Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | The drawing shows three possibilities for the potentials at two points, A and B. In each case, the same positive charge is moved from A to B. In which case, if any, is the most work done on the positive charge by the electric force? Account for your answer.

| 2 | A positive point charge and a negative point charge have equal magnitudes. One charge is fixed to one corner of a square, and the other is fixed to another corner. On which corners should the charges be placed, so that the same potential exists at the empty corners? Give your reasoning.

| 3 | Three point charges have identical magnitudes, but two of the charges are positive and one is negative. These charges are fixed to the corners of a square, one to a corner. No matter how the charges are arranged, the potential at the empty corner is positive. Explain why.

| 4 | ssm What point charges, all having the same magnitude, would you place at the corners of a square (one charge per corner), so that both the electric field and the electric potential (assuming a zero reference value at infinity) are zero at the center of the square? Account for the fact that the charge distribution gives rise to both a zero field and a zero potential.

| 5 | Positive charge is spread uniformly around a circular ring, as the drawing illus trates. Equation 19.6 gives the correct potential at points along the line perpendicular to the plane of the ring at its center. However, the equation does not give the correct potential at points that do not lie on this line. In the equation, q represents the total charge on the ring. Why does Equation 19.6 apply for points on the line, but not for points off the line?

| 6 | The electric field at a single location is zero. Does this fact necessarily mean that the electric potential at the same place is zero? Use a spot on the line between two identical point charges as an example to support your reasoning.

| 7 | ssm An electric potential energy exists when two protons are separated by a certain distance. Does the electric potential energy in crease, decrease, or remain the same when (a) both protons are replaced by electrons, and (b) only one of the protons is replaced by an electron? Justify your answers.

| 8 | A proton is fixed in place. An electron is released from rest and allowed to collide with the proton. Then the roles of the proton and electron are interchanged, and the same experiment is repeated. Which is traveling faster when the collision occurs, the proton or the electron? Justify your answer.

| 9 | The potential is constant throughout a given region of space. Is the electric field zero or nonzero in this region? Explain.

| 10 | In a region of space where the electric field is constant every where, as it is inside a parallel plate capacitor, is the potential constant everywhere? Account for your answer.

| 11 | A positive test charge is placed in an electric field. In what direction should the charge be moved relative to the field, such that the charge experiences a constant electric potential? Explain.

| 12 | The location marked P in the drawing lies midway between the point charges +q and −q. The blue lines labeled A, B, and C are edge-on views of three planes. Which one of these planes is an equipotential surface? Why?

Question 12

| 13 | Imagine that you are moving a positive test charge along the line between two identical point charges. With regard to the electric potential, is the midpoint on the line analogous to the top of a mountain or the bottom of a valley when the two point charges are (a) positive and (b) negative? In each case, explain your answer.

| 14 | Repeat question 13, assuming that you are moving a negative instead of a positive test charge.

| 15 | The potential at a point in space has a certain value, which is not zero. Is the electric potential energy the same for every charge that is placed at that point? Give your reasoning.

| 16 | ssm A proton and an electron are released from rest at the midpoint between the plates of a charged parallel plate capacitor. Except for these particles, nothing else is between the plates. Ignore the attraction between the proton and the electron, and decide which particle strikes a capacitor plate first. Why?

| 17 | A parallel plate capacitor is charged up by a battery. The battery is then disconnected, but the charge remains on the plates. The plates are then pulled apart. Explain whether each of the following quantities increases, decreases, or remains the same while the distance between the plates increases: (a) the capacitance of the capacitor, (b) the potential difference between the plates, (c) the electric field between the plates, and (d) the electric potential energy stored by the capacitor. Give reasons for your answers.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.