straight rod for its antenna. Estimate the operating frequency of the phone.

4

1105.8 2 mL

L4 )105.8()4( 2 m

m34.0

fv

v

f m

sm

34.0

/100.3 3

MHzHzf 880108.8 8

Ch 22 10

22.7 Radio and Television; Wireless Communication

The mixing of signal and carrier can be done two ways. First, by using the signal to modify the amplitude of the carrier (AM):

Ch 22 11

22.7 Radio and Television; Wireless Communication

Second, by using the signal to modify the frequency of the carrier (FM):