Your Comments. What does signify?

Size: px

Start display at page:

Download "Your Comments. What does signify?"

Dorothy Wilkerson
5 years ago
Views:

1 Your Comments WHAT? "POYNTING" VECTOR NOT "POINTING" VECTOR? I THOUGHT I COUGHT A SPELLING MISTAKE!!!!!!!! I must now see i<clicker waves! I feel like the material is very, very abstract and general and is not as easy to intuitively grasp. Hopefully this vagueness will go away after seeing it in class. Also, I feel like the last class we had (the first one on E-M waves) did not really give us what we needed to solve the homework problems. I like where we are going. Light and optics sounds really cool, and I'm excited. Just got to to remember to not forget the physics along the way. EVERYONE RUN TOWARDS THEIR I-CLICKERS. What does signify? You know what I dont get? this particle-wave duality business. I mean it just doesn't make any sense, you have physics that apply to the natural world but when you go quantum its a whole different game. This pre-lecture wasn't too bad, I'm excited to c what we do with it in class!!!! 05

2 Physics 1 Lecture 3 Electricity & Magnetism Lecture 3, Slide

3 Plane Waves from Last Time E and B are perpendicular and in phase Oscillate in time and space Direction of propagation given by E X B E 0 = cb 0 Argument of sin/cos gives direction of propagation Electricity & Magnetism Lecture 3, Slide 3

4 Understanding the speed and direction of the wave E x E o sin(kz - t) E x z t 0 E x sin( kz- ) -cos( kz) speed / 4 c f / z t /() What has happened to the wave form in this time interval? It has MOVED TO THE RIGHT by l/4 Electricity & Magnetism Lecture 3, Slide 4

5 CheckPoint 1a No moving in the minus z direction No has E y rather than E x Electricity & Magnetism Lecture 3, Slide 5

6 CheckPoint a C 3.0 x 10 8 m/s Wavelength is equal to the speed of light divided by the frequency. f c 300,000, ,000, Check: Look at size of antenna on base unit Electricity & Magnetism Lecture 3, Slide 6

7 Doppler Shift The Big Idea As source approaches: Wavelength decreases Frequency Increases Electricity & Magnetism Lecture 3, Slide 7

8 Doppler Shift for E-M Waves What s Different from Sound or Water Waves? Sound /Water Waves : You can calculate (no relativity needed) BUT Result is somewhat complicated: is source or observer moving wrt medium? Electromagnetic Waves : You need relativity (time dilation) to calculate BUT Result is simple: only depends on relative motion of source & observer f ' 1 b f 1- b 1 b v/c b > 0 if source & observer are approaching b < 0 if source & observer are separating Electricity & Magnetism Lecture 3, Slide 8

9 Doppler Shift for E-M Waves f f v or f f The Doppler Shift is the SAME for both cases! f /f only depends on the relative velocity f ' 1 b f 1- b 1 v Electricity & Magnetism Lecture 3, Slide 9

10 Doppler Shift for E-M Waves A Note on Approximations 1 1 b f ' f b << 1 f ' f 1 b 1- b why? 1 b Taylor Series: Expand Fb around b 0 1- b Evaluate: F( 0) 1 F( 0) 1 F b F'(0) F (0) F( 0) b b 1!! 1 F( b) 1 b NOTE: b) (1 F( b) 1/ 1 F( b) 1 b Electricity & Magnetism Lecture 3, Slide 10

11 wavelength Red Shift Wavelengths shifted higher Frequencies shifted lower Star separating from us (Expanding Universe) Our Sun Star in a distant galaxy Electricity & Magnetism Lecture 3, Slide 11

4,000,000,000 Hz (k-band radar gun) c = 300,000,000 m/s 1.

12 Example Police radars get twice the effect since the EM waves make a round trip: f f 1 b v b f f -f 30 m/s (67 mph) 31 m/s (69 mph) If f = 4,000,000,000 Hz (k-band radar gun) c = 300,000,000 m/s x ,000,004, Hz x ,000,004, Hz Electricity & Magnetism Lecture 3, Slide 1

13 CheckPoint b A) B) C) f iclicker = 900 MHz Need to shift frequency UP Need to approach i>clicker b > 0 How fast would you need to run to see the i>clicker radiation? f f b 1- b 1/ 1 b 1- b 10 b Approximation Exercise: b 1- (10 ) Electricity & Magnetism Lecture 3, Slide 13

14 Waves Carry Energy Electricity & Magnetism Lecture 3, Slide 14

15 Intensity Intensity Average energy delivered per unit time, per unit area I 1 A du dt Length c dt Area A du u volume u Acdt I c u Sunlight on Earth: I ~ 1000J/s/m ~ 1 kw/m Electricity & Magnetism Lecture 3, Slide 15

