Physics Lab #15

 

Electric Charge, Force and Conduction

Theory

The theory for this lab is very simple mathematically and conceptually and I am challenging you to figure out most of it on your own.  You will be expected to develop a viable theory as to how charge by induction and conduction works and how to use the apparatus to figure out the sign of the charge being considered. You will make charges by rubbing, for example, silk along a piece of glass or felt along a piece of plastic.  You will also be using a gold foil and a pith ball electroscope to analyze the charges created.

 

You will recognize the gold foil electroscope by the presence of the gold foil.  The pith ball electroscope consists of two pith balls suspended from the same L shaped hanger.  You should also have two single pith ball set ups.  These are not  electroscopes.

 

Also remember: metals acts as electrical conductors while things like rubber are electrical insulators.  Like charges repel and unlike charges attract.

 

 

Procedure

1. Rub a glass graduated cylinder with a piece of silk and place the cylinder near BUT NOT IN CONTACT WITH the top of the gold foil electroscope.  Note what happens.  Develop a reasonable explanation.  What do the results this tell you about the materials that make up the pith ball electroscope?  (Hint: are they conductors or insulators?)  Draw a picture of the apparatus and label the parts by their material and its conducting properties.   
2. Now do the same thing to the pith ball electroscope.  Is it the same result?  Does this apparatus work better or worse than the gold foil electroscope?  Explain why.  What do the results this tell you about the materials that make up the pith ball electroscope?  (Hint: are they conductors or insulators?)  Draw a picture of the apparatus and label the parts by their material and its conducting properties.   
3. Now use the single pith ball apparatus.  Charge the glass again and hold it close to the pith ball but do not touch it.  Describe and explain what happens.
4. Repeat the actions (but do not write the answers) for 1 to 3 with a plastic cylinder being rubbed with a wool cloth.  Note any differences and similarities.
5. Take a pair of single pith balls and following step 3 charge one with the glass and one with the plastic.  Describe and explain what you see.
6. Can you think of a method to tell when you have a positive or a negative charge?  Explain.  If I tell you that silk on glass leaves the glass with a positive charge can you tell me the charge on the plastic?  Use this logic to develop a way to see what charges you are creating by using different cloths and materials rubbed together.  (I will provide various things but you can try others including your clothing and hair.) Make a table up of the materials you test and the results from the electroscope and pith ball test to see if they become charged and if so with what sign charge.