Applied Physics 322 demonstrations

Electrostatics I: Van Der Graaf Generator demos

These demos are all owned by Physics, and are stored up in Sloane or Gibbs.

Maximum arc to grounded sphere
Volta's hailstorm (styrofoam popcorn in a capacitor on top of VDG)
Longhaired individual stands on insulating stool while touching VDG ball. The way to discharge without pain is by putting a paperclip on the end of the discharge rod so that charge can be gradually drained without an arc.
Leiden jar
Gauss' law demo (pipe with pith ball electrometers on both the inside and the outside).
Pie plates stacked on top of ball which fly off
Wimsurst Machine
pink flamingo and soda can: charge up the plastic bird and use it to pull the can around on a table with electrostatic action at a distance.

How to repair VDG belt

VDG belts can break, and also can start to get slightly conducting. The conductivity of a little bit of finger grease is enough to almost ruin the effect of the generator, and replacing the belt is a good idea if the performance starts to become dissapointing. I used 0.030 inch thick latex rubber sheet, purchased from McMaster Carr. Cut sheet with scisors to get a belt of the right size, and glue together with 5 minute epoxy. Always handle the latex with clean latex gloves on to avoid getting hand grease on it.

Electrostatics II: Electrostatic motor

I got this off of the web, from http://amasci.com/emotor/emot1.html. Construction details can be found on the above site. To operate, just connect one stator bottle to ground and the other one to the ball of the VDG using an alligator clip lead. If the wires are in the right places, the rotor bottle will spin. It's good to demonstrate the the sign of the voltage and the initial spin of the bottle doesn't matter in spin direction; the direction is purely determined by the geometry of the arcing wires.

Another good demo to do with the electrostatic motor is the ion wind. Tape a paperclip or wire to the VDG ball with some aluminum tape, pointing at another ball on the other side of the room, which is left floating. This will charge up the ball, which can then arc to a grounded ball next to it. This ion wind can also be demonstrated with the hair-raising demo, since the pointed ends of the hair can charge up metal objects around the room. Arcs between the metal cart and the chalkboard were observed several meters from the charged person's hair.

Magnetostatics

Magnetic field demonstrator--get this from physics

Put the plastic pieces on the surface of the slide projector so that the image of the dipoles gets projected on the big screen, move magnet around and demonstrate field.
Ferrofluid magnetic field demonstrator. Get this from Dan Prober. Little tube containing a fluid with tiny magnetic particles. Pass this around to the class with a rare earth magnet so they can get a feel for where the field l ines are.
Current carrying wires:
Get the big, high current power supply from the room behind Davies. This beast used to be kept up at Gibbs by the teaching lab, but since they were not using it in Physics, and since it is insanely heavy, I've put it down there for AP use. This power supply goes up to 10 Amps, and you want all the current you can get.
, where I₁=I₂=10 Amps are the currents in the wires, L= 1 meter is the length of the wires and d= 0.01 m is the distance between the wires. This gives a force between wires of 0.002 Newtons = .00045 pounds. Not much to work with!
The apparatus consists of a pair of very thin hollow aluminum tubes hung from the cieling by dental floss. 26 AWG insulated wire is attatched to both tubes with cable ties and electrical tape, fixed so that the wires are on the side of the tube facing eachother to minimize distance. Tubes are hung at just the right height to suspend them right over the surface of the slide projector so that their shadows are magnified up on the big screen in the classroom.

The apparatus is passed over one of the cross pieces of the hung cieling in the classroom, which should leave the wires at a good height when they are made even. One wire is permanently hooked accross the leads of the power supply, and the other is connected through the knife switched shown above so that the direction of current may be reversed. Attraction and repulsion may hence both be demonstrated. Once the wire-wire interaction has been shown, bring up a great big permanent magnet to watch much more dramatic attraction and repulsion.

