Download - “…and then there was light.”
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“…and then there was light.”
Physics 100
Chapt 16
James Clerk Maxwell
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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
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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
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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
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Maxwell’s equations
B-fields are produced by changing E-fields
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Fields from an electric charge
+
xE
+
E
Is the change in Einstantaneous?
Does it occur onlyafter some time?
M.E.s can tell us?
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fun in the bathtub
Water level will increase
but not instantaneously
1st waves will propagatefrom her entrance pointto the edge of the tub
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According to Maxwell’s eqs:
+
xE
+
E
The change in Eis not instantaneous
1st waves made of E-fields & B-fields propagate thru space.
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Wave solutions to Maxwell’s Eqs:
Fc = kq1q2
r2
k = 9.0 x 109 Nm2/C2
k ”strength” of electric force
FM = I1I2 ld
= 2 x 10-7NA2
”strength” of magnetic force
2k
Wave speed =
2x9x109Nm2/C2
2x10-7N/A2
=
9x109+7(m2/C2)xA2
=
9x1016m2/s2=
= 3x108m/s Speed of
light!!
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“…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.
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EM waves
-
+
-+
+
-
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antennaE
B
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antenna
E
B
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Light wave
E-field
B-field
wave velocity
+
-
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Light wave animation
E
B
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Electro-magnetic “spectrum”
Visible light: freq (c/)
Red 0.75x10-6m 4.0x1014 Hz
Green 0.55x10-6m 5.5x1014 Hz
Violet 0.4x10-6m 7.5x1014 Hz
Ult
ra-
vio
let
Infr
a-
red
X-
rays
- rays
mic
ro
wave
srad
io
wave
sTV
/FM
AM