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Short Version : 29. Maxwell’s Equations & EM Waves.

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1 Short Version : 29. Maxwell’s Equations & EM Waves

2 29.1. The Four Laws of Electromagnetism LawMathematical Statement Physical Meaning Gauss for E How q produces E; E lines begin & end on q’s. Gauss for B Faraday Ampere (Steady I only) 4 Laws of EM (incomplete) No magnetic monopole; B lines form loops. Changing  B gives emf. Moving charges give B. Note E-B asymmetry between the Faraday & Ampere laws.

3 29.2. Ambiguity in Ampere’s Law B in a RC circuit. Ampere’s law: I is current through any open surface S bounded by C. Current flows through surfaces 1,2,& 4. But not 3.  Ampere’s law fails ( for non-steady current ). Maxwell’s modification: Changing  E gives I, which in turn gives B.

4 29.3. Maxwell’s Equations LawMathematical Statement Physical Meaning Gauss for E How q produces E; E lines begin & end on q’s. Gauss for B Faraday Ampere- Maxwell No magnetic monopole; B lines form loops. Changing  B gives emf. Moving charges & changing  E give B. Maxwell’s Eqs (1864). Classical electromagnetism.

5 Maxwell’s Equations in Vacuum Gauss for E Gauss for B Faraday Ampere-Maxwell

6 29.4. Electromagnetic Waves  Electromagnetic (EM) waves Faraday’s law: Ampere-Maxwell’s law: changing B gives E. changing E gives B.

7 Plane Electromagnetic Wave EM wave in vacuum is transverse: E  B  k (direction of propagation). For uniqueness, see Prob 46 Sinusoidal plane waves going in x-direction: Right-hand rule

8 Gauss’s Laws Both E & B field lines are straightlines, so their flux over any closed surfaces vanish identically. Hence the Gauss’s laws are satisfied. Plane wave :

9 Faraday’s Law For loop at x of height h & width dx : Faraday’s Law : Faraday’s law expressed as a differential eq :

10 Ampere-Maxwell Law I = 0 For loop at x of height h & width dx : Ampere-Maxwell Law : Ampere-Maxwell law expressed as a differential eq : in vacuum

11 Conditions on Wave Fields Faraday’s Law : Ampere-Maxwell Law : For E = E(x,t) j & B = B(x,t) k, For a plane wave Faraday’s Law :  Ampere-Maxwell Law : 

12 29.5. Properties of Electromagnetic Waves  speed of wave = = speed of light in vacuum= c Maxwell: light is EM wave. 1983: meter is defined so that c is exactly 299,792,458 m/s. Hence,  0 = 1 / (4  c 2  10  7 ) C 2 /N  m 2, where c = 299,792,458.

13 Example Laser Light A laser beam with wavelength 633 nm is propagating through air in the +z direction. Its electric field is parallel to the x axis and has magnitude 6.0 kV/m. Find (a) the wave frequency, (b) the amplitude of the magnetic field, and (c) the direction of the magnetic field. (a) (b) (c)y axis.

14 Polarization Polarization  // E. Radiation from antennas are polarized. E.g., radio, TV, …. Light from hot sources are unpolarized. E.g., sun, light bulb, … Reflection from surfaces polarizes. E.g., light reflecting off car hoods is partially polarized in horizontal direction. Transmission through crystal / some plastics polarizes. E.g., Polaroid sunglasses, … Law of Malus : Only component of E // preferred direction e is transmitted.  = angle between E inc & . or

15 2 polarizers with mutually perpendicular transmission axes. No light gets through where they overlap. Polarization of EM wave gives info about its source & the medium it passes through. Applications: astronomy, geological survey, material stress analysis, … Liquid crystal display (LCD) Vertical polarizer passes only E v. Unpolarized light LC molecules rotate polarization to horizontal direction. Horizontal polarizer passes light. Applied V aligns molecules; polarization not rotated. Horizontal polarizer blocks light.

16 Conceptual Example Crossed Polarizers Unpolarized light shines on a pair of polarizers with their transmission axes perpendicular, so no light gets through the combination. What happens when a third polarizer is sandwiched in between, with its transmission axes at 45  to the others? 1 st & middle polarizers not  so some light passes through. Passed light’s polarization not  to axis of last polarizer so some light passes through.

17 Making the Connection How does the intensity of light emerging from this polarizer “sandwich” compare with the intensity of the incident unpolarized light? Intensity of light emerging from 1 st polarizer : ( polarized along axis of 1 st polarizer ) Intensity of light emerging from middle polarizer : Intensity of light emerging from ensemble : ( polarized along axis of middle polarizer.) ( polarized along axis of 3 rd polarizer.)

18 29.6. The Electromagnetic Spectrum Earth’s atmosphere: Transparent to:most radio, visible light. Opaque to:most IR, upper UV, X-rays,  rays. UV is absorbed by ozone layer IR by green house gases.

19 29.7. Producing Electromagetic Waves Any changing E or B will create EM waves. Any accelerated charge produces radiation. Radio transmitter: e’s oscillate in antenna driven by LC circuit. X-ray tube: accelerated e’s slammed into target. MW magnetron tube: e’s circle in B. EM wave : f = f of q motion Most efficient: ~ dimension of emitter / reciever Waves emit / receive  axis of dipole. Source replenishes radiated energy LC oscillator drives I in antenna Outgoing EM waves

20 29.8. Energy & Momentum in Electromagetic Waves Consider box of thickness dx, & face A  k of EM wave. Energy densities: Energy in box: Rate of energy moving through box: Intensity S = rate of energy flow per unit area Plane waves: 

21 In general: Poynting vector see Prob 64 Average intensity for plane waves : E, B in phase

22 Example Solar Energy The average intensity of noontime sunlight on a clear day is about 1 kW/m 2. (a) What are the peak electric & magnetic fields in sunlight ? (b) At this intensity, what area of 40% efficient solar collectors would you need to replace a 4.8-kW water heater ? (a) (b) Area needed is

23 Waves from Localized Sources Afar from localized source, wave is spherical : Intensity = power / area   wave fields dominates static fields away from the sources.

24 Example Cell Phone Reception A cell phone’s typical average power output is about 0.6 W. If the receiver at a cell tower can handle signals with peak electric fields as weak as 1.2 mV/m, what is the maximum allowable distance from cell phone to tower ? 

25 Application: Cell Phones Hexagonal cell area  25 km 2. ~ circle of radius Transmission & reception are at different frequencies.

26 Momentum & Radiation Pressure Maxwell : radiation momentum radiation pressure on absorbing surface radiation pressure on reflecting surface Cosmos 1, a solar light-sailing spacecraft, failed at launch in 2005.


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