# 18.1 Static electricity and charge: conservation of charge  (Page 6/10)

 Page 6 / 10

In an experiment, three microscopic latex spheres are sprayed into a chamber and become charged with +3 e , +5 e , and −3 e , respectively. Later, all three spheres collide simultaneously and then separate. Which of the following are possible values for the final charges on the spheres? Select two answers.

X Y Z
(a) +4 e −4 e +5 e
(b) −4 e +4.5 e +5.5 e
(c) +5 e −8 e +7 e
(d) +6 e +6 e −7 e

If objects X and Y attract each other, which of the following will be false?

1. X has positive charge and Y has negative charge.
2. X has negative charge and Y has positive charge.
3. X and Y both have positive charge.
4. X is neutral and Y has a charge.

(c)

Suppose a positively charged object A is brought in contact with an uncharged object B in a closed system. What type of charge will be left on object B?

1. negative
2. positive
3. neutral
4. cannot be determined

What will be the net charge on an object which attracts neutral pieces of paper but repels a negatively charged balloon?

1. negative
2. positive
3. neutral
4. cannot be determined

(a)

When two neutral objects are rubbed against each other, the first one gains a net charge of 3 e. Which of the following statements is true?

1. The second object gains 3 e and is negatively charged.
2. The second object loses 3 e and is negatively charged.
3. The second object gains 3 e and is positively charged.
4. The second object loses 3 e and is positively charged.

In an experiment, a student runs a comb through his hair several times and brings it close to small pieces of paper. Which of the following will he observe?

1. Pieces of paper repel the comb.
2. Pieces of paper are attracted to the comb.
3. Some pieces of paper are attracted and some repel the comb.
4. There is no attraction or repulsion between the pieces of paper and the comb.

(b)

In an experiment a negatively charged balloon (balloon X) is repelled by another charged balloon Y. However, an object Z is attracted to balloon Y. Which of the following can be the charge on Z? Select two answers.

1. negative
2. positive
3. neutral
4. cannot be determined

Suppose an object has a charge of 1 C and gains 6.88×10 18 electrons.

1. What will be the net charge of the object?
2. If the object has gained electrons from a neutral object, what will be the charge on the neutral object?
3. Find and explain the relationship between the total charges of the two objects before and after the transfer.
4. When a third object is brought in contact with the first object (after it gains the electrons), the resulting charge on the third object is 0.4 C. What was its initial charge?

(a) -0.1 C, (b) 1.1 C, (c) Both charges will be equal to 1 C, law of conservation of charge, (d) 0.9 C

The charges on two identical metal spheres (placed in a closed system) are -2.4×10 −17 C and -4.8×10 −17 C.

1. How many electrons will be equivalent to the charge on each sphere?
2. If the two spheres are brought in contact and then separated, find the charge on each sphere.
3. Calculate the number of electrons that would be equivalent to the resulting charge on each sphere.

In an experiment the following observations are made by a student for four charged objects W, X, Y, and Z:

• A glass rod rubbed with silk attracts W.
• W attracts Z but repels X.
• X attracts Z but repels Y.
• Y attracts W and Z.

Estimate whether the charges on each of the four objects are positive, negative, or neutral.

W is negative, X is positive, Y is negative, Z is neutral.

## Section summary

• There are only two types of charge, which we call positive and negative.
• Like charges repel, unlike charges attract, and the force between charges decreases with the square of the distance.
• The vast majority of positive charge in nature is carried by protons, while the vast majority of negative charge is carried by electrons.
• The electric charge of one electron is equal in magnitude and opposite in sign to the charge of one proton.
• An ion is an atom or molecule that has nonzero total charge due to having unequal numbers of electrons and protons.
• The SI unit for charge is the coulomb (C), with protons and electrons having charges of opposite sign but equal magnitude; the magnitude of this basic charge $\mid {q}_{e}\mid$ is
$\mid {q}_{e}\mid =1.60×{\text{10}}^{-\text{19}}\phantom{\rule{0.25em}{0ex}}\text{C}.$
• Whenever charge is created or destroyed, equal amounts of positive and negative are involved.
• Most often, existing charges are separated from neutral objects to obtain some net charge.
• Both positive and negative charges exist in neutral objects and can be separated by rubbing one object with another. For macroscopic objects, negatively charged means an excess of electrons and positively charged means a depletion of electrons.
• The law of conservation of charge ensures that whenever a charge is created, an equal charge of the opposite sign is created at the same time.

## Conceptual questions

There are very large numbers of charged particles in most objects. Why, then, don't most objects exhibit static electricity?

Why do most objects tend to contain nearly equal numbers of positive and negative charges?

## Problems&Exercises

Common static electricity involves charges ranging from nanocoulombs to microcoulombs. (a) How many electrons are needed to form a charge of $–2.00\phantom{\rule{0.25em}{0ex}}\text{nC}$ (b) How many electrons must be removed from a neutral object to leave a net charge of $0.500\phantom{\rule{0.25em}{0ex}}µ\text{C}$ ?

(a) $1.25×{\text{10}}^{\text{10}}$

(b) $3.13×{\text{10}}^{\text{12}}$

If $1\text{.}\text{80}×{\text{10}}^{\text{20}}$ electrons move through a pocket calculator during a full day's operation, how many coulombs of charge moved through it?

To start a car engine, the car battery moves $3\text{.}\text{75}×{\text{10}}^{\text{21}}$ electrons through the starter motor. How many coulombs of charge were moved?

-600 C

A certain lightning bolt moves 40.0 C of charge. How many fundamental units of charge $\mid {q}_{e}\mid$ is this?

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