prof. paul steinhardtsteinh/ph115/slides_11_12_19.pdf · prof. paul steinhardt r physics 115...

“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