Conceptual Physics

to the right have the same charge, either positive or negative, they repel each other. The old cliché í opposites attract and likes repel í proves true in physics. When dealing with issues of attraction and repulsion, it really is important to know your sign. Two charged objects exert equal but opposite forces on each other. In other words, if they attract, they pull toward each other with the same force. If they repel, they push against each other with equal force. The forces act along a straight line between the centers of the two charges. For instance, if they attract, each force points directly toward the other charge, as illustrated in Concept 2. If they repel, each force points directly away from the other charge.

Charged balloons repel each

other

Charged objects can attract or repel

Electrostatic force

Opposites attract Likes repel

Will the balloons attract or repel?

Same charge

Balloons repel

22.7 - Inducing an electric charge

Inducing an electric charge: Creating a charged

object or region of an object without direct

contact.

Objects can become electrically charged when they are put into contact with each other, for example, by rubbing glass and silk together, or by touching a charged rod to a neutral one. In this process electrons flow from one object to the other. Objects can also become charged without touching. Like gravity, electrostatic forces act at a distance, so charges cause other charges to move without direct contact. When a charged object, like the nonconducting sphere shown in Concept 1, is placed near a neutral object in which electrons are free to move, such as the joined pair of conducting metal rods to its right, the charged object causes electrons to move in the neutral object. Charges in the rods, initially evenly distributed throughout the pair, end up in the asymmetrical configuration you see in Concept 1. The rod pair as a whole is still

Inducing an electrical charge

Creating charged objects without direct contact