What are the two ways to charge an object?

When a rubber rod is rubbed with fur, the fur transfers electrons to the rubber rod. The rod and fur, originally neutral, are now charged. If an object touches the rod some of the excess electrons on the rod can move to the object, charging it. The rod, which is now negatively charged because it has excess electrons, can attract positive charges. This method is called charging by contact.

But, as you observed with the cellophane tapes, your hand and other neutral objects attract both positively and negatively charged objects. How does this happen? The rod attracts positive charges and repels negative charges. Neutral objects contain equal numbers of positive and negative charges. In a conductor the charges are free to move and so the electrons can be pushed to the far end of the object making it negatively charged and leaving the close end positively charged. An object that is neutral but has separated charges is polarized. Is there a net force on a polarized object? And can it exert a net force on the charged object, like the cellophane tape? Yes, because the electrostatic force is stronger at closer distances. Thus the attractive force between the unlike charges is stronger than the repulsive force between the like charges, and there is a net attractive force.

In non-conducting materials the charges cannot be widely separated, but they can move within the atoms or molecules. So insulators, like pieces of paper, dust, or hair, can also be attracted, even though they are neutral.

Did you ever see a piece of paper attracted to a charged rod, touch it, and then jump away? How would that happen? If it touched the rod, it became charged with a charge like that of the rod, and so it would now be repelled. A conductor can also be charged after being polarized, but without touching the charged object. If you bring a large metal object, like a pie plate, near a charged rod, the positive charges will move to the far end of the plate. If you now touch this end briefly with your finger the positive charges will be pushed even further away into your finger. When you remove your finger the pie plate is negatively charged. This process is called charging by induction.

Rubbing a glass rod with silk will achieve the same effect. The glass rod is positively charged, while the silk receives the excess negative electrons. The glass rod can still pick up small objects, but attracts the negative charges in those objects instead of the positive charges. When the pie plate is charged by induction it will be positively charged.


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