QUIZ: Coloumb's Law and Charges

Which of the following provides evidence that there must be at least two types of electrical charge, but that there is only one type of mass?

• The magnitude of the Coulomb's law constant differs greatly from the magnitude of the universal gravitational constant.

• Contact forces such as friction are caused primarily by electrostatic forces and not by gravitational ones.

• The electric force has a relative strength that is much greater than the strength of the gravitational force.

• On the scale of the solar system, electric forces cancel out and gravitational forces dominate.

Two small conducting spheres hang from insulating strings and are in contact, as shown in Figure 1. A positively charged rod then touches the left sphere and the two spheres repel, as shown in Figure 2. Which of the following correctly identifies the charges on the left and right spheres as the two spheres hang as shown in Figure 2?

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Two pieces of electrically neutral tape are stuck together. The sticky side of the top piece is stuck to the back side of the bottom piece, as shown in the diagram. A student pulls them apart quickly. Afterward, the student holds the backs (nonsticky sides) of the two pieces of tape near one another, and they attract. The attraction of the pieces is evidence of which of the following?

• The two pieces of tape have equal amounts of excess charge.

• There are two types of charge, and these two types attract each other.

• The charge on the top piece is negative, and the charge on the bottom piece is positive.

• The charge on the pieces is due to the electrons moving from one piece to the other.

The figure shows two identical, initially uncharged conducting spheres. Sphere B is grounded and sphere A is not grounded. A positively charged rod is brought close to but does not touch sphere A, as shown. The rod is then removed. A student claims that both spheres are now charged. Is the student correct? Why or why not?

• Yes. When the rod is brought near, both spheres become negatively charged

• Yes. The ground on sphere B ensures that there is no movement of charge on either sphere when the rod is removed.

• No. When the rod is brought near, sphere A is polarized and sphere B gains a net negative charge. When the rod is removed, sphere B remains charged but sphere A does not remain polarized.

• No. When the rod is removed, the situation is identical to before the rod was brought near, so both spheres are uncharged.

The figure represents the charge distribution on one side of a neutral aluminum disk. A large negatively charged rubber rod is touched to the disk, separated from the disk, and finally held near the right side of the disk. Which of the following best represents the final charge distribution on the side of the disk?

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Answer explanation

The figure shows a negatively charged conducting rod and two uncharged conducting spheres on insulating stands set near each other. The rod is brought near sphere 1, sphere 1 is grounded, the ground is removed, and then the rod is moved away. The rod is then touched to sphere 2 and removed. Which of the following cross sections best shows the resulting distribution of the excess charge on the spheres?

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Three identical metal spheres are mounted on insulating stands. Initially, sphere A has no charge, sphere B has a charge of −4q, and sphere C has a charge of +2q. Spheres A and C are touched together and then separated. Then spheres A and B are touched together and separated. What is the resulting charge on each sphere?

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Students have four identical, hollow, uncharged conducting spheres, W, X, Y, and Z.

Sphere Z is given a positive charge of +40μC. Sphere Z is touched first to sphere W, then sphere X, and finally to sphere Y. What is the resulting charge on sphere Y?

• +5 μC

• +10 μC

• +20 μC

• +40 μC

Students have four identical, hollow, uncharged conducting spheres, W, X, Y, and Z.

The separate spheres are given the charges shown in the figure and held at the corners of a square. Which of the following relationships between electric forces can be explained with the concept of action-reaction pairs of forces?

• The force that sphere W exerts on sphere X has the same magnitude as the force that sphere Y exerts on sphere X.

• The force that sphere Y exerts on sphere X has the same magnitude as the force that sphere X exerts on sphere Y.

• The force that sphere Y exerts on sphere X has the same magnitude as the force that sphere Y exerts on sphere Z.

• The force that sphere W exerts on sphere Y has the same magnitude as the force that sphere Z exerts on sphere X.

Two solid conducting spheres, sphere A and sphere B, are made of the same material. The radius of sphere B is two times the radius of sphere A. Initially sphere A is negatively charged while sphere B is uncharged, as shown in the figure. The spheres are brought into contact and then separated. Which of the following is the best representation of the distribution of charge on the two spheres after the two spheres have been separated? Assume the net charge of each sphere is proportional to the number of charges drawn on its surface.

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