Lecture 17

Summary

Faraday's law
Lenz's law
Transformers
Maxwell's equations

Electromagnetic radiation

electromagnetic waves applet
production by accelerating charges

Click to Run

the speed of light

Example #1

electromagnetic spectrum

Example #2

Example #3

EM Spectrum from Chaisson 5th ed Figure 3.9

the Doppler effect for light

Example #4

Analysis of radar returns for the May 31, 2013 tornado near El Reno, OK. This image depicts the locations of rain drops based on the time delay between when the radar waves were emitted and when they returned to the antenna. Credit: Steve Horstmeyer

This second image, taken at about the same time as the previous, indicates the velocity of the rain drops by using the Doppler effect. Red is leftward air movement (away from the radar antenna), green is rightward air movement. The tornado (red triangle) is centered at the boundary between the two. The yellow shape indicates the damage path of the tornado (determined later, after the storm had passed).

energy delivered by EM waves

Example #5

This animation of dipole radiation is from Wikipedia. You can also explore an interactive applet by Paul Falstadt. Choose "Oscillating dipole" from the Setup list. There is also a 3-D version but it's a little more difficult to see what's going on. Choose "dipole source" from the Setup list.

Lecture learning outcomes
A student who masters the topics in this lecture will be able to:

explain the production of electromagnetic waves in terms of accelerating charges
use the speed of light to predict distance from a source or time elapsed during propagation
use algebra to find the wave speed c, frequency f, or wavelength λ when any two of these quantities are given
describe the frequency shift (higher or lower) detected by an observer when there is relative radial motion between the observer and the source
use algebra to find the observed frequency f', source frequency f, or relative radial velocity u when any two of these quantities are given
use algebra to find the electric field amplitude E, magnetic field amplitude B, or speed of propagation c when any two of these quantities are given
use algebra to find the average intensity I_av or electric field amplitude E when either of these quantities is given

Practice:
Try these additional examples

Prepare:
Read textbook section 25-5 before the next lecture

klm
In March 2005 Steve Fossett flew the Virgin Atlantic GlobalFlyer 36,912 km in 67 hours to circumnavigate the world. In how much time would an EM wave complete the same trip?
A. 0.123 s
B. 8.10 s
C. 76.7 µs
D. 3.62 h
Answer

klm
What is the wavelength of radio waves broadcast by AM 620 WTMJ? (AM station numbers correspond to the broadcast frequency in kHz.)
A. 2.07 mm
B. 484 m
C. 1.86×10¹⁴ m
D. 3.00×10⁸ m
Answer

klm
What would be the beat frequency if radio waves of wavelength 3.141 m and 3.135 m are mixed?
A. 50 GHz
B. 6.0 MHz
C. 609 kHz
D. 183 kHz
Answer

Walker4 25.21
Suppose a particular galaxy emits orange light with a frequency of 5.000×10¹⁴ Hz. If the galaxy is receding from Earth with a speed of 3325 km/s, what is the frequency of the light when it reaches Earth?
A. 5.109×10¹⁴ Hz
B. 4.889×10¹⁴ Hz
C. 5.055×10¹⁴ Hz
D. 4.945×10¹⁴ Hz
Answer

sj6 34.16
What is B_max for an EM wave emitted by a 10 kW radio station 5 km away?
A. 33.3 µT
B. 338 nT
C. 0.516 nT
D. 19.5 mT
Answer

Walker5e 25.14a
When an airplane uses microwave communication with a satellite at a frequency of 1.535×10⁹ Hz, the signal received by the satellite is shifted higher by 207 Hz. Is the airplane moving toward or away from the satellite?
A. toward
B. away
Answer

Walker5e 25.14a2
When an airplane uses microwave communication with a satellite at a frequency of 1.535×10⁹ Hz, the signal received by the satellite is shifted higher by 207 Hz. Is the speed of the microwaves detected by the satellite greater than, less than, or equal to c?
A. greater than c
B. less than c
C. equal to c
Answer

Walker5e 25.14b
When an airplane communicates with a satellite at a frequency of 1.535×10⁹ Hz, the signal received by the satellite is shifted higher by 207 Hz. What is the component of the airplane's velocity toward the satellite?
A. 0.0202 m/s
B. 40.5 m/s
C. 318 km/s
D. 2.22×10¹⁵ m/s
Answer

Walker5e 25.49
What is the maximum value of the electric field in an electromagnetic wave whose average intensity is 7.55 W/m²?
A. 75.4 V/m
B. 107 V/m
C. 302 V/m
D. 5690 V/m
Answer

A. 0.123 s
solution equation

B. 484 m

D. 183 kHz
solution equation

D. 4.945×10¹⁴ Hz
solution equation

C. 0.516 nT
solution equation

A. toward
A component of the airplane's velocity must be toward the satellite because the received frequency is higher than the emitted frequency.

C. equal to c
The speed of light is equal to c, independent of the speed of the source.

B. 40.5 m/s
solution equation

A. 75.4 V/m
solution equation