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Electromagnetic Waves

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Jie Zou PHY 1161 * Propagation of an Electromagnetic Wave E and B are ... of the circuit matches the frequency of the radio wave. Dr. Jie Zou PHY 1161 ... – PowerPoint PPT presentation

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Title: Electromagnetic Waves


1
Chapter 25
  • Electromagnetic Waves

2
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

3
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

4
Propagation of an Electromagnetic Wave
  • 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

5
Production of Electromagnetic Waves
Example Production of electromagnetic waves by
means of an electric circuit and an antenna
  • Accelerated charges radiate electromagnetic waves.

6
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.

7
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.

8
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
  • 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
9
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
10
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
11
Homework 10
  • Chapter 25, P. 901, Problems 23, 24 (Physics,
    Walker, 4th edition).

Dr. Jie Zou
PHY 1161
11
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