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

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Presentation on theme: "“…and then there was light.” Physics 100 Chapt 16 James Clerk Maxwell."— Presentation transcript:

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 + x E + E 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 1 st waves will propagate from her entrance point to the edge of the tub

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

9 Wave solutions to Maxwell’s Eqs: F c = k q1q2r2q1q2r2 k = 9.0 x 10 9 Nm 2 /C 2 k  ” strength” of electric force F M =  I1I2 ldI1I2 ld  = 2 x NA2NA2   ”strength” of magnetic force  2k  Wave speed =  2x9x10 9 Nm 2 /C 2 2x10 -7 N/A 2 =  9x (m 2 /C 2 )xA 2 =  9x10 16 m 2 /s 2 = = 3x10 8 m/s Speed of light!!

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 E B

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

15 Light wave animation E B

16 Electro-magnetic “spectrum” Visible light:  freq (c/ ) Red 0.75x10 -6 m 4.0x10 14 Hz Green 0.55x10 -6 m 5.5x10 14 Hz Violet 0.4x10 -6 m 7.5x10 14 Hz Ultra- violet Infra- red X-rays  -rays micro waves radio waves TV/FM AM


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