1. Dr. Jie ZouPHY 11611 Chapter 25 Electromagnetic Waves

2. Dr. Jie ZouPHY 11612 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. Dr. Jie ZouPHY 11613 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. Dr. Jie ZouPHY 11614 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 10 8 m/s

5. Dr. Jie ZouPHY 11615 Accelerated charges radiate electromagnetic waves. Production of Electromagnetic Waves Example: Production of electromagnetic waves by means of an electric circuit and an antenna

6. Dr. Jie ZouPHY 11616 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. Dr. Jie ZouPHY 11617 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. Dr. Jie ZouPHY 11618Dr. Jie ZouPHY 11618 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.

9. Dr. Jie ZouPHY 11619Dr. Jie ZouPHY 11619 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.

10. Dr. Jie ZouPHY 116110 Dr. Jie ZouPHY 116110 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)

11. Dr. Jie ZouPHY 116111Dr. Jie ZouPHY 116111 Homework #10 Chapter 25, P. 901, Problems: #23, 24 (Physics, Walker, 4 th edition).