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Electrostatics Problems (all with worked out answers but wrong)

  1. Dec 12, 2009 #1
    1. The problem statement, all variables and given/known data

    1. A thin rod bent in the shape of a semicircle of radius 20 cm is uniformly charged along its length with a total charge of 8.0 microC. What is the electric field at the center of the semicircle?
    a. 7.3 x 10^4 N/C
    b. 8.4 x 10^4
    c. 5.7 x 10^4
    d. 3.9 x 10^4
    e. None
    Answer: C

    2. Two charges, q1 = +2.0 microC and q2 = -3.0 microC are placed on the two corners A and B of a square as shown. What should be the value a charge q3 placed at the corner D such that the net electrostatic force on a test charge Q on corner C is zero?
    a. 3.0 microC
    b. -3.0
    c. -2.0
    d. +2.0
    e. None
    Answer: E

    3. Two charges of +5.0 microC are placed on two corners of an equilateral triangle of side 0.50 m. The magnitude of the electric field at the third corner is:
    a. 3.9 x 10^3 N/C
    b. 2.8 x 10^4
    c. 1.6 x 10^5
    d. Zero
    e. None
    Answer: C

    4. A charge of +3.0 microC is located at the origin and another charge of -4.0 microC is located on the y axis at y = 0.30 m. The magnitude of the electric field at 0.40 m is:
    a. 2.5 x 10^3 N/C
    b. 3.2 x 10^4
    c. 5.7 x 10^4
    d. 9.5 x 10^4
    e. None
    Answer: D

    5. The figure shows two 5.0 g spheres suspended from 1.0-m-long threads. The spheres repel each other after being charged to +31 nC. What is the angle theta?
    a. 4.0 degrees
    b. 6.0
    c. 2.0
    d. 7.9
    Answer: C

    6. Three charges of magnitude 9.0 x 10^-4 C each are located at x = 1.0m, y = 0.0 m, at x = 0.0 m, y = 0.0 m, and a t x = -1.0 m, y = 0.0 m. The one in the middle is negative, while the other two are positive. What is the net Coulombic force exerted by them on a negative 9.0 x 10^-5 C charge located at x = 0.0 m, y = -2.0 m?
    a. 790 N repulsive
    b. 1100 N attractive
    c. 1100 N repulsive
    d. 790 N attractive
    Answer: D

    2. Relevant equations

    3. The attempt at a solution

    *see pictures*

    For second 3:

    http://img695.imageshack.us/img695/8262/electrostatics4.jpg [Broken]
    http://img189.imageshack.us/img189/3112/electrostatics5.jpg [Broken]
    http://img706.imageshack.us/img706/4039/electrostatics6.jpg [Broken]

    Attached Files:

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 12, 2009 #2
    Please, I know there's a lot, but my exam is coming up and I would really appreciate if someone could show me what I'm doing wrong.
  4. Dec 13, 2009 #3
    I don't think i see anything wrong with your working for question 1. Are the questions from a reliable source?
  5. Dec 13, 2009 #4
    I think so; they are the questions that I got wrong on my last test for electrostatics. But it may be that the test bank is wrong, that has happened before. I'll have to speak with my teacher.
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