Download 22.2 Production of Electromagnetic Waves Oscillating charges will

Chapter 22
Electromagnetic Waves
© 2006, B.J. Lieb
Some figures electronically reproduced by permission of
Pearson Education, Inc., Upper Saddle River, New Jersey
Giancoli, PHYSICS,6/E © 2004.
Ch 22
1
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
Ch 22
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.
2
22.2 Production of Electromagnetic Waves
Oscillating charges will produce
electromagnetic waves:
Ch 22
3
Reception of Radio Waves
•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
Antenna
4
22.2 Production of Electromagnetic Waves
The electric and magnetic waves are
perpendicular to each other, and to the
direction of propagation.
Ch 22
5
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 naturesome 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
6
Electromagnetic Spectrum
c  f
Ch 22
7
Electromagnetic Spectrum
c  f
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
Ch 22
8
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.
1
L  8.5 10 m    
4
2
 4L
 (4) (8.5  10 m)
2
  0.34 m
v f
v
3.0  10 m / s

f
0.34 m

3
f  8.8 108 Hz  880 MHz
Ch 22
9
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
10
22.7 Radio and Television;
Wireless Communication
Second, by using the signal to modify the
frequency of the carrier (FM):
Ch 22
11