Homework #1Due: 11:59pm on Wednesday, July 9, 2014 You will receive no credit for items you complete after the assignment is due. Grading Policy Item 1 The Charge on Adhesive Tape When adhesive tape is pulled from a dispenser, the detached tape acquires a positive charge and the remaining tape in the dispenser acquires a negative charge. Part A If the tape pulled from the dispenser has 0.14μC of charge per centimeter, what length of tape must be pulled to transfer 1.9×1013 electrons to the remaining tape? Express your answer using two significant figures. ANSWER: L = 22 cm Correct Item 2 Consider the three electric charges, A, B, and C, shown in the figure. Part A Rank the charges in order of decreasing magnitude of the net force they experience. Rank charges from largest to smallest magnitude of the net force they experience. To rank items as equivalent, overlap them. ANSWER: Correct Item 3 Part A Given that q = 14μC and d = 16cm , find the direction and magnitude of the net electrostatic force exerted on the point charge q 1 in the figure. Express your answer using two significant figures. ANSWER: F = 86 N Correct Part B ANSWER: toward the left toward the right Correct Item 4 A point charge q = -0.32nC is fixed at the origin. Part A Where must a proton be placed in order for the electric force acting on it to be exactly opposite to its weight? (Let the y axis be vertical and the x axis be horizontal.) ANSWER: above the q below the q Correct Part B Express your answer using two significant figures. ANSWER: r = 5.3 km Correct Item 5 Part A Find the direction and magnitude of the net electrostatic force exerted on the point charge q 2 in the figure. Let Express your answer using two significant figures. q = +2.3μC and d = 36cm. ANSWER: Fnet = 3.3 N Correct Part B Express your answer using three significant figures. ANSWER: θ = 175 ∘ counterclockwise from q 2 -q3 direction Correct Item 6 μC μC IP Two point charges lie on the 10.7cm axis. A charge of 9.2μC is at the origin, and a charge of -4.4μC is at x = 10.7cm . Part A At what position x would a third charge q 3 be in equilibrium? Express your answer using two significant figures. ANSWER: x = 35 cm Correct Part B Does your answer to part A depend on whether q 3 is positive or negative? Explain. ANSWER: 3508 Character(s) remaining No, either way it would still be in equilibrium at 35 cm. F13 - F23 and F23 - F13 both equal zero as Coulomb's law uses the absolute value of the charge. The assignment of positive or negative is arbitrary as long as the relationship between the charges and all other factors remain constant. Submitted, grade pending Item 7 Two point charges lie on the x axis. A charge of 6.2μC is at the origin, and a charge of -9.6μC is at x = −4.0 cm and at (b) x = +4.0 cm? Part A x = 10.0 cm . What is the net electric field at (a) Express your answer using two significant figures. ANSWER: E = −3.0×107 N/C Correct Part B Express your answer using two significant figures. ANSWER: E = 5.9×107 N/C Correct Item 8 An object with a charge of -4.0μC and a mass of 1.7×10−2kg experiences an upward electric force, due to a uniform electric field, equal in magnitude to its weight. Part A Find the magnitude of the electric field. Express your answer using two significant figures. ANSWER: E = 4.2×104 N/C Correct Part B Find the direction of the electric field. ANSWER: upward downward to the left to the right Correct Part C If the electric charge on the object is doubled while its mass remains the same, find the direction and magnitude of its acceleration. Express your answer using three significant figures. ANSWER: a = 9.81 2 m/s Correct Part D If the electric charge on the object is doubled while its mass remains the same, find the direction and magnitude of its acceleration. ANSWER: upward downward to the left to the right Correct Item 9 Two point charges of equal magnitude are 8.3cm apart. At the midpoint of the line connecting them, their combined electric field has a magnitude of 41N/C . Part A Find the magnitude of the charges. Express your answer using two significant figures. ANSWER: q = 3.9 pC Correct Item 10 Two small plastic balls hang from threads of negligible mass. Each ball has a mass of 0.180g and a charge of magnitude q . The balls are attracted to each other, and the threads attached to the balls make an angle of 20.0 ∘ with the vertical, as shown in the figure. Part A Find the magnitude of the electric force acting on each ball. ANSWER: F = 6.42×10−4 N Correct Part B Find the tension in each of the threads. ANSWER: T = 1.88×10−3 N Correct Part C Find the magnitude of the charge on the balls. ANSWER: q = 5.48 nC Correct Item 11 The table below presents the relative charging due to rubbing - also known as triboelectric charging - for a variety of materials. The more plus signs associated with a material, the more readily it gives up electrons. Similarly, the more minus signs for a material, the more readily it acquires electrons. Material Relative charging with rubbing Rabbit fur ++++++ Glass Human hair Nylon Silk +++++ ++++ +++ ++ Paper + Cotton - Wood -- Amber --- Rubber ---- PVC ----- Teflon ------ Part A If you rub a rubber balloon against a cloth surface, it can attract a stream of water. Based on the materials listed in table, is the charge of the rubber balloon more likely to be positive or negative? ANSWER: positive negative Correct Part B Explain. ANSWER: Essay answers are limited to about 500 words (3800 characters maximum, including spaces). 3785 Character(s) remaining (none provided) Part C If the charge on the balloon is reversed, will the stream of water deflect toward or away from the balloon? ANSWER: toward away Correct Part D Explain. ANSWER: Essay answers are limited to about 500 words (3800 characters maximum, including spaces). 3785 Character(s) remaining (none provided) Item 12 Part A If rabbit fur is rubbed against glass, what is the sign of the charge each acquires? (Refer to the table.) Material Rabbit fur Glass Rubber ANSWER: Relative charging with rubbing ++++++ +++++ ---- rabbit fur - positive, glass - negative rabbit fur - negative, glass - positive Correct Part B Explain. ANSWER: Essay answers are limited to about 500 words (3800 characters maximum, including spaces). 3785 Character(s) remaining (none provided) Part C Repeat part A for the case of glass and rubber. ANSWER: rubber - positive, glass - negative rubber - negative, glass - positive Part D This question will be shown after you complete previous question(s). Part E Comparing the situations described in parts A and C, in which case is the magnitude of the triboelectric charge the greatest? ANSWER: in part (a) in part (c) Part F This question will be shown after you complete previous question(s). Item 13 Four pairs of conducting spheres, all with the same radius, are shown in the figure, along with the net charge placed on them initially. The spheres in each pair are now brought into contact, allowing charge to transfer between them. Part A Rank the pairs of spheres in order of decreasing magnitude of the charge transferred. Rank from largest to smallest. To rank items as equivalent, overlap them. ANSWER: Correct Item 14 Suppose the charge q 2 in the figure can be moved left or right along the line connecting the charges q and q . 1 3 Part A Given that q = +12 μC , find the distance from q 1 where q 2 experiences a net electrostatic force of zero. (The charges fixed distance of 42cm .) Express your answer using two significant figures. ANSWER: x = 15 cm Correct Item 15 IP The point charges in the figure have the following values: q1 = 2.2μC , q2 = 6.3μC , q3 = -0.86μC . q 1 and q 3 are separated by a Part A Given that the distance d in the figure is 4.57cm , find the direction and magnitude of the net electrostatic force exerted on the point charge q 1 . Express your answer using two significant figures. ANSWER: Fnet = N Part B Express your answer using three significant figures. ANSWER: θ ∘ = counterclockwise from - q1 q3 direction Part C How would your answer to part A change if the distance d were doubled? ANSWER: increased by the factor of 2 increased by the factor of 4 decreased by the factor of 2 decreased by the factor of 4 stay the same Part D This question will be shown after you complete previous question(s). Score Summary: Your score on this assignment is 97.2%. You received 19.43 out of a possible total of 20 points.
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