1. Van de Graaf generator
Russell Chamberlain, Max Johnson, Morgan Short, Jacob Sterrett & Claire Harris
STEM Spring 2016

2. So What is a Van De Graaff Generator?
It’s A Giant Shocky Thingy!
It was designed to generate large voltages for particle accelerators
Induces large potential on a hollow sphere
Designed in 1929 by Robert J. Van De Graaff
Exchange of charge between insulating surfaces transports charge carriers to top of sphere.

3. Theory Triboelectric effect Gauss’ Law Fun Fact: Voltage vs. Radius
Electric charge with frictional contact.
Negative charge picked up by lower rod, carried by rubber belt to upper rod.
Gauss’ Law
Positive charge will be distributed evenly across surface
The Electric Field and Voltage are proportional to total charge.
Fun Fact: Voltage vs. Radius
Voltage from an object is proportional to the charge on the object and inversely proportional to the radius of a sphere enclosing that charge.
The smaller of two conductors has greater voltage

4. How it works
Uses two rollers on opposite sides of the triboelectric series
Runs an insulating belt over the rollers
Combs “spray” charges onto the belt
These charges are carried up on the belt to another comb where they are discharged onto the globe
The charges are released once the globe is grounded

5. Materials Science Teflon:C2F4
Carbon and fluorine have 4 and 7 electrons in their respective outer shells
When these two atoms bond, all outer shells have 8 electrons
With the outer shells fully satisfied, teflon will not give away any electrons, it will however, gain electrons
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6. Materials Science Nylon:C12H22N2O2
Carbon, Hydrogen, Nitrogen, and Oxygen have 4,1,5, and 6 valence electrons respectively
These 4 elements form bonds which satisfy almost every outer electron shell
However, you end up with 2 Nitrogen atoms with 2 unpaired electrons each, and 4 oxygen atoms with 4 unpaired electrons each
The result is 12 unpaired electrons for each Nylon molecule, causing this material to easily give up unpaired or “extra” electrons
With Nylon ready to give electrons, and Teflon ready to take them, these two materials work perfectly for the static transfer of electrons in our Van de Graaff generator

7. But how much juice? Dielectric eqn:
Dielectric strength, Es Varies widely with Humidity
Dry air: 3.0 MV/m
~40% humidity: MV/m depending on spark gap
1-inch gap: MV
1-foot gap: MV

8. How much Power? Seems like A lot of volts, right?
Well...the energy associated with this voltage even under perfect conditions (humidity = 0%) is about 460 Joules, about enough energy to raise a coffee cup of water 1 degree celsius, or melt one ice cube.

9. Safety First! Safety Rules for Van de Graaff generator:
Do not touch generator without first discharging it.
Make sure you are on the rubber mat when interacting with the generator.
No really, ALWAYS discharge the generator before touching it.
Dr. T will now demonstrate how not to use the generator

11. Resources
Valavala, Madhu & Kanchanapalli, Bhavya (date).Measurement of Air Breakdown Voltage Using Standard Sphere Gap Method. Journal of Electronics Engineering.
Bonin, Asheley. Teflon. Retrieved from shape-and-lewis-structure.html.