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“…and then there was light.”

“…and then there was light.”. Physics 100 Chapt 16. James Clerk Maxwell. Properties of E & B fields. Coulomb’s law: E-field lines start on + charge & end on – charge Ampere’s law: B-fields are produced by electric currents Faraday’s law: Changing B-fields produce E-fields

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“…and then there was light.”

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  1. “…and then there was light.” Physics 100 Chapt 16 James Clerk Maxwell

  2. Properties of E & B fields • Coulomb’s law: E-field lines start on + charge & end on – charge • Ampere’s law: B-fields are produced by electric currents • Faraday’s law: Changing B-fields produce E-fields • (un-named law): B-field lines never end

  3. In equation form: E-field lines start on +charges & end on - charges B-field lines never end E-fields are produced by changing B fields B-fields are produced by electric currents

  4. Maxwell The previous equations, as written, are mathematically inconsistent with the conservation of electric charge. He found he could fix this by adding one more term: B-fields are produced by changing E-fields

  5. Maxwell’s equations B-fields are produced by changing E-fields

  6. Fields from an electric charge E E x Is the change in E instantaneous? Does it occur only after some time? M.E.s can tell us? + +

  7. fun in the bathtub Water level will increase but not instantaneously 1st waves will propagate from her entrance point to the edge of the tub

  8. According to Maxwell’s eqs: E E x The change in E is not instantaneous 1st waves made of E-fields & B-fields propagate thru space. + +

  9. Wave solutions to Maxwell’s Eqs:  2kk k”strength” of electric force Wave speed = q1q2 r2 Fc= k  2x9x109Nm2/C2 2x10-7N/A2 = k = 9.0 x 109 Nm2/C2  = 9x109+7(m2/C2)xA2 k”strength” of magnetic force  = 9x1016m2/s2 Speed of light!! I1I2l d FM =k = 3x108m/s N A2 k = 2 x 10-7

  10. “…let there be light.” Maxwell’s equations have solutions that are waves of oscillating E- & B-fields that travel at the speed of light. Faraday & Maxwell made the immediate (& correct) inference that these waves are, in fact, light waves.

  11. EM waves - + - + + -

  12. antenna E B

  13. antenna E B

  14. Light wave B-field + - E-field wave velocity

  15. Light wave animation E B

  16. Electro-magnetic “spectrum” Infra- red Ultra- violet radio waves micro waves X-rays g-rays TV/FM AM Visible light: l freq (c/l) Red 0.75x10-6m 4.0x1014Hz Green 0.55x10-6m 5.5x1014Hz Violet 0.4x10-6m 7.5x1014Hz

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