Induction and transformers, and eddy currents

Spinning coil in magnetic field: display induced voltage straight on scope--no pre-amp needed. I made this, and it's in the closet.
The Demo That Shouldn't Work

Connect to power supply with about a volt, twirl rotor with finger to get it started, carefully adjust magnet and coil until it spins forever. This works because of bouncing, rocking, and possibly periodic breaking of the circuit. I made this, and it's in the closet.
Rare earth magnet dropped through a copper tube(we have this in closet)
Big rare earth magnet moved over a slab of copper(get from Dan Prober)
Dan Probers toy racecar maglev demo with spinning metal disk
Jumping ring demo: get this from physics

Note that compared to many jumping ring demos this is pretty weak, even after cooling the ring in liquid nitrogen. I'm not sure why this is, but it's probably worth someone's time to build/buy a new one for applied physics that has a higher velocity. I think students' enjoyment of the demo scales with the square of projectile velocity. It's only a matter of time before the old physics demo dissintegrates given its current state anyway.
Magnet moved through a coil connected to a current meter: inspiration for relativity (On 4th floor)
Jacob's ladder(4th floor demo closet)
Future addition: rehabilitate old sparking coil demo from demo graveyard behind Davies auditorium

RLC circuits

Connect a function generator and scope as follows:

The pot should be somewhere between 5k and 100k, and the inductor and capacitor are taken from the bin of demo components to have a resonance in the low KHz range. The function generator can either have a sinusoidal output with a frequency sweep to show both the amplitude and phase response of an RLC circuit above and below resonance or a square wave output that produces a bounce at the resonant frequency. In the latter mode, one can then change the resistance continuously to show critical damping.

Transmission Lines

Long transmission line demo. Connect the long transmission line (big black coil of coax) to a scope and function generator with a 5 MHz frequency, 2 V_pp amplitude, 8 ns pulse width. The pulse will reflect back, reflect back with opposite sign or be absorbed depending on what terminates the line. Demonstrate open, short, 50 Ohm match, and loads above and below match. Show that the speed of light and the delay give you the length of the line. You have to use the Agilent function generator for this; the DS345 won't do the pulses as well.
Microstrip on board: Show that it can be 50 Ohms or different from 50 Ohms. Demo with network analyzer.
Microstrip cavity resonator with capacitive breaks at each end.
50 Ohm coax to 300 Ohm twinax transition with 1 meter resonator. Also a netowork analyzer demo.
Another demo to add in the future: quarter wave transformer.

Resonators

Barrel resonator: demonstrate resonances on network analyzer. Show frequency and Q of fundamental mode. The barrel should be in the storage space behind Davies Auditorium.
Helmholtz resonator: Same as barrel. Illustrate that it's physically much smaller, but has similar frequencies because it's like a discrete L and C. This is in the 4th floor closet.

Take it apart, put it back together, show how simple it is, and how it works.
Future demo to add: ratio of speed of light to speed of sound. Coax with big hollow tubes that can contain either air or helium gas. Simultaneously measure accoustic resonances with speakers and amplifiers and RF resonances on network analyzer. Ratio of frequencies gives ratio of speed of sound in air or helium to speed of light. Can also bang on the pipe with something metal to make a ringing noise the frequency of which gives the speed of sound in the metal of the outer pipe.

Waveguides

Pass around various waveguide components. They can be found in the fourth floor closet.
Measure the throughput of a waveguide on the network analyzer, explain the cutoff frequency.

Antennas and free fields

Everything has the following setup for detection:

PC speakers are plugged into the wall, and with the volume turned up they can be quite loud if needed. The diode is a broadband detector that should be with the antenna demos in the 4th floor closet. Any incoming RF signal with an audio amplitude modulation will produce an audio signal from the speakers. The dipole antenna in the above diagram is a female press fit SMA connector with wires soldered on.

Use a dipole antenna for the source, connected to 2.5 GHz or so microwave source with a 1 kHz amplitude modulation. Several of the microwave generators in the lab will do this with the internal oscillator. With both the source and the detector as dipoles, the shape and polarization of the diple field can be demonstrated.
Use the 10 GHz horn as a source and demonstrate polarization. This can be found in a box of demos in the 4th floor closet, and comes with an audio generator. You just plug it all in and it works, with no added equipment.
Play around with polarizers, showing both transmission and reflection, make a periscope out of two polarizers.
Show that the signal can ricochet off of the chalkboard
Show scattering from x-ray diffraction solid. This is a styrofoam block with ball bearings in it which can be found in the 4th floor closet.
Demonstrate Pringles can directional antenna with 2.5 GHz radiation. This is also in the closet.