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What causes a point charge to move in an electric field?

  1. Mar 24, 2017 #1
    1. The problem statement, all variables and given/known data:
    STATEMENT:
    A point charge at rest is released in an electric field. It will move along the electric field line....(1)
    REASON: The force on a point charge is along the field line...(2)
    State whether:
    A) 2 is the correct reason for(1)
    B) Both (1) and (2) are correct but 2 is not correct justification of 1
    C) 1 is correct 2 is wrong
    D) 2is correct 1 is wrong



    2. Relevant equations: No equation is required here..this is based plainly on concept


    3. The attempt at a solution:
    Option A.
    Text says its option B.
    Well, if that is the case, then what causes the charge to move?
     
    Last edited: Mar 24, 2017
  2. jcsd
  3. Mar 24, 2017 #2

    Doc Al

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    What text is this? I'd say you are correct.
     
  4. Mar 24, 2017 #3
    must be a printing error then...
    Its an adaptation of halliday rasnik..The mcq exercises are framed by the adapters not the original author of the books..I tried my best to get a hard copy of the original text but everywhere in my country, I find an adaptation...
     
  5. Mar 24, 2017 #4

    TSny

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    randomgamenerd,

    Can we clarify what statement (1) is saying? Is it saying that if you let a positively charged particle start from rest at point "a" in the figure below, then it would move along the electric field line and reach point "b"?

    upload_2017-3-24_20-42-43.png

    Also, note that options (C) and (D) in the problem statement say the same thing.
     
    Last edited: Mar 24, 2017
  6. Mar 24, 2017 #5
    I think so..I dont know.
    And regarding option C and D..i made a typing error
     
  7. Mar 24, 2017 #6

    TSny

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    Something to consider: Is it possible for a particle's velocity vector to change direction if the force is always in the direction of the velocity?
     
  8. Mar 25, 2017 #7

    Doc Al

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    If you interpret statement 1 to mean something like "at that instant, will the charge begin to move along the field line?", then answer A would be OK.

    But as TSny notes, statement 1 is a more general statement about the charge's future motion. (Consider: Does a mass always move in the direction of the net force on it? Even as the direction of that force changes?)

    Either way, answer B is incorrect. (But one of the other answers is correct.)
     
  9. Mar 25, 2017 #8
    I guess not
     
  10. Mar 25, 2017 #9
    yea..I interprete the question just as you said..if I just release a charge which was previously at rest, then it will definitely just begin to move..
     
  11. Mar 25, 2017 #10
    If i say the electric field lines are parallel, then probably the force along field line is causing the charge to move, right?
     
  12. Mar 25, 2017 #11

    TSny

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    Why not?
     
  13. Mar 25, 2017 #12
    Because there is no component of force acting in some other direction and hence no acceleration in any other direction. we know from law of inertia that a body tends to remain at rest or continues its motion with uniform velocity unless some external unbalanced force acts..So we can imagine that the body is at "rest" with respect to other directions except the direction in which force is acting.
    I mean no component of force acts in any other direction. There is no velocity in any other direction(assuming for the sake of convenience that at at present force and velocity are along +ve X axis) and to change direction there must be come new velocity in some other direction.And to bring a new velocity we need some acceleration but that is absent. Or atleast the force must oppose the velocity.

    I mean I dont know how to express...I can just..I mean till date I never tried to explain this..
     
  14. Mar 25, 2017 #13

    TSny

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    You explanation is good. So, would you say that statement (1) in the problem is correct?
     
  15. Mar 25, 2017 #14
    I think it will be correct only if the field lines are straight
     
  16. Mar 25, 2017 #15

    TSny

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    Yes. That's right.
     
  17. Mar 25, 2017 #16
    okay, so can we say that for a more generalised case, statement 1 is wrong 2 is correct⇒option D


    if the question was:it will start moving along field line, then option A.
    option B and Cdoes not hold true for any case, right?
     
  18. Mar 25, 2017 #17

    TSny

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    Yes.

    Yes, that's right.
     
  19. Mar 26, 2017 #18
    okay, thanks for helping me out..teachers usually dont attend to our doubts with so much patience as is done here....thats why I love PF
     
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