http://www.ck12.org Chapter 15. Electrostatics
some simple demonstrations involving objects that have contain an unbalanced amount of charge, called “a charged
object.”
Charging by Friction: Demonstrating that Like Charges Repel
We have already explained that an imbalance of charge can be achieved by rubbing one material against another. In
any such exchange, both objects are oppositely charged. One object will gain negative charge and the other object
will “gain” positive charge (by losing the negative charge the other received).
Try this experiment:
Inflate two balloons and tie each one with strings measuring about 50-cm long. Give one balloon to a friend and
keep one for yourself. Have your friend rub her balloon with her hair while you do the same with yours. Whatever
charge the balloons now have will be of the same kind.
Holding the balloons by the strings, slowly bring them together until they are a few centimeters apart. You should
see the balloons move away from each other. The balloons now both carry an excess of the same type of charge and
repel each other, which helps confirm to us that like charges repel.
(Note: If the air has too much moisture, our efforts may not succeed. The charges will be attracted by the water
molecules in the air and “leak off” the balloons. Electrostatic experiments work best when the air is dry, not humid.)
http://demonstrations.wolfram.com/RepulsionOfChargedPithBalls/
Charging by Conduction
Charging by conduction is the same as charging by contact. For example, if a neutral (uncharged) metal object, say
a coin, is brought into contact with a coin that is positively charged, electrons will move from the uncharged coin to
the charged coin, seeFigure15.4 andFigure15.5.
FIGURE 15.4
A charged and neutral coin.The positively charged coin inFigure15.4 lacks electrons. The neutral coin inFigure15.4 has weakly held electrons
that move to the charged coin upon contact,Figure15.5. Using this very simplified example (typically there would
be many trillions of electrons), four electrons from the neutral coin move to the charged coin. The coin on the left
has a final charge of(+ 8 )+(− 4 ) = +4 elementary charges→ 4 ( 1. 6 × 10 −^19 C).
The coin on the right has lost four electrons and now has a net positive charge 0−(− 4 ) = +4. After contact, both
coins have the same charge. Do you think both coins would have the same charge if they were not identical?