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1 30 Outline Maxwell’s Equations and the Displacement Current Electromagnetic Waves Polarization
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2 Maxwell’s Equations Gauss’ Law: E & B Faraday’s Law Ampere’s Law
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3 displacement current ‘explains’ existence of B around E
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4 EM waves accelerating charges produce ‘waves’ of E and B can be ‘pulse’ or ‘harmonic wave’
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5 dipole radiation
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6 EM waves transverse: E perpendicular to B E and B are in phase speed: c = f = 3 10 8 m/s
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7 Electric Dipole Radiation I(r = 1.0m, angle = 90) is 12 W/m 2. I at 2.0m and angle of 30 degrees is: Example:
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8 antennas can respond to E or B
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9 Light Light is an electromagnetic wave c = f ≈ 3 10 8 m/s As light waves travel through space they: »transport energy and/or information »transport momentum
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11 EM Waves carry energy and momentum, shared equally between the electric and magnetic fields.
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12 Energy and Momentum in EM Waves Intensity: energy/area/time [watts/sq.meter]
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13 Example 50W Bulb Assume that 5.00% of the electrical power consumed by the bulb is converted to light. The average intensity at a distance of r = 1.00m: The rms value of E:
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14 Polarization Unpolarized light is the superposition of many waves with random direction of E. Linearly Polarized light has only one direction of E.
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15 Polarizing Filters Polarizing material only allows the passage of only one direction of E Malus ’ Law:
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16 Two Filters (incident light unpolarized) 1 st reduces intensity by 1/2. 2 nd reduces according to Malus’ Law
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17 Polarization by Scattering and Reflection Light scattered at 90 degrees is 100% polarized.
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18 Summary displacement current added to Ampere’s Law: completes Maxwell Eqs., which explain ‘light’ properties (transverse EM wave with speed c) visible light small segment of spectrum energy density and pressure polarization by reflection/scattering Malus’ Law
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19 30-4 The Wave Equation for Electromagnetic Waves Omit End
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20 Momentum momentum = U/c The total energy received in the time by an area A The momentum received The average force Radiation pressure
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23 Example (cont.) Part (b) 2. Use to find 3. Use to find
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24 Example: Consider a laser that puts out an average power of P=1.0 milliwatt in a beam having a diameter of 1.0 mm. What is the peak amplitude of the electric field? The area of the laser beam is The electromagnetic flux S is Recall so that Substitution of the numerical values yields and thus
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