Ch. 18

Question 1

An electricity neutral object acquires a net electric charge. Which one of the following statements concerning the mass of the object is true?

Answer 1

The mass increases if the charge is negative and decreases if it is positive.

Question 2

Object A and object B are each electrically neutral. Two million electrons are removed from object A and placed on object B. Expressed in coulombs, what is the resulting charge (algebraic sign and magnitude) on object A and on object B?

Answer 2

Object A +3.2x10^-13 C

Object B -3.2x10^-13 C

Question 3

Object A has a charge of -1.6x10^-13 C, and object B is electrically neutral. Two million electrons are removed from object A and placed on object B. Expressed in coulombs, what is the resulting charge on object A and on object B?

Answer 3

Object A +1.6x10^-13 C

Object B -3.2x10^-13 C

Question 4

Two metal spheres are identical. They are electrically neutral and are touching. An electrically charged ebonite rod is then brought near the spheres without touching them. After a while, with the rod held in place, the spheres are separated, and the rod is then removed. The following statements refer to the masses mA and mB of the spheres are they are separated and the rod is removed. Which one or more of the statements is true?

Answer 4

mA > mB if the rod is positive

mA < mB is the rod is negative

Question 5

Blow up a balloon, tie it shut, and rub it against your shirt a number of times, so that the balloon acquires a net electric charge. Now touch the balloon to the ceiling. When released, will the balloon remain stuck to the ceiling?

Answer 5

Yes, because the charge on the balloon will induce a slight charge of opposite polarity in the surface of the ceiling.

Question 6

A rod made from insulating material carries a net charge (which may be positive or negative), whereas a copper sphere is electrically neutral. The rod is held close to the sphere but does not touch it. Which statement concerning the forces that the rod and sphere exert on each other is true?

Answer 6

The forces are always attractive.

Question 7

Identical point charges are fixed to diagonally opposite corners of a square. Where does a third point charge experience the greater force?

Answer 7

At one of the empty corners.

Question 8

A proton and an electron are held in place on the x axis. The proton is at x = -d, while the electron is at x = +d. They are released simultaneously, and the only force that affects their motions significantly is the electrostatic force of attraction that each applies to the other. Which particle reaches the origin first?

Answer 8

The electron, because, being less massive, it has the greater acceleration

Question 9

A particle is attached to one end of a horizontal spring, and the other end of the spring is attached to a wall. When the particle is pushed so that the spring is compressed more and more, the particle experiences a greater and greater force from the spring. Similarly, a charged particle experiences a greater and greater force when pushed closer and closer to another particle that is fixed in position and has a charge of the same polarity. Considering this similarity, will the charged particle exhibit simple harmonic motion on being released, as will the particle on the spring?

Answer 9

No, because the force of the spring changes direction when the spring is stretched compared to when it’s compressed, while the electrostatic force does not have this characteristic

Question 10

There is an electric field at point P. A very small positive charge is placed at this point and experiences a force. Then the positive charge is replaced by a very small negative charge that has a magnitude different from that of the positive charge. Which one of the statements is true concerning the forces that these charges experience at P?

Answer 10

They have different magnitudes and different directions.

Question 11

A positive point charge is located to the left of a negative point charge. When both charges have the same magnitude, there is no place on the line passing through both charges where the net electric field due to the two charges is zero. Suppose, however, that the negative charge has a greater magnitude than the positive charge. On which part of the line, if any, is a place of zero net electric field now located?

Answer 11

To the left of the positive charge.

Question 12

Three point charges are fixed to the corners of a square, one to a corner, in such a way that the net electric field at the empty corner is zero.

Do these charges all have the same sign and the same magnitude?

Answer 12

No, no

Question 13

Consider two identical, thin, and nonconducting rods, A and B. On rod A, positive charge is spread evenly, so that there is the same amount of charge per unit length at every point. On rod B, positive charge is spread evenly over only the left half, and the same amount of negative charge is spread evenly over the right half. For each rod deduce the direction of the electric field at a point that is located directly above the midpoint of the rod.

Answer 13

For rod A, the field points perpendicularly away from the rod.

For rod B, it points parallel to the rod and is directed from the positive toward the negative half.

Question 14

A Gaussian surface contains a single charge within it, and as a result an electric flux passes through the surface. Suppose that the charge is then moved to another spot within the Gaussian surface. Does the flux through the surface change?

Answer 14

The flux does not change, as long as the charge remains the Gaussian surface.

Question 15

Each of three objects carries a charge. Objects A and B attract each other, and objects C and A also attract each other. What is true about objects B and C?

Answer 15

They repel each other.

Question 16

Each of two identical objects carries a net charge. The objects are made from conducting material. One object is attracted to a positively charged ebonite rod, and the other is repelled by the rod. After the objects are touched together, it is found that they are each repelled by the rod. What can be concluded about the initial charges on the objects?

Answer 16

Initially one object is positive and one is negative, with the positive charge having a greater magnitude than the negative charge.

Question 17

Only one of three balls A, B, and C carries a net charge q. The balls are made from conducting material and are identical. One of the uncharged balls can become charged by touching it to the charged ball and then separating the two. This process of touching one ball to another and then separating the two balls can be repeated over and over again, with the result that the three balls can take on a variety of charges. Which one of the following distributions of charges could not possibly be achieved in this fashion, even if the process were repeated a large number of times?

Answer 17

qA = (1/2)q
qB = (3/8)q
qC = (1/4)q 

Not possible because of the law of conservation of electric charge. The total charge on the three objects here is (9/8)q

Question 18

A positive point charge q1 creates an electric field of magnitude E1 at a spot located at a distance r1 from the charge. The charge is replaced by another positive point charge q2, which creates a field of magnitude E2 = E1, at a distance of r2 = 2r1. How is q2 related to q1?

Answer 18

q2 = 4q1