#4 #6 #14 #18 #28 #32 #36 #42 #44 #484. Could you use fur, silk, or wool to charge a metal rod that is held in your hand? Explain.
No. Since the rod is a conductor, you would have to insulate the rod from your hand to do so.
6. Before an aircraft is fueled from a truck, a wire from the truck is attached to the aircraft. Why?
This is done to prevent the possible build-up of charge, that might in turn cause sparks.
14. When a charged comb is brought near bits of paper, the bits are first attracted to the comb and then repeled. Describe how the charge on the comb and on the bits of paper change during this process.
Initially, the bits of paper have no net charge. The bits of paper are attacted to the comb because of the induced separation of the charges within the neutrally charged paper bits. When these paper bits come in contact with the comb, they acquire some of the comb's charge and are then repelled from the comb since the comb and paper bits are both positively or negatively charged.
18. Why does a freshly wiped phonograph record attract dust?
The act of rubbing the phonograph record produces a net charge on the record. The charged record then induces charges on dust particles.
28. The nucleus of a helium atom contains two protons and two uncharged neutrons. If the nucleus is surrounded by two electrons, what is the total charge of the atom?
The total charge Q of the atom is the sum of the charges within it. Since Q = +2e +(-2e) = 0, the total charge is zero.
32. Assume that you have two identically charged objects separated by a certain distance. How would the force change if the objects were three times as far away from each other?
The electric force is proportional to the reciprocal of the distance between the two charges squared. If the charges were 3 times further apart, the new force would be one-ninth the old force.
36. Two parallel plates have equal but opposite charges. What is the direction of the force on a positive charge placed near the center of the plates?
The positive charge would have a force that points directly toward the plate with the negative charge.
42. What force holds our bodies together, gravitational or electric?
The electric forces within our bodies are much stronger than the gravitational forces, so electric forces hold our bodies together.
44. In the chapter we compared the electric and gravitational forces between an electron and a proton. Why is the result valid for all separations?
If you look back at the calculation of the ratio of the electric and gravitation force between the electron and proton in the hydrogen atom that we did in class, you will see that the distance between the electron and proton is squared in the denominators of both the electric and gravitional force. When we took the ratio, this distance squared cancelled. So, we didn't need to know the distance between the two particles to find the ratio.
48. Describe the electric field in the space between two parallel plates containing equal charges of opposite sign.
The electric field points directly away from the positive plate toward the negative one.