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Homework Help: Electric Fields and Charges Fill in the blanks

  1. Feb 23, 2015 #1
    1. The problem statement, all variables and given/known data

    1. Select the answers which complete the statements below (e.g., if answer A completes the first statement, and answer B the others, enter ABBBB).

      A) positive B) negative C) neutral D) cannot tell

      1. A positive point charge is brought near the outside surface of a neutral conducting sphere and released. The initial position of the point charge is on the positive x axis and the sphere is centred on the origin. The point charge moves in the ____ x direction. (If the point charge does not move, select answer C.)
      2. Two neutral metal spheres are mounted on insulating supports. They are connected by a wire. A positively charged glass rod is brought near sphere 1. The wire connecting the spheres is removed and the charged rod is then taken away. Now the charge on sphere 1 is ____ .
      3. A positively charged glass rod repels an object suspended by a non-conducting thread. The charge on the object is ____ .
      4. A positively charged ball is brought close to a neutral isolated conductor. The conductor is then grounded, while the ball is kept close. If the ball is first taken away and then the ground connection is removed, the conductor has a ____ charge.
      5. A negatively charged glass rod attracts an object suspended by a non-conducting thread. The charge on the object is ____ .
    2. Relevant equations

    3. The attempt at a solution
    i) This is the one which gave me the most trouble, but the way I see it, either it doesn’t move as the sphere is neutral, or it does move towards the sphere (Negative direction) as the electrons all move towards the point charge and they pull each other towards one another. So either B or C

    ii) This is pretty straight forward. By connecting the two spheres, electrons can be transferred between them, and when the positively charged rod is brought next to sphere 1, all of the electrons in both spheres go towards it making sphere 1 negative and sphere 2 positive. The wire is then cut and the charged rod is removed, meaning sphere 1 will be negative. So B

    iii) Again, pretty straight forward. Positively charged rod repels an object, so the object must be positively charged or it wouldn’t be repelled. So A

    iv) This one also gave me a bit of trouble but I think I got it. A positively charge ball is brought close to a conductor, so the electrons go to the charged ball, it is then grounded, meaning electrons will come up from the ground to balance the positive side of the conductor. The charged is then removed so then electrons will flow out through the ground to bring the conductor back to being neutral, and the ground connection is then remove, so in the end, the conductor remains neutral. So C

    v) Very similar to iii, this time it is a negatively charged rod that is attracting an object, therefore the object must be positive in order to be attracted to the negative charge. So A

    The only issue is that I’ve tried all of those combinations, as well as all of the combinations with _BACA, with no success which means one or more of my answers must be wrong, but I cannot figure out which one(s) it could be. Any help would be much appreciated!
    Last edited by a moderator: Feb 23, 2015
  2. jcsd
  3. Feb 23, 2015 #2


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    Hello Jeff&o, welcome to PF :smile:

    I'm with you almost all the way. In case i) I don't understand why you hesitate between B and C.

    And case v) is where I smell the proverbial rat. Compare with case i): in v) they mention an 'object', so you don't know if it's an insulator or a conductor. If assuming the former leads to a different conclusion than assuming the latter, there's no more A , and no B or C either ....

    For the sake of other readers, let us know if it works -- or not !
  4. Feb 23, 2015 #3
    Thank you for the help! It ended up being BBACD, I guess the wording of the question got me, but again thank you! If you wouldn't mind, why can we not tell for v) but we can tell for iii), as the wording is almost identical, they just switch the charge on the rod and what happens to the object?
  5. Feb 23, 2015 #4


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    Switching the charge isn't important. Switching what happens to the object is. A charge can induce polarization in a neutral object and attract it. Can it repel it?
  6. Feb 24, 2015 #5
    That makes sense! Thanks!
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