Transcript Welcome to 1161 Principles of Physics II

Chapter 25
Electromagnetic Waves
Dr. Jie Zou
PHY 1161
1
Outline






Introduction
Propagation of electromagnetic waves
Production of electromagnetic waves
 Receiving radio waves
Electromagnetic spectrum
Doppler Effect
Common applications of the Doppler effect:


Radar units used to measure the speed of automobiles
Doppler radar used to monitor weather
Dr. Jie Zou
PHY 1161
2
Introduction



Electromagnetism: we refer to the
phenomena of electricity and magnetism
together as electromagnetism.
Electromagnetic waves: the electric and
magnetic fields can work together to create
traveling waves called electromagnetic
waves.
Examples:



Radio and TV signals
Visible light we see all around us
X rays that reveal our internal structure…
Dr. Jie Zou
PHY 1161
3
Propagation of an
Electromagnetic Wave




Dr. Jie Zou
E and B are perpendicular to
each other at all times and are
both perpendicular to the
direction of propagation.
Electromagnetic waves are
transverse waves.
All electromagnetic waves
travel through a vacuum with
the same speed, c.
c: The speed of light in a
vacuum, c = 3.00 x 108 m/s
PHY 1161
4
Production of Electromagnetic
Waves
Example: Production of electromagnetic waves by means of an
electric circuit and an antenna

Accelerated charges radiate electromagnetic waves.
Dr. Jie Zou
PHY 1161
5
Receiving Radio Waves


The electromagnetic field makes the receiving antenna
behave much like an ac generator.
The resulting current can be large if the resonant frequency
(determined by L and C) of the circuit matches the
frequency of the radio wave.
Dr. Jie Zou
PHY 1161
6
Electromagnetic Spectrum


All electromagnetic waves in a vacuum have the same
speed, c.
The frequency and wavelength are related as follows:
c = f .

As the frequency of an electromagnetic wave increases, its
wavelength decreases.
Dr. Jie Zou
PHY 1161
7
The Doppler Effect

Doppler effect for electromagnetic waves: For
source speeds u that are small compared with the
speed of light, the observed frequency f’ from a
source with frequency f is
 u
f '  f 1  
 c



“+”: When the source is approaching the observer.
“-”: When the source is receding from the observer.
u: a relative speed between the source and observer, both
of which may be moving.
Dr. Jie Zou
PHY 1161
8
Exercise 25-2


An FM radio station broadcasts at a
frequency of 88.5 MHz. If you drive
your car toward the station at 32.0 m/s,
what change in frequency do you
observer?
Answer: 9.44 Hz.
Dr. Jie Zou
PHY 1161
9
Example 25-2: Doppler
Weather Radar

The Doppler weather radar used
by the National Weather Service
(Nexrad ) commonly operates at
a frequency of 2.7 GHz. If a
Nexrad wave reflects from an
approaching weather system
moving with a speed of 28 m/s,
find the difference in frequency
between the outgoing and
returning waves. (Answer: 500
Hz)
Dr. Jie Zou
PHY 1161
10
Homework #10

Chapter 25, P. 901, Problems: #23, 24
(Physics, Walker, 4th edition).
Dr. Jie Zou
PHY 1161
11