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Thread: Work and Electric Potential

  1. #1
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    Work and Electric Potential

    How much work is needed to move a -7 μC charge from ground to a point whose potential is +6 V higher?

    I am confused of this problem.

    Do I use ΔU = qΔV or ΔU = -qΔV? Why?

    And after I get the result, how do I know if the work was done by the field (electric force) or the agent (external force)?
    Last edited by joshuaa; Mar 29th 2017 at 07:52 AM.
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  2. #2
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    Re: Work and Electric Potential

    the work done equals the increase in potential regardless of the charge.
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  3. #3
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    Re: Work and Electric Potential

    Which formula do I use?
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  4. #4
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    Re: Work and Electric Potential

    Quote Originally Posted by joshuaa View Post
    Which formula do I use?
    sigh...

    what's the increase in potential here?
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  5. #5
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    Re: Work and Electric Potential

    This one ΔU = qΔV

    but why is it also used for the decrease in potential as well?

    then when to use ΔU = -qΔV?
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  6. #6
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    Re: Work and Electric Potential

    Quote Originally Posted by joshuaa View Post
    This one ΔU = qΔV

    but why is it also used for the decrease in potential as well?

    then when to use ΔU = -qΔV?
    Ok, I see that potential is defined as the work done moving a positive charge

    $\Delta U = q \Delta V$

    $W = -\Delta U$

    so

    $W = -q \Delta V$

    Here we have

    $\Delta U = -6V$

    $q=-7 \mu C$

    $W = -(-7 )(U_0 - (U_0+6)) \mu J= 7 (-6)\mu J = -42 \mu J$

    You have to do work to move a positive charge to a higher potential. Work is done on a positive charge moving to a lower potential

    Work is done on a negative charge moving to a higher potential. You have to do work on a negative charge to move it to a lower potential.
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  7. #7
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    Re: Work and Electric Potential

    Just want to clarify one thing -6