Electrostatics Basics

Read chapter 32 of your text.

Read my short description of static electricity.

1. Compare the charge of a proton to that of an electron.

The magnitude of a proton's charge is the same as that of an electron. The proton's charge is opposite that of the electron though, positive instead of negative.

2. What is the SI (metric) unit of charge?

Charge is measured and expressed in Coulomb's abbreviated C.

3. Explain why, even though electrostatic forces are much stronger than gravitational forces at the level of small particles, we feel gravity all the time (we call it our "weight"), but only notice electric forces occasionally and rarely do we think of them as very strong.

The gravity force is in one direction only. Each particle of the earth that has mass exerts a force on each particle of our own mass and the part of each of these tiny forces that points towards the center of the earth is added together into one modest force that we call our weight. On the other hand electric forces occur as attractions and repulsions. Even though each electric force is much larger than any individual gravity force, the electric repulsions occur in quantities and at magnitudes equal to the electric attractions -- their effect is cancelled.

4. Answer this concept test question (and explain how you know):

Based on only what is recorded here we can't conclude anything. The conditions described could be met by either the positive charges moving out (by repulsion) or some extra negative charges moving in (by attraction). Even though we know that generally the latter case is what happens in nature, we can't extend that inference to this case when we are limited by what the problem gives us.

5. Answer this concept test question (and explain how you know):

All three must carry the same charge. If 1 and 2 repel, they are the same. If 2 and 3 repel they are the same. Since 2 is the same as 1 and 3 we must conclude 1 and 3 are the same.

6. Answer this concept test question (and explain how you know):

One and three must be opposite. One and two attract and are therefore opposite. Two and three repel and are the same. So one would attract either two or three and must be opposite each.

7. Answer this concept test question (and explain how you know):

Number 5 is correct. Newton's third law requires the forces to be the same size (equal and opposite). The fundamental law of electrostatics requires it to be repulsion if both are positive.

8. Explain how an electroscope works.

The leaves of an electroscope are very lightweight, flexible conductors. They are attached to a shaft that is connected on its other end to a metal ball. All components are conductors of electric charge. When a charged object is brought near the ball of an electroscope, electrons within it either flee the ball (into the leaves of a neutral electroscope diverge) or move from the leaves to the ball (making the leaves of a neutral electroscope diverge). If the electroscope is given a known charge prior to being used to test another it can detect the sign as well as the presence of a charge.