16 Waves Carry Energy Electricity & Magnetism Lecture 3, Slide 16

17 Comment on Poynting Vector Just another way to keep track of all this: Its magnitude is equal to I Its direction is the direction of propagation of the wave Electricity & Magnetism Lecture 3, Slide 17

18 Power in EM Waves: Example A cell phone tower has a transmitter with a power of 100 W. What is the magnitude of the peak electric field a distance 1500 m (~ 1 mile) from the tower? Assume the transmitter is a point source. What is the intensity of the wave 1500 m from the tower? A) 1.5 nw/m B) 3.5 W/m C) 6 mw/m I S E B I P 100 W W 3.5 4r m m What is the peak value of the electric field? 0 E E c 0 1 c 0 E 0 E ci 0 0 E / mv m

Checkpoint 1 b Which of the following actions will increase the energy carried by an electromagnetic wave? A. Increase E keeping constant B. Increase keeping E constant C.

19 Checkpoint 1 b Which of the following actions will increase the energy carried by an electromagnetic wave? A. Increase E keeping constant B. Increase keeping E constant C. Both of the above will increase the energy D. Neither of the above will increase the energy But then again, what are we keeping constant here? WHAT ABOUT PHOTONS? The energy of one photon is E photon = hf = h/ U wave = N photons x E photon E photon / Volume = 1/ e 0 E 0 Electricity & Magnetism Lecture 3, Slide 1

20 Photons We believe the energy in an e-m wave is carried by photons Question: What are Photons? Answer: Photons are Photons. Photons possess both wave and particle properties Particle: Energy and Momentum localized Wave: They have definite frequency & wavelength ( f c) Connections seen in equations: E hf p h/ Planck s constant h 6.63e -34 J - s Question: How can something be both a particle and a wave? Answer: It can t (when we observe it) What we see depends on how we choose to measure it! The mystery of quantum mechanics: More on this in PHYS 14 Electricity & Magnetism Lecture 3, Slide

Exercise An electromagnetic wave is described by:

Which of the following graphs represents the z -

ˆjE 0 cos kz-t y z x X X A B C D E and B are in

moves in z direction y E x E B Points in direction

21 Exercise An electromagnetic wave is described by: E where ĵ is the unit vector in the y direction. Which of the following graphs represents the z - dependence of B x at t 0? ˆjE 0 cos kz-t y z x X X A B C D E and B are in phase (or 180 o out of phase) E ˆjE 0 coskz-t Wave moves in z direction y E x E B Points in direction of propagation B z B -ib ˆ 0 cos kz-t Electricity & Magnetism Lecture 3, Slide 3

22 An electromagnetic wave is described by: Exercise E iˆ ˆj E 0 cos kzt y x What is the form of B for this wave? z A) B iˆ ˆj E / ccoskzt 0 C) B -iˆ ˆj E / ccoskzt 0 B) B iˆ - ˆj E / ccoskzt 0 D) B -iˆ - ˆj E / ccoskzt 0 E iˆ ˆj E 0 cos kzt Wave moves in -z direction y E x z points out of screen -z points into screen E B B Points in direction of propagation Electricity & Magnetism Lecture 3, Slide 4

23 Exercise A certain unnamed physics professor was arrested for running a stoplight. He said the light was green. A pedestrian said it was red. The professor then said: We are both being truthful; you just need to account for the Doppler effect! Is it possible that the professor s argument is correct? ( green 500 nm, red 600 nm) As professor approaches stoplight, the frequency of its emitted light will be shifted UP The speed of light does not change A) YES B) NO Therefore, the wavelength (c/f) would be shifted DOWN If he goes fast enough, he could observe a green light! Electricity & Magnetism Lecture 3, Slide 6

24 Follow-Up A certain unnamed physics professor was arrested for running a stoplight. He said the light was green. A pedestrian said it was red. The professor then said: We are both being truthful; you just need to account for the Doppler effect! How fast would the professor have to go to see the light as green? ( green 500 nm, red 600 nm) A) 540 m/s B) 5.4 x 10 4 m/s C) 5.4 x 10 7 m/s D) 5.4 x 10 8 m/s Relativistic Doppler effect: f f 1 b 1- b f f b 1- b 11 36(1 - b) 5(1 b) b Note approximation for small b is not bad: f f ( 1 b) 1 b 5 0. c 3 x 10 8 m/s -> v 5.4 x 10 7 m/s Change the charge to speeding! Electricity & Magnetism Lecture 3, Slide 7

Physics 212. Lecture 23. Physics 212 Lecture 23, Slide 1

Physics 212. Lecture 23. Physics 212 Lecture 23, Slide 1 Physics 212 Lecture 23 Physics 212 Lecture 23, Slide 1 Music Who is the Artist? A) Soul Rebels Brass Band B) John Boutte C) New Orleans Nightcrawlers D) Paul Sanchez & Shammar Allen E) Alex McMurray and