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
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
speed is not uniform over spaceDuring day, you can see peopleacross the lake,but you can’t hear them
cold/slow
hot/fast
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
Sound Shadow effect
During the 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
T
altitudeozone
absorbsa lot of UV
groundabsorbs heatand radiates
(below cloud details depend on day/night)
Atmospherics and the sound channel
Project Mogul
1947 – Flying Disks Fall to ground
1949 – Russian A-bomb test detected
Area 51
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
Dispersion Example: deep water surface waves(Speed depends on wavelength)
DEEP WATERsurface waveswavelength < 2 x depth
)(~)/(v msm l
“dispersive”
NO DISPERSION
SHALLOW surface waveswavelength > 2 x depth )(3~)/(v mDepthsm
“non-dispersive”
SHALLOW water waves
)(3~)/(v mDepthsm
Tsunamis
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
silk
acrylic rod
bunny
rubber rod15
Two view of STATIC Electric forces
FORCE VIEW
Coulomb’s Law: The force between two charges is
+ –
2rQQkForce -+=
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
F = k q1q2r2
18
red, N
bar magnet
Two view of STATIC Electric forces
+
FIELD VIEW: A charge creates a FIELD everywhere in space
2rQkE +=
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
21
iron filings
bar magnet
wire perpendicular to page
current off
current on,coming up
out of page
22
v
v
v
Induction summary
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
Maxwell’s Equations (1873)
1831-1879
magnetic fieldchangesnearby
Changing electric and magnetic fields can make patterns on their own:
magnetic fieldchangesnearby
electric fieldchangesnearby
electric fieldchangesnearby
electric fieldchanges
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