Discovering Charges

In the early 1700s, scientists studied what happened when various materials came in contact with one another, resulting in attraction or even sparks. To explain what was going on, they came up with the model that the world contains positive and negative charges, and that they liked to be together, so if they were separated, they would have the tendency to jump back together. In other words, there is an attractive force between positive and negative charges.

Most things in the world have an equal number of positive and negative charges, but when you rub 2 materials together, it is possible that some of these charges can transfer from one material to the other. This balloon was rubbed this balloon vigorously on Yosh’s sweater, and then it became quite attracted to Clark’s hair!

Students observed that when 2 pieces of tape were pulled rapidly off the table at the same time, they were similarly charged, so they repelled one another.

The students experimented with other materials. They were given plastic, rubber and glass rods and several kinds of cloth. They tried to change the charges on the rods, so the rods would actually pick up paper, and were challenged to discover the best combination. Now we see that a charged rod can attract something with a neutral charge.

One student announced wondered if a charged rod could bend water. He had seen it done before with a charged balloon. He was right!

Water has no net electrical charge, but the electrons in the water can move somewhat. When the negatively charged rod comes near the water, it repels the electrons, so that the side of the water nearest the rod then has a positive charge. The attraction between this positive charge and the negatively charged rod results in a net force on the water, bending the stream.