When you rubbed the balloon with the wool sweater earlier in this activity, you were charging both of these objects by friction. When two objects are charged this way, one always develops a positive charge while the other develops a negative charge. Why is this always the case? The answer has to do with the law of conservation of charge.

The illustration shows electrons transferring from a cloth that is rubbed back and forth over a piece of plastic. Note that the total number of positives and negatives must be the same. 



Charging by Friction: the process of charging an object by transferring electrons from one object to another by rubbing the objects together.


Read Methods of Charging Objects
Beginning on page 517 and ending on page 519 to learn more about the law of conservation of charge and charging by friction.

A balloon is made from a material that holds its electrons more tightly than wool.

  1. If the balloon is rubbed with wool, state the resulting charge on the balloon and on the wool. 

  2. Explain why the process of rubbing the balloon with the wool does not generate additional electrons. 

  3. Explain why it is essential for the balloon to be an insulator in order for the excess charges on its surface to remain in place.

  1. The process of rubbing removes electrons from the wool and transfers them to the balloon. This leaves the wool positively charged and the balloon negatively charged.  

  2. Rubbing the balloon with the wool did not generate additional electrons. The electrons were simply rearranged, since they were transferred from the wool to the balloon. 

  3. Charging by friction involves transferring electrons from one object to another by rubbing. If the balloon were a conductor instead of an insulator, then the excess electrons would simply migrate across its surface and possibly return to the wool.


Answer "Concept Check" question 1 on the bottom of page 518 of your textbook. As you answer this question, be sure to include the charge that ebonite acquires.

Fur tends to hold its electrons more loosely than silk. When a material that tends to hold its electrons tightly, such as ebonite, comes into contact with fur, it will be able to remove more electrons from the fur than the silk. The ebonite acquires a larger negative charge after being rubbed by fur than by silk. Conversely fur will have a larger positive charge than silk after being rubbed by ebonite.