E&M Demo and More

Chris Tully

Princeton

NJAAPT, March 16, 2013

1

The ElectrophorusA device that uses induction to produce a substantial charge.

Insulating handle

Metal disk Hard rubber

1. The hard rubber pallet is charged (-)by rubbing with fur.Positive charge movesdown and negative moves up.

2. The electrophorus is moved to contact the metal case. Negative charge, repelled by the negative charge on the rubber, tries to get as far away as possible.

3. Once again isolatedfrom the case, the electrophorus carriesa substantial net (+)charge.

Metal plate

2

The Electroscope

Demo: Charge flows onto the conductor and causes the foil to deflect.

A simple device for measuring charge.

3

Charging an electroscope by induction.

1. Bring charge close, without touching.

2. Momentarily ground the electroscope, thus allowing some of the positive charge to escape.

3. Move the charge away, leaving the electroscope charged.

4

Wimshurst machine

5

Rainforest

+ + + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Negative Charge

Positive Charge

+++

6

Ohm’s Law

7

Charging a Capacitor

8

I

V

I

Sunlight

Current Source(For example, solar panel)Voltage Source

Q3. Starting from the same initial charging current and ending at the same maximum voltage, which will charge up a capacitor faster?

a) b)

c) Tie – no difference in charging rate

9

Storing energy in capacitors

Capacitors can store energy

The stored energy can be quickly released with spectacular effect.

10

11

Field of Permanent Dipole Magnet

There are no“magnetic charges” analogous to electric charges (monopoles).

Fields are due to currents and do not terminate on “magnetic charges”

Field lines are continuous inside objects.

North pole: where field comes outSouth pole: where the field goes in

Magnetic Levitation

12

The “Ring Toss”

13

Force on a current ring

B increasingI clockwise from aboveForce up

B

BxI

F

B

Binduced 14

Alternating current with Inductor

15

Alternating current with capacitor

V =Vmax cos(t)

V Q

C0

Q VC = VmaxC cos(t)

I dQ

dtVmaxC sin(t)

Vmax

1/Csin(t)

Vmax

X c

sin(t)

X c 1

C "capacitive reactance"

"Current leads voltage"

16

Transporting and Rotating B field flux

I1(t)

I2(t)

I3(t)

17

3-Phase Power

18

3-Phase Power

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47Ω connected from V1 to ground 33Ω to ground

3.3KΩ to ground3.3KΩ to ground

1µF connected to V1

100mH connected to V2 .15µF connected

to V2

Eddy Currents

• Lenz’s Law holds for “eddy currents”.

Here, part of the current loop is outside the B-field and has no force on it, and part is inside the B-field with a force.

• The net result is a force that tries to oppose the change in flux, acting opposite the velocity.

No force here (B=0)

20

Tesla Coil

21

EM waves from accelerating charge on antenna

B

22

Anatomy of a EM Wave

23

Note that the velocityis in the direction of ExB:

r v

r E

r B

The dipole antenna oscillator and receiver

The 200 MHz oscillator(at rear) generates EMwaves with the dipoleantenna. The polarization(E-field) is horizontal.

The dipole receiver (infront) detects the waves.The bulb lights up when thesignal is strong. Expected polarization and intensity patternare observed.

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Polarization

The orientation of the E field could be vertical, horizontal, “circular,” or random.

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Polarization

Radio waves will be polarized along the direction of the radiating element.

The receiving antenna should have the same polarization to work efficiently.

26

Standing Waves

Just as one can have standing waves on a string, one can have standing electromagnetic waves.

27

Standing Waves- Photo

28

29

Heat Dissipation of Light Bulbs

• Heat capacity of water:– Takes 4.2 Joules to raise 1gram=1cm3 of water

by 1 degree C– 1 Watt = 1 Joule/sec

Pdissipated (4.2J /goC)(1000g)(25oC)

3000sec35 Watts

30

• The Universe was not always as cold and dark as it is today – there are a host of landmark measurements that track the history of the universe

• None of these measurements, however, reach back as far in time as ~1 second after the Big Bang– At ~1 second the hot, expanding universe is

believed to have become transparent to neutrinos

– In the present universe, relic neutrinos are predicted to be at a temperature of 1.9K (1.7x10-4 eV) and to have an average number density of ~56/cm3 of electron neutrinos EVERYWHERE IN SPACE!

Dicke, Peebles, Roll, Wilkinson (1965)

1 sec

Looking Back in Time

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Princeton Tritium Observatory for Light, Early-Universe, Massive-Neutrino Yield