PHYSICS 272 Electric & Magnetic Interactions

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1 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions PHYSICS 7 Electric & Magnetic Interactions Lecture 7 (last lecture) Electromagnetic Waves and Radiation [Chap 4] Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 1/

2 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions A Few Reminders Final Exam: 1/16 Thursday 1:-1: STEW 183 Special accommodation students: watch s Students with conflicts: contact me asap by and indicate if availability on 1/16 Thurs morning Equation sheet will be provided/posted There will be recitation and lab this week; the lab will be review with some final practice problems Grading issues/complaints bring to TA by next Tues (1/14) and Prof by 1/16 Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide /

3 B dl PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Integral form B nda ˆ div( B) dv B dl curl( B ) nda Maxwell s Equations Gauss s law Differential form q E nda ˆ inside div( E) E Gauss s law for magnetism B nˆda div( B) B E dl d dt Faraday s law B nˆ da curl( E ) Ampere-Maxwell law d elec Iinside_ path dt curl( B) B + Lorentz force F qe qv B Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 3/ E B t J E t

4 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Summary Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 4/

5 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions B-field Radiate outward in (almost) all directions x. B = qa c r 1 E radiative 4 The derivation* is given in The direction of the field is opposite to qa. The electric field falls off at a rate 1/r Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 5/

6 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Sinusoidal Electromagnetic Radiation a Acceleration: d y dt y max sin t qa c r 1 E radiative 4 f T 1/ f E radiative 1 4 qy c max r sint ĵ Sinusoidal E/M field Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 6/

PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions http://www.physics.purdue.

7 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Sinusoidal E/M Radiation: Wavelength Instead of period can use wavelength: ct c f f T 1/ f Freeze picture in time: Example of sinusoidal E/M radiation: atoms radio stations E/M noise from AC wires Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 7/

8 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Sinusoidal electromagnetic radiation An accelerated charge emits a single brief pulse of electromagnetic radiation The charge emits continuous radiation if it is moved sinusoidally Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 8/

9 Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 9/ PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Energy density in E & M fields: ) ( Volume Energy E c E E B E 1 c

10 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions E cb c 1 In a time t, a volume Volume A( ct) of E & M fields passes through the area A. The amount of energy is Energy E ( Act) Define energy flux as J/sec/m energy flux E c EBc Define the Poynting vector EB Its direction is the energy flow direction. 1 S E B 1 Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 1/

11 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Electromagnetic Radiation Carries Momentum Einstein s equation: E (pc) (mc ) For E&M radiation: E pc Define momentum flux is 1/c times the energy flux given by the Pointing vector S c 1 c E B in N/m The units of momentum flux are the same as pressure Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 11/

12 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 1/

13 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Incident radiation Re-radiation Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 13/

14 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Cardboard Why there is no light going through a cardboard? Electric fields are not blocked by matter Electrons and nucleus in cardboard reradiate light Behind the cardboard reradiated E/M field cancels original field Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 14/

15 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Effect of E/M Radiation on Matter 1. Radiative pressure too small to be observed in most cases. E/M fields can affect charged particles: nucleus and electrons (electrons in conductors, insulators, molecules ) Both fields (E and M) are always present they feed each other But usually only electric field is considered (B=E/c) Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 15/

16 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Interaction is particularly strong if incident radiation in resonance with electrons in matter E/M radiation waves with frequency ~1 6 Hz has big effect on mobile electrons in the metal of radio antenna: can tune radio to a single frequency E/M radiation with frequency ~ 1 15 Hz has big effect on organic molecules: retina in your eye responds to visible light but not radio waves Very high frequency (X-rays) has little effect on atoms and can pass through matter (your body): X-ray imaging Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 16/

17 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Polarized E/M Radiation AC voltage (~3 MHz) no light E/M radiation can be polarized along one axis and it can be unpolarized: polarized Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 17/

18 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Making polarized light Polarized Light Turning polarization Polaroid sunglasses and camera filters: reflected light is highly polarized: can block it Considered: using polarized car lights and polarizers-windshields Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 18/

19 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions In which of these situations will the bulb light? A) A B) B C) C D) None E) B and C Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 19/

20 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide /

21 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions Why there is light coming from the sky? Why is it polarized? Why is it blue? x xa sin t E ~ a d x dt Energy flux: Why the Sky is Blue y sin t A 4 ~ E ~ z y N,O Ratio of blue/red frequency is ~ scattering intensity ratio is 16 Why is sun red at sunset? Why are distant mountains blue? Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide 1/

22 PHYS 7: Matter and Interactions II -- Electric And Magnetic Interactions A Few Reminders Final Exam: 1/16 Thursday 1:-1: STEW 183 Special accommodation students: watch s Students with conflicts: contact me asap by and indicate if availability on 1/16 Thurs morning Equation sheet will be provided/posted There will be recitation and lab this week; the lab will be review with some final practice problems Grading issues/complaints bring to TA by next Tues (1/14) and Prof by 1/16 Fall 1 Prof. Yong Chen (yongchen@purdue.edu) Prof. Michael Manfra (mmanfra@purdue.edu) Lec7 Slide /

Re-radiation: Scattering. Electric fields are not blocked by matter: how can E decrease?

Re-radiation: Scattering. Electric fields are not blocked by matter: how can E decrease? Re-radiation: Scattering lectric fields are not blocked by matter: how can decrease? Cardboard Why there is no light going through a cardboard? lectric fields are not blocked by matter lectrons and nucleus