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“…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 - PowerPoint PPT Presentation
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“…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
• (un-named law): B-field lines never end
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
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
Maxwell’s equations
B-fields are produced by changing E-fields
Fields from an electric charge
+
xE
+
E
Is the change in Einstantaneous?
Does it occur onlyafter some time?
M.E.s can tell us?
fun in the bathtub
Water level will increase
but not instantaneously
1st waves will propagatefrom her entrance pointto the edge of the tub
According to Maxwell’s eqs:
+
xE
+
E
The change in Eis not instantaneous
1st waves made of E-fields & B-fields propagate thru space.
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!!
“…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.
EM waves
-
+
-+
+
-
antennaE
B
antenna
E
B
Light wave
E-field
B-field
wave velocity
+
-
Light wave animation
E
B
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