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Maxwell's Equations

Electromagnetic Waves. Maxwell's Equations. Chapter 34. Ampere’s Law is Wrong!. Maxwell realized Ampere’s Law is not self-consistent This isn’t an experimental argument, but a theoretical one Consider a parallel plate capacitor getting charged by a wire

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Maxwell's Equations

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  1. Electromagnetic Waves Maxwell's Equations Chapter 34

  2. Ampere’s Law is Wrong! • Maxwell realized Ampere’s Law is not self-consistent • This isn’t an experimental argument, but a theoretical one • Consider a parallel plate capacitor getting charged by a wire • Consider an Ampere surface between the plates • Consider an Ampere surface in front of plates • But they must give the same answer! • There must be something else that creates B-fields • Note that for the first surface, there is also an electric field accumulating in capacitor • Maybe electric fields? • Take the time derivative of this formula • Speculate : This replaces I for first surface I I

  3. Ampere’s Law (New Recipe) I2 I1 E1 E2 • Is this self-consistent? • Consider two surfaces with the same boundary • Gauss’s Law for electric fields • This makes sense! B

  4. Maxwell’s Equations • We now have four formulas that describe how to get electric and magnetic fields from charges and currents • Gauss’s Law • Gauss’s Law for Magnetism • Ampere’s Law (final version) • Faraday’s Law • Collectively, these are calledMaxwell’s Equations • There is also a formula for forces on charges • Called Lorentz Force All of electricity and magnetism is somewhere on this page • Inside materials, these formulas are sometimes modified to include the effects of the reaction of the materials (like dielectric constants) • These formulas commonly rewritten in derivative form

  5. Electromagnetic Waves Wave solutions • We can solve Maxwell’s Equations • (take my word for it) and come up with two “simple” differential equations. • I could have used cosine instead, it makes no difference • I chose arbitrarily to make it move in the x-direction • These are waves where we have .

  6. Show that satisfies On board

  7. Wave Equations summarized: B0 E0 E0 B0 • Waves look like: • Related by: • Two independent solutions to these equations: • Note that E, B, and direction of travel are all mutually perpendicular • The two solutions are called polarizations • We describe polariza-tion by telling which way E-field points • Note EB is in direction of motion

  8. Understanding Directions for Waves • The wave can go in any direction you want • The electric field must be perpendicular to the wave direction • The magnetic field is perpendicular to both of them • Recall: E  B is in direction of motion A wave has an electric field given by E = j E0 sin(kz – t). What does the magnetic field look like? A) B = i(E0/c) sin(kz - t) B) B = k (E0/c) sin(kz - t) C) B = - i (E0/c) sin(kz - t) D) B = - k (E0/c) sin(kz - t) • The magnitude of the wave is B0 = E0 / c • The wave is traveling in the z-direction, because of sin(kz - t). • The wave must be perpendicular to the E-field, so perpendicular to j • The wave must be perpendicular to direction of motion, to k • It must be in either +idirection or –i direction • If in +i direction, then E  B would be in direction j  i = - k, wrong • So it had better be in the –i direction

  9. The meaning of c: • Waves traveling at constant speed • Keep track of where they vanish • c is the velocity of these waves • This is the speed of light • Light is electromagnetic waves! • But there are also many other types of EM waves • The constant c is one of the most important fundamental constants of the universe

  10. Wavelength and wave number • The quantity k is called the wave number • The wave repeats in time • It also repeats in space  • EM waves most commonly described in terms of frequency or wavelength • Some of these equations must be modified when inside a material

  11. Solve on Board

  12. The Electromagnetic Spectrum f Increasing  Increasing • Different types of waves are classified by their frequency (or wavelength) • Boundaries are arbitrary and overlap • Visible is 380-740 nm Radio Waves Microwaves Infrared Visible Ultraviolet X-rays Gamma Rays Red Orange Yellow Green Blue Violet Which of the following waves has the highest speed in vacuum? A) Infrared B) Orange C) Green D) Blue E) It’s a tie F) Not enough info Vermillion Saffron Chartreuse Turquoise Indigo

  13. Energy and the Poynting Vector • Let’s find the energy density in the wave • Now let’s define the Poynting vector: • It is energy density times the speed at which the wave is moving • It points in the direction energy is moving • It represents the flow of energy in a particular direction, energy/area/time • Units:

  14. Intensity and the Poynting vector • The time-averaged Poynting vector is called the Intensity • Power per unit area In Richard Williams’ lab, a laser can (briefly) produce 50 GW of power and be focused onto a region 1 m2 in area. How big are the electric and magnetic fields?

  15. Momentum and Pressure • Light carries energy – can it carry momentum? • Yes – but it’s hard to prove • p is the total momentum of a wave and U the total energy • Suppose we have a wave, moving into a perfect absorber (black body) • As they are absorbed, they transfer momentum • Intensity: • As waves hit the wall they transfer their momentum • Pressure on a perfect absorber: • When a wave bounces off a mirror, the momentum is reversed • The change in momentum is doubled • The pressure is doubled

  16. Cross-Section: • To calculate the power falling on an object, all that matters is the light that hits it • Example, a rectangle parallel to the light feels no pressure • Ask yourself: what area does the light see? • This is called the cross section If light of intensity S hits an absorbing sphere of radius a, what is the force on that sphere? A) a2S/c B) 2a2S/c C) 4a2S/c • As viewed from any side, a sphere looks like a circle of radius a • The cross section for a sphere, then, is a2

  17. Sample Problem: A 150 W bulb is burning at 6% efficiency. What is the force on a mirror square mirror 10 cm on a side 1 m away from the bulb perpendicular to the light hitting it? 1 m • Light is distributed in all directions equally over the sphere of radius 1 m

  18. Solve on Board

  19. Sources of EM waves – + • A charge at rest produces no EM waves • There’s no magnetic field • A charge moving at uniform velocity produces no EM waves • Obvious if you were moving with the charge • An accelerating charge produces electromagnetic waves • Consider a current that changes suddenly • Current stops – magnetic field diminishes • Changing B-field produces E-field • Changing E-field produces B-field • You have a wave

  20. Simple Antennas ++++++ – – – – – – • To produce long wavelength waves, easiest to use an antenna • AC source plus two metal rods • Some charge accumulates on each rod • This creates an electric field • The charging involves a current • This creates a magnetic field • It constantly reverses, creating a wave • Works best if each rod is ¼ of awavelength long • The power in any direction is  distance r

  21. Common Sources of EM Radiation Radio Waves Antennas Microwaves Klystron, Magnetron Infrared Hot objects Visible Outerelectronsin atoms Ultraviolet Inner electrons in atoms X-rays Accelerated electrons Gamma Rays Nuclear reactions

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