Objectives: The student should be able to:
1. tell how to produce an electrostatic charge
2. state the law of electrostatic charges
3. explain common electrostatic phenomena
4. compare insulators and conductors
Materials: plastic wrap, various rods, fur, pencil, electroscope, pith ball
Background: Often when two different types of materials are rubbed together, static electricity is often produced. One of the materials develops a positive charge, which is an absence of electrons. The other material develops a negative charge, which is an excess of electrons.
Procedure: In your lab notebook, record your observations and answer the questions below. Use complete sentences when answering questions!
1. Take a piece of plastic wrap (Saran Wrap is excellent) about 30 cm x 6 cm, and fold it in half lengthwise if it is not folded so already. Hang this strip over a pencil or your finger so that 2 strips of equal length are formed. Charge the strips by placing one finger between the strips with a finger on the outside of each strip. Stroke down gently. Describe the results. If all worked correctly, you have charged the strips with anegativecharge. The strips are charged alike. Do similar charges attract or repel? Your fingers that charged the strips have apositivecharge. Place your fingers between the strips. Observe. Do opposite charges attract or repel? If your strips lose their charge, they may be easily recharged.
2. Find out what kind of charge is produced on objects when you rub them. Bring them close to the charged strips to determine their charge. Try this with the ebonite, polystyrene, polyethylene, wooden, and copper rods after they have been rubbed with fur. Then try them after they have been rubbed with silk. Record all observation in a data table.
3. Bring your charged strips near some tiny bits of paper that are not charged. Describe what happens. Explain what is going on using what you have learned about electrostatics. (Note, the paper is neutral and very light.)
4. An electroscope is a device used for demonstrating the presence of an electrostatic charge. A pith ball suspended on a string can be used as an electroscope. A strip of foil in a flask suspended against a metal bar makes a very good electroscope. You may find several types of electroscopes available. Try each type of electroscope to observe its characteristics. Rub an ebonite rod with fur and bring it near an electroscope. Describe what occurs? To remove any charge on the glass electroscope, touch the metal at the top with your finger. Is your finger a conductor or an insulator? To remove the charge from a pith ball electroscope it is necessary to ground the pith ball. If your finger does not remove the charge, touch the ball to the sink faucet.
5. Briefly touch the metal at the top of the electroscope with a charged rod. You have now charged your electroscope. What charge is it? Several types of plastic materials are available. Some, like polystyrene rubbed with silk, will have a positive charge while others, like polyethylene rubbed with fur, will have a negative charge. Try all the materials available against your charged electroscope. How do these results compare with the data you collected in step 2?
6. Bring a negatively charged object near the top of the (uncharged) glass electroscope while touching the opposite side of the metal with your finger. Remove your finger and then remove the charged rod. The electroscope should be charged. If not, try the procedure again. Explain your observations. This is called charging by induction.
7. Again charge the electroscope by induction. Bring the negatively charged rod near the electroscope top. Describe what happens. What is the charge on the electroscope? Confirm this by testing the charged electroscope with a positively charged rod.
8. See which materials available conduct the static charge from a charged object to the electroscope. Classify the materials as conductors or insulators.
9. Further investigations are possible. Record any observations in your data book.
1. How many different kinds of charges have been observed in this experiment?
2. How do electric charges behave towards each other?
3. Which materials that we investigated were conductors? Which were insulators? How do the materials differ?
4. When any object is charged by induction, does it have the same charge as the object used to charge it? Explain.
5. How could an electric charge be used to remove smoke from the air?
6. Describe the operation of an electroscope.
7. Describe the operation of an electrophorus.
8. Explain (in detail) how it is possible to stick a balloon on the wall by first rubbing the balloon.