Reading QuizA metal sphere has a positive electric

charge. A small piece of paper is initially attracted to it. This can be explained by:

1. The paper might be negatively charged, or it might have no net charge on it

2. The paper must be positively charged.3. The paper must be negatively charged.4. The paper must have no net charge on it.

Like charges repel and unlike charges attract. The plus and minus signs were introduced

in Franklin’s single-fluid model.

Touching the ball on top of a charged electroscope with either your finger or a metal rod causes the electroscope to discharge. Touching it with an

uncharged glass rod produces no effect.

Quiz 1

Two identical balls have opposite charges, one with q1= +2 C and one with q2= -2 C. These two balls are now brought into contact with each other, and then separated.

1. If both balls are insulating, they each now have no charge.

2. If both are conducting, they each now have no charge.

3. Because of charging by induction, each ball now has twice as much charge as before.

4. If both are conducting, there is no change in their charge.

Figure 12.9The negative charge in the atoms is attracted to the

positively charged glass rod, while the positive charge is repelled. This produces a polarization of the charge in the atoms. This is called an induced electric dipole moment.

The size of the atoms is grossly exaggerated.

Quiz 2

All materials can be classified as

• Conductors and Insulators

• Conductors, Insulators and Semiconductors

• Semiconductors

• Conductors, Insulators, Semiconductors and polarized

Quiz 3

• An example of a semiconductor is

• Iron

• Glass

• Plastic

• Carbon

Figure 12.12Two positive charges exert equal but oppositely directed

forces upon one another, according to Coulomb’s law and Newton’s third law of motion.

Electric field “flows” form the positive charge and drains into the negative charge. (almost (?) acts like there is a hidden dimension

Coulomb’s Law:

€

F =kq1q2

r2

Note: looks a lot like Newton’s law of Gravitation.

q = charge in coulombsr = distance in meters k = 9x109 N-m2/C2

The two forces acting upon q0 are in opposite directions, yielding a net force of 9 N (see box 12.2).

Quiz 4

Two equal charges are 1 meter apart. If the distance between them is doubled, what increase in each charge is require to keep the force between them the same.

1. 2 times2. Square root 23. 4 times4. 8 times