QUICK QUIZ 15.1 If a suspended object A is attracted to object B, which is charged, we can conclude that (a) object A is uncharged, (b) object A is charged, (c) object B is positively charged, or (d) object A may be either charged or uncharged.

QUICK QUIZ 15.1 ANSWER (d). Object A could possess a net charge whose sign is opposite that of the excess charge on B. If object A is neutral, B would also attract it by creating an induced charge on the surface of A. This situation is illustrated in Figure 15.5 of the textbook.

QUICK QUIZ 15.2 Object A has a charge of +2 µC, and object B has a charge of +6 µC. Which statement is true: (a) FAB = –3FBA, (b) FAB = –FBA, or (c) 3FAB = –FBA

QUICK QUIZ 15.2 ANSWER (b). By Newton’s third law, the two objects will exert forces having equal magnitudes but opposite directions on each other.

QUICK QUIZ 15.3 A test charge of +3 µC is at a point P where the electric field due to other charges is directed to the right and has a magnitude of 4  106 N/C. If the test charge is replaced with a –3 µC charge, the electric field at P (a) has the same magnitude but changes direction, (b) increases in magnitude and changes direction, (c) remains the same, or (d) decreases in magnitude and changes direction.

QUICK QUIZ 15.3 ANSWER (c). The electric field at point P is due to charges other than the test charge. Thus, it is unchanged when the test charge is altered. However, the direction of the force this field exerts on the test change is reversed when the sign of the test charge is changed.

QUICK QUIZ 15.4 A Styrofoam ball covered with a conducting paint has a mass of 5.0 ´ 10-3 kg and has a charge of 4.0 C. What electric field directed upward will produce an electric force on the ball that will balance the weight of the ball? (a) 8.2 ´ 102 N/C (b) 1.2 ´ 104 N/C (c) 2.0 ´ 10-2 N/C (d) 5.1 ´ 106 N/C

QUICK QUIZ 15.4 ANSWER (b). The magnitude of the upward electrical force must equal the weight of the ball. That is: qE = mg, so = 1.2 × 104 N/C

QUICK QUIZ 15.5 A circular ring of radius b has a total charge q uniformly distributed around it. The magnitude of the electric field at the center of the ring is (a) 0 (b) keq/b2 (c) keq2/b2 (d) keq2/b (e) none of these.

QUICK QUIZ 15.5 ANSWER (a). If a test charge is at the center of the ring, the force exerted on the test charge by charge on any small segment of the ring will be balanced by the force exerted by charge on the diametrically opposite segment of the ring. The net force on the test charge, and hence the electric field at this location, must then be zero.

QUICK QUIZ 15.6 A "free" electron and "free" proton are placed in an identical electric field. Which of the following statements are true? (a) Each particle experiences the same electric force and the same acceleration. (b) The electric force on the proton is greater in magnitude than the force on the electron but in the opposite direction. (c) The electric force on the proton is equal in magnitude to the force on the electron, but in the opposite direction. (d) The magnitude of the acceleration of the electron is greater than that of the proton. (e) Both particles experience the same acceleration.

QUICK QUIZ 15.6 ANSWERS (c) and (d). The electron and the proton have equal magnitude charges of opposite signs. The forces exerted on these particles by the electric field have equal magnitude and opposite directions. The electron experiences an acceleration of greater magnitude than does the proton because the electron’s mass is much smaller than that of the proton.

QUICK QUIZ 15.7 Rank the magnitudes of the electric field at points A, B, and C in Figure 15.15, largest magnitude first.

QUICK QUIZ 15.7 ANSWERS A, B, and C. The field is greatest at point A because this is where the field lines are closest together. The absence of lines at point C indicates that the electric field there is zero.

QUICK QUIZ 15.8 For a surface through which the net flux is zero, the following four statements could be true. Which of the statements must be true? (a) There are no charges inside the surface. (b) The net charge inside the surface is zero. (c) The electric field is zero everywhere on the surface. (d) The number of electric field lines entering the surface equals the number leaving the surface.

QUICK QUIZ 15.8 ANSWERS Statements (b) and (d) are true and follow from Gauss’s law. Statement (a) is not necessarily true because Gauss’s law says that the net flux through any closed surface equals the net charge inside the surface divided by ε0. For example, a positive and a negative charge could be inside the surface. Statement (c) is not necessarily true. Although the net flux through the surface is zero, the electric field in that region may not be zero.