Physics Demo Number: 140

Approximate

Run Time: 15 min

Demonstrations To Show The Basis of Modeling Charges in Materials

Demo Description

Initial demonstrations (for majors advanced course) in electrostatics concepts.

Scientific Principles

• Basic notions of the detailed electrostatic interaction of insulators and conductors and the charges contained within them and shared between them.

Equipment

• Comb

• 3 pith balls hung from a horizontal rod on a stand

• Rubber rods/ fur patches

• Glass rods/ silk patches

• Large copper tube

• An insulated stand for holding rods horizontally

• Two metal spheres each on its own insulated stand

• Foil-leaf electroscope

• Electrometer with projection display slave

Equipment Location

• In Kit (140) on [A-4-1]

Instructions

The first photo shows the equipment ready to travel to building 6.

The second photo gives a better view of the major elements.

One can give a rubber rod a negative charge by rubbing it with a fur patch.

One can give a glass rod a positive charge by rubbing it with a silk patch.

Then the following steps can be demonstrated:

1. Set the soda can on its side and pull it into horizontal rolling motion with each of the charged rods.

2. Attract the uncharged pith balls with each of the charged rods.

3. Charge two pith balls with the rubber rod and show that the balls then repel each other.

4. Charge two pith balls with opposite charges to see that the balls then attract each other.

5. Place a rod (metal, glass, or rubber) in the insulated clamp stand and slide an insulated sphere into contact with the rod.

6. Then place a charged rod into contact with the opposite end of the rod in the clamp stand.

7. Use the pith balls to establish that the sphere becomes charged when the horizontal linking rod is metal, but not when the linking rod is glass or plastic.

8. The two spheres can be used with the pith balls to show charging by induction.

Writeup created by David A. Burba

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