Electrostatics Sample Multiple Choice Test1. A conducting sphere has a net charge of −4.8 × 10−17 C. What is the approximate number of excess electrons on the sphere? A) 100 B) 200 C) 300 D) 400 E) 500 2. A charge Q exerts a 12 N force on another charge q. If the distance between the charges is doubled, what is the magnitude of the force exerted on Q by q? A) 3 N B) 6 N C) 24 N D) 36 N E) 48 N 3. At what separation will two charges, each of magnitude 6.0 µC, exert a force of 1.4 N on each other? A) 5.1 × 10−6 m B) 0.23 m C) 0.48 m D) 2.0 m E) 40 m 4. Determine the ratio of the electrostatic force to the gravitational force between a proton and an electron, FE/FG. Note: k = 8.99 × 109 N•m2 /C2 ; G = 6.672 × 10−11 N•m2 /kg2 ; me = 9.109 × 10−31 kg; and mp = 1.672 × 10−27 kg. A) 1.24 × 1023 B) 2.52 × 1029 C) 1.15 × 1031 D) 2.26 × 1039 E) 1.42 × 1058 Page 1 Page 2 . 5. The electric field is zero somewhere on the x axis between the two charges. The electric field is zero somewhere on the x axis to the right of the −2q charge. B. E) The electric field is zero at two points along the x axis. because the forces on Q are vectors and three vectors can add to zero. 6. The electric field is zero somewhere on the x axis to the left of the +4q charge. because the electric force at the mid-point between B and C is zero whether a charge is placed there or not. At which point (or points) is the electric field zero N/C for the two point charges shown on the x axis? A) B) C) D) The electric field is never zero in the vicinity of these charges. but this point is nearer to the −2q charge. because the forces on Q due to the charges at B and C point in the same direction. B) No. because a fourth charge would be needed to cancel the force on Q due to the charge at A.Use the following to answer question 55: The figure shows an equilateral triangle ABC. one such point is to the right of the −2q charge and the other is to the left of the +4q charge. A positive point charge +q is located at each of the three vertices A. A point charge Q (that may be positive or negative) is placed at the mid-point between B and C. D) No. A) Yes. E) No. because the forces on Q are vectors and three vectors can never add to zero. C) Yes. Each side of the triangle is of length a. Is it possible to choose the value of Q (that is non-zero) such that the force on Q is zero? Explain why or why not. and C. An electron traveling horizontally enters a region where a uniform electric field is directed upward.7. What is the direction of the force exerted on the electron once it has entered the field? A) B) C) D) E) to the left to the right upward downward out of the page. The numbered lines show the paths taken by the five particles. A) B) C) D) E) In which direction does the electric field point? toward the top of the page toward the left of the page toward the right of the page toward the bottom of the page out of the page. A negatively charged particle with a charge −3Q follows path 2 while it moves through this field. Do not consider any effects due to gravity. toward the reader Page 3 . 8. toward the reader Use the following to answer questions 58-59: Five particles are shot from the left into a region that contains a uniform electric field. 7. 9. 5. 4. C A C D E C D D E Page 4 .9. 8. A) B) C) D) E) Which path would be followed by a charge +6Q? path 1 path 2 path 3 path 4 path 5 Answer Key 1. 6. 2. 3.