prof. paul steinhardtsteinh/ph115/slides_11_12_19.pdf · prof. paul steinhardt r physics 115...
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“Physics for FUTURE LEADERS”Prof. Paul Steinhardt
r
Physics 115
Princeton in the Nation’s ServiceWoodrow Wilson, 1896
SPECTACULAR on Tuesday November 26
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Speed and Temperature
Common experience:
During day, you can see peopleacross the lake,but you can’t hear them
During night, you can hear peopleacross the lake,but you can’t see them
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speed is not uniform over spaceDuring day, you can see peopleacross the lake,but you can’t hear them
cold/slow
hot/fast
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What if speed is not uniform over space?During day, you can see peopleacross the lake,but you can’t hear them
cold/slow
hot/fast
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Sound Shadow effect
During the day, you can see peopleacross the lake,
but you can’t hear them
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During night, you can hear peopleacross the lake,
but you can’t see them
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T
altitudeozone
absorbsa lot of UV
groundabsorbs heatand radiates
(below cloud details depend on day/night)
Atmospherics and the sound channel
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Project Mogul
1947 – Flying Disks Fall to ground
1949 – Russian A-bomb test detected
Area 51
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Another speed effect
Echo (reflect)
Disperse (speed varies with wavelength)
Refract (bend going from one medium to another)
Interfere (when two or more waves add)
- diffract- resonate
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Dispersion Example: deep water surface waves(Speed depends on wavelength)
DEEP WATERsurface waveswavelength < 2 x depth
)(~)/(v msm l
“dispersive”
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NO DISPERSION
SHALLOW surface waveswavelength > 2 x depth )(3~)/(v mDepthsm
“non-dispersive”
SHALLOW water waves
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)(3~)/(v mDepthsm
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Tsunamis
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Light
400 nm
7.5×1014 Hz
500 nm
6.0×1014 Hz
600 nm
5.0×1014 Hz
700 nm
4.3×1014 Hz
infraredultraviolet
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silk
acrylic rod
bunny
rubber rod15
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Two view of STATIC Electric forces
FORCE VIEW
Coulomb’s Law: The force between two charges is
+ –
2rQQkForce -+=
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17
If the force between the two charges is 1 newton when 1 m apart,what is the force if we replace the + charge with +2 and move it
4 m away _____ ?
+ –
2rQQkForce -+=
1 m
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F = k q1q2r2
18
red, N
bar magnet
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Two view of STATIC Electric forces
+
FIELD VIEW: A charge creates a FIELD everywhere in space
2rQkE +=
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Two view of STATIC Electric forces
+
FIELD VIEW: A charge creates a FIELD everywhere in space
2rQkE +=
–
Force feltby 2nd charge is
Q- x E
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21
iron filings
bar magnet
wire perpendicular to page
current off
current on,coming up
out of page
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22
v
v
v
Induction summary
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Fields can “mix”
–
Moving charges generate magnetic fields,
v
It turns out that• Changing electric fields can generate magnetic fields• Changing magnetic fields can generate electric fields
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Maxwell’s Equations (1873)
1831-1879
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magnetic fieldchangesnearby
Changing electric and magnetic fields can make patterns on their own:
magnetic fieldchangesnearby
electric fieldchangesnearby
electric fieldchangesnearby
electric fieldchanges