The above figures indicate a sequence of events in time that involve a positivel

Question

The above figures indicate a sequence of events in time that involve a positively charged rod, wires, a conducting sphere, a switch, and the "ground". Which is the correct statement? (A) At (c) the sphere carries a net positive charge. (B) At (e) the sphere is uncharged. (C) At (e) the sphere carries a net negative charge. The next two questions pertain to the following situation Three negative charges are arranged as shown to the right. The magnitude of the force on q_2 due to charge q_1 is F_21. The magnitude of the force on q_2 due to charge q_2 is F_23. The ratio F_21/F_23 is (A) F_21/F_23 = 2/3 (B) F_21/F_23 = 4/3 (C) F_21/F_23 =2 (D) F_21/F_23 = 4 (E) F_21/F_23 = 6 Calculate the potential energy U stored in the configuration of charges shown above. (A) U = +9.5 times 10^-1J (B) U = +1.6 times 10^-2 J (C) U = +2.2 times 10^-2J (D) U = +3.6 times 10^-2 J (E) U = +5.7 times 10^-2 J

Explanation / Answer

2) Magnitude of force on q2 by q1 = kq1q2 / r^2

= 9*10^9*1*2*10^-12 / 1^2

= 0.018 N

magntiude of force on q2 by q3 = kq2q3 / r^2

= 9*10^9*2*3*10^-12 / 2^2

= 0.0135 N

= 0.018/0.0135 = 4/3

option B) is the correct answer

3) Potential energy of the system = U12 + U23 + U31

= kq1q2 / r1 + kq2q3 / r2 + kq3q1 / r3

= 9*10^9*10^-12 ( -1*-2 / 1 + -2*-3/2 + -3*-1 / sqrt 5)

= 0.057 J

= 5.7*10^-2 J

so option E) is the correct answer

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