Lecture Demos: Electricity and Magnetism

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Parallel Plate Capacitors

Parallel Plate Capacitors

Two parallel plate capacitors are charged and hooked up to an electroscope. When you change the distance between the plates, the electroscope pointer will move because the potential difference between the two plates changes.

PIRA Code(s): 
5C10.20
Set up time: 
none
Physics and Science Concepts: 
Capacitance
Operation: 
Charge the plates using the Van de Graaff. Disconnect wire going from top of sphere to fixed plate at plate end. Change the spacing of a charged parallel plate capacitor while it is attached to an electroscope. Observe changes in electroscope pointer.
Safety: 
Electroscope can be obtained from demo 5A22.22. The Van de Graaff can be obtained from demo 5A50.30.

Charged Ovoid

 Charged Ovoid

A tear-drop shaped conductor demonstrates that c harges tend to accumulate at sharply curved points. In order to maintain uniform electric potential over a whole conductor, the surface charge density must be greater at sharply curved points.

PIRA Code(s): 
5B30.20
Set up time: 
5 minutes
Physics and Science Concepts: 
Electrostatics, Surface Charge Density
Operation: 
Use the Van der Graaf or Wimshurst generator to charge the ovoid. Touch a proof plane to the pointed end of the ovoid and another to the spherical end. Place the proof planes on the electroscope plate. The deflection will be greater from the proof plane that touched the pointed end.
Safety: 
Electroscope can be obtained from demonstration number 5A22.22.

Van de Graaff Generator

Van de Graaff Generator

The Van de Graaff generator has the ability to produce extremely high voltages when the belt picks up negative charges via friction and transfers them to the sphere. The excess charge accumulates on the outside surface of the sphere.

PIRA Code(s): 
5A50.30
Set up time: 
10 minutes
Physics and Science Concepts: 
Static Electricity, Friction
Operation: 
Plug in the generator. Turn on the rocker switch and adjust speed using potentiometer on the base of the Van de Graaff. Other suggested uses include combining this with other electrostatic demonstrations found under a 5A50 heading. Also, throwing packing peanuts or taping strips of paper to the Van de Graaff are popular demonstrations.
Safety: 
The rod works best on dry days with the blue wig. Care should be taken when operating. Do not approach the generator while it is running. Use the grounding rod to discharge.

Wimshurst Generator

Wimshurst Generator

This is a Wimshurst static electricity generator which can be used to produce large charges for other experiments or may be used directly to view "mini" lightning bolts of +/- 10 cm. Works best in the winter time and on dry days.

PIRA Code(s): 
5A50.10
Set up time: 
2 minutes
Physics and Science Concepts: 
Electrostatics
Operation: 
Place the four (two large, two small) balls on the poles in the front about 1 inch apart. Turn the crank.
Safety: 
Do not touch any metal portions of the generator

Electroscope

Electroscope

The electroscope is used to show electrostatic charge. When a charged rod is brought near the plate, the needle deflects. If you scrape the plate with the rod, it becomes charged and the needle remains deflected. The deflection decreases when you bring an object of opposite charge near the plate.

PIRA Code(s): 
5A22.22
Set up time: 
3 minutes
Physics and Science Concepts: 
Electrostatics
Operation: 
First, show the electroscope in resting stage. Then, charge a rod or other material by rubbing the object in a cloth. Place this near the plate of the electroscope and observe the deflection of the needle. To keep the needle deflected, scrape the plate with the charged object. A decrease in deflection occurs when an oppositely charged object is placed near the plate.
Safety: 
None

Pith Balls

 Pith Balls

A charged PVC or acrylic rod is used to charge the pith balls. The two will repel each other.

PIRA Code(s): 
5A20.20
Set up time: 
1 minute
Physics and Science Concepts: 
Electrostatics, Coulomb’s Law
Operation: 
Charge the rod by rubbing it the through the cloth provide or on your shirt. Hold the rod near a pith ball. The two balls will repel each other. The PVC gives a positive charge and the acrylic rod gives negative charge.
Safety: 
None

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