18.2 Conductors and insulators  (Page 4/11)

Some students experimenting with an uncharged metal sphere want to give the sphere a net charge using a charged aluminum pie plate. Which of the following steps would give the sphere a net charge of the same sign as the pie plate?

1. bringing the pie plate close to, but not touching, the metal sphere, then moving the pie plate away.
2. bringing the pie plate close to, but not touching, the metal sphere, then momentarily touching a grounding wire to the metal sphere.
3. bringing the pie plate close to, but not touching, the metal sphere, then momentarily touching a grounding wire to the pie plate.
4. touching the pie plate to the metal sphere.

When the balloon is brought closer to the sphere, there will be a redistribution of charges. What is this phenomenon called?

1. electrostatic repulsion
2. conduction
3. polarization
4. none of the above

What will be the charge at Y (i.e., the part of the sphere furthest from the balloon)?

1. positive
2. negative
3. zero
4. It can be positive or negative depending on the material.

What will be the net charge on the sphere?

1. positive
2. negative
3. zero
4. It can be positive or negative depending on the material.

If Y is grounded while the balloon is still close to X, which of the following will be true?

1. Electrons will flow from the sphere to the ground.
2. Electrons will flow from the ground to the sphere.
3. Protons will flow from the sphere to the ground.
4. Protons will flow from the ground to the sphere.

If the balloon is moved away after grounding, what will be the net charge on the sphere?

1. positive
2. negative
3. zero
4. It can be positive or negative depending on the material.

A positively charged rod is used to charge a sphere by induction. Which of the following is true?

1. The sphere must be a conductor.
2. The sphere must be an insulator.
3. The sphere can be a conductor or insulator but must be connected to ground.
4. The sphere can be a conductor or insulator but must be already charged.

As shown in the figure above, two metal balls are suspended and a negatively charged rod is brought close to them.

1. If the two balls are in contact with each other what will be the charges on each ball?
2. Explain how the balls get these charges.
3. What will happen to the charge on the second ball (i.e., the ball further away from the rod) if it is momentarily grounded while the rod is still there?
4. If (instead of grounding) the second ball is moved away and then the rod is removed from the first ball, will the two balls have induced charges? If yes, what will be the charges? If no, why not?

Two experiments are performed using positively charged glass rods and neutral electroscopes. In the first experiment the rod is brought in contact with the electroscope. In the second experiment the rod is only brought close to the electroscope but not in contact. However, while the rod is close, the electroscope is momentarily grounded and then the rod is removed. In both experiments the needles of the electroscopes deflect, which indicates the presence of charges.

1. What is the charging method in each of the two experiments?
2. What is the net charge on the electroscope in the first experiment? Explain how the electroscope obtains that charge.
3. Is the net charge on the electroscope in the second experiment different from that of the first experiment? Explain why.

An eccentric inventor attempts to levitate by first placing a large negative charge on himself and then putting a large positive charge on the ceiling of his workshop. Instead, while attempting to place a large negative charge on himself, his clothes fly off. Explain.

If you have charged an electroscope by contact with a positively charged object, describe how you could use it to determine the charge of other objects. Specifically, what would the leaves of the electroscope do if other charged objects were brought near its knob?

When a glass rod is rubbed with silk, it becomes positive and the silk becomes negative—yet both attract dust. Does the dust have a third type of charge that is attracted to both positive and negative? Explain.

Why does a car always attract dust right after it is polished? (Note that car wax and car tires are insulators.)

Describe how a positively charged object can be used to give another object a negative charge. What is the name of this process?

What is grounding? What effect does it have on a charged conductor? On a charged insulator?

Suppose a speck of dust in an electrostatic precipitator has $1\text{.}\text{0000}\times {\text{10}}^{\text{12}}$ protons in it and has a net charge of –5.00 nC (a very large charge for a small speck). How many electrons does it have?

An amoeba has $1.00\times {10}^{16}$ protons and a net charge of 0.300 pC. (a) How many fewer electrons are there than protons? (b) If you paired them up, what fraction of the protons would have no electrons?

A 50.0 g ball of copper has a net charge of $2.00\mu \text{C}$ . What fraction of the copper's electrons has been removed? (Each copper atom has 29 protons, and copper has an atomic mass of 63.5.)

What net charge would you place on a 100 g piece of sulfur if you put an extra electron on 1 in ${\text{10}}^{\text{12}}$ of its atoms? (Sulfur has an atomic mass of 32.1.)

How many coulombs of positive charge are there in 4.00 kg of plutonium, given its atomic mass is 244 and that each plutonium atom has 94 protons?