Chapter 25. Electromagnetic Waves

Transcription

1 Chapter 25 Electromagnetic Waves

2 EXAM # 3 Nov Chapter 23 Chapter 25 Powerpoint Nov. 4 Problems from previous exams Physics in Perspective (pg )

3 Chapter 25 Electromagnetic Waves

4 Units of Chapter 25 The Production of Electromagnetic Waves The Propagation of Electromagnetic Waves The Electromagnetic Spectrum Energy and Momentum in Electromagnetic Waves Polarization

5 A transformer for home use of a portable radio reduces 120-V ac to 9.0 V. The secondary coil contains 30 turns and the radio draws 400 ma. Calculate (a) the number of turns on the primary coil, (b) the current in the primary, and the power transformed. If the power in the primary coil is the same as the power in the secondary coil. There is 100 % efficiency

6 James Clerk Maxwell Electricity and magnetism were originally thought to be unrelated in 1865, James Clerk Maxwell provided a mathematical theory that showed a close relationship between all electric and magnetic phenomena

7 Maxwell s Starting Points Electric field lines originate on positive charges and terminate on negative charges Magnetic field lines always form closed loops they do not begin or end anywhere A varying magnetic field induces an emf and hence an electric field (Faraday s Law) Magnetic fields are generated by moving charges or currents (Ampère s Law)

8 Maxwell s Predictions Maxwell used these starting points and a corresponding mathematical framework to prove that electric and magnetic fields play symmetric roles in nature He hypothesized that a changing electric field would produce a magnetic field Maxwell calculated the speed of light to be 3x10 8 m/s He concluded that visible light and all other electromagnetic waves consist of fluctuating electric and magnetic fields, with each varying field inducing the other

9 Hertz s Confirmation of Maxwell s Predictions Heinrich Hertz was the first to generate and detect electromagnetic waves in a laboratory setting

10 Hertz s Basic LC Circuit When the switch is closed, oscillations occur in the current and in the charge on the capacitor When the capacitor is fully charged, the total energy of the circuit is stored in the electric field of the capacitor At this time, the current is zero and no energy is stored in the inductor

11 LC Circuit, cont As the capacitor discharges, the energy stored in the electric field decreases At the same time, the current increases and the energy stored in the magnetic field increases When the capacitor is fully discharged, there is no energy stored in its electric field The current is at a maximum and all the energy is stored in the magnetic field in the inductor The process repeats in the opposite direction There is a continuous transfer of energy between the inductor and the capacitor

12 Hertz s Conclusions Hertz hypothesized the energy transfer was in the form of waves These are now known to be electromagnetic waves Hertz confirmed Maxwell s theory by showing the waves existed and had all the properties of light waves They had different frequencies and wavelengths

13 Hertz s Measure of the Speed of the Waves Hertz measured the speed of the waves from the transmitter He used the waves to form an interference pattern and calculated the wavelength From v = f λ, v was found v was very close to 3 x 10 8 m/s, the known speed of light This provided evidence in support of Maxwell s theory

14 Production of Electromagnetic Waves Produced by an Antenna When a charged particle undergoes an acceleration, it must radiate energy If currents in an ac circuit change rapidly, some energy is lost in the form of em waves EM waves are radiated by any circuit carrying alternating current An alternating voltage applied to the wires of an antenna forces the electric charge in the antenna to oscillate

15 EM Waves by an Antenna, cont Two rods are connected to an ac source, charges oscillate between the rods (a) As oscillations continue, the rods become less charged, the field near the charges decreases and the field produced at t = 0 moves away from the rod (b) The charges and field reverse (c) The oscillations continue (d)

16 Electromagnetic fields are produced by oscillating charges.

17 EM Waves by an Antenna Because the oscillating charges in the rod produce a current, there is also a magnetic field generated As the current changes, the magnetic field spreads out from the antenna

18 Electromagnetic Waves are Transverse Waves The E and B fields are perpendicular to each other Both fields are perpendicular to the direction of motion Therefore, em waves are transverse waves

19 An electromagnetic wave propagating in the positive x direction, showing the electric and magnetic fields:

20 The direction of propagation and the directions of the electric and magnetic fields in an electromagnetic wave can be determined using a right-hand rule: Point the fingers of your right hand in the direction of E, curl your fingers toward B, and your thumb will point in the direction of propagation.

21 Any time an electric charge is accelerated, it will radiate: Accelerated charges radiate electromagnetic waves.

22 Charges and Fields, Summary Stationary charges produce only electric fields Charges in uniform motion (constant velocity) produce electric and magnetic fields Charges that are accelerated produce electric and magnetic fields and electromagnetic waves

23 Electromagnetic Waves, Summary A changing magnetic field produces an electric field A changing electric field produces a magnetic field These fields are in phase At any point, both fields reach their maximum value at the same time

24 At a certain point in space, the electric and magnetic fields of an electromagnetic wave at a certain instant are given by E = +x 1. positive x-direction. 2. negative x-direction. 3. positive y-direction 4. negative y-direction r 5. None of the above B = + z This wave is propagating in the 21% 3% 26% 50% 0%

25 25-2 The Propagation of Electromagnetic Waves All electromagnetic waves propagate through a vacuum at the same rate: In materials, such as air and water, light slows down. In dense materials, such as glass or water, the speed of light is reduced to about half the above speed.

26 when you see the light that comes out of a glass of water the velocity of the light is 1. Less than 2. More than 3. Equal to 0% 0% 100% Than the speed of the light in vacuum

27 What good are electromagnetic waves? 1. They are good for nothing. 2. They are idealizations that physics students never see. 3. They come from outer space 4. Equal to An example is in this room 0% 3% 9% 88%

28 This speed is so large that it is very hard to measure; the first measurements were done in the late 1600s, using the eclipses of the moons of Jupiter.

29 Properties of EM Waves Electromagnetic waves are transverse waves Electromagnetic waves travel at the speed of light c Because em waves travel at a speed that is precisely the speed of light, light is an electromagnetic wave 1 o o

30 25-2 The Propagation of Electromagnetic Waves Light from the Andromeda Galaxy, left, takes about 2 million years to reach us. From the most distant galaxies in the Hubble Deep Field image, right, it takes 13 billion years.

31 Astronomers think that they have detected radiation from a star 15x10 9 light years away. How far is the star in m? d m/ s m 31 11/9/2013

32 25-2 The Propagation of Electromagnetic Waves The Doppler effect applies to electromagnetic waves as well as to sound waves. The speed of the waves in vacuum does not change, but as the observer and source move with respect to one another, the frequency does change.

33 25-2 The Propagation of Electromagnetic Waves Difference: a) Sound waves require a medium through which to travel b) The speed of sound can be different for different observers. The speed of electromagnetic waves in independent of the motion of the source and observer.

34 Which of the following states comparing sound waves to electromagnetic waves are false? 1. They both satisfy the wave equation 2. For both, the wave speed c is related to frequency and wavelength according to c=lf 3. They both travel through a medium 1. Equal to An example is in this room 2. The Doppler effect for sound waves is different in electromagnetic waves and in sound waves 0% 0% 0% 0% 0%