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Direction of a positive charge's velocity in an electric field.

  1. Apr 18, 2012 #1
    1. The problem statement, all variables and given/known data
    Here is the question with the answer:
    http://dl.dropbox.com/u/64325990/phys153Q/21.5.PNG [Broken]

    3. The attempt at a solution
    I initially thought it would be A but it isn't and does anyone know why?
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Apr 18, 2012 #2

    tiny-tim

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    hi theBEAST! :smile:
    wouldn't it only be A if the electric field was uniform? :wink:
     
  4. Apr 18, 2012 #3
    Ohhhh thanks that makes sense.

    I have another one:
    http://dl.dropbox.com/u/64325990/phys153Q/23.9.PNG [Broken]

    I thought this was A, if you put a point charge there... How could it feel a force?
     
    Last edited by a moderator: May 5, 2017
  5. Apr 18, 2012 #4

    tiny-tim

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    what is the equation relating electric potential and electric field? :wink:
     
  6. Apr 18, 2012 #5
    Hmmmm one is 1/r and one is 1/r^2 so would this mean E ≠ 0? Thus F is not zero? Not really sure.
     
  7. Apr 18, 2012 #6

    tiny-tim

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    you should know this definition :redface:

    the electric field is minus the gradient of the (scalar) potential …

    E = -φ ​

    (what did you think the electric potential was for? :confused:)
     
  8. Apr 18, 2012 #7
    Thanks Tim, I kind of know the definition... The concept was potential gradients was not taught in our physics class but I did read some of it on my own online. They are related in that the derivative of potential with respect to x is the electric field. So I am not sure how I can relate it to the multiple choice question :S.

    I hope I'm not asking too many question at once but it is getting late and I would love to get a hint at what I am doing wrong for this question as well (once I wake up :P):
    http://dl.dropbox.com/u/64325990/phys153Q/26.9.PNG [Broken]

    I thought R would affect the maximum charge stored as well because it affects the V of the battery and so the equation C=Q/V?
     
    Last edited by a moderator: May 5, 2017
  9. Apr 18, 2012 #8

    tiny-tim

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    hint: when the capacitor C is fully charged, what is the voltage drop across the resistor R ? :wink:
     
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