PHYSICS 272 Electric & Magnetic Interactions Lecture 27 (last lecture) Electromagnetic Waves and Radiation [Chap 24]

A Few Reminders Final Exam: 12/16 Thursday 10:20-12:20 STEW 183 Special accommodation students: watch emails Students with conflicts: contact me asap by email and indicate if availability on 12/16 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 (12/14) and Prof by 12/16

Gauss’s law for magnetism Maxwell’s Equations Integral form Differential form Gauss’s law Gauss’s law for magnetism Faraday’s law Ampere-Maxwell law + Lorentz force 3

Summary

. x 1. The direction of the field is opposite to qa Radiate outward in (almost) all directions B-field . x B = 0 The derivation* is given in 24.11 1. The direction of the field is opposite to qa 2. The electric field falls off at a rate 1/r

Sinusoidal Electromagnetic Radiation Acceleration: Sinusoidal E/M field 6

Sinusoidal E/M Radiation: Wavelength Instead of period can use wavelength: Freeze picture in time: Example of sinusoidal E/M radiation: atoms radio stations E/M noise from AC wires 7

Sinusoidal electromagnetic radiation An accelerated charge emits a single brief pulse of electromagnetic radiation The charge emits continuous radiation if it is moved sinusoidally

Energy density in E & M fields:

In a time ∆t, a volume of E & M fields passes through the area A. The amount of energy is Define energy flux as J/sec/m2 Define the “Poynting vector” Its direction is the energy flow direction.

Electromagnetic Radiation Carries Momentum Einstein’s equation: For E&M radiation: Define momentum flux is 1/c times the energy flux given by the Pointing vector The units of momentum flux are the same as pressure in N/m2

Incident radiation Re-radiation

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 14

Effect of E/M Radiation on Matter 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) 15

Interaction is particularly strong if incident radiation “in resonance” with electrons in matter E/M radiation waves with frequency ~106 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 ~ 1015 Hz has big effect on organic molecules: retina in your eye responds to visible light but not radio waves Organic molecules – electronic system is in resonance with E/M frequencies corresponding to ‘visible’ light Very high frequency (X-rays) has little effect on atoms and can pass through matter (your body): X-ray imaging 16

Polarized E/M Radiation AC voltage (~300 MHz) E/M radiation can be polarized along one axis… no light …and it can be unpolarized: At every single time E is of course in just one direction, but it changes randomly and rapidly over time Light from the sun is not polarized! polarized 17

Polarized Light Making polarized light Turning polarization Polaroid sunglasses and camera filters: Show trick with three polarizers – two crossed ones, add one at 45 degrees in the middle Sheet of special plastic whose long molecules are aligned with each other Sunglasses reflected light is highly polarized: can block it Considered: using polarized car lights and polarizers-windshields 18

In which of these situations will the bulb light? None B and C A 19

C 20

Why the Sky is Blue Why there is light coming from the sky? N2,O2 Why is it polarized? Why is it blue? z A A Energy flux: No such effect on moon - black sky even if sun can be seen. It is polarized – use polaroid sunglasses to check it Why are distant mountains blueish? Blue: High frequency Ratio of blue/red frequency is ~2  scattering intensity ratio is 16 Why is sun red at sunset? Why are distant mountains blue? 21

A Few Reminders Final Exam: 12/16 Thursday 10:20-12:20 STEW 183 Special accommodation students: watch emails Students with conflicts: contact me asap by email and indicate if availability on 12/16 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 (12/14) and Prof by 12/16