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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
• 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
• 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
• 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
88.5 MHz. If you drive your car toward the
station at 32.0 m/s, what change in frequency do
you observer?

Dr. Jie Zou
PHY 1161
9
10
• 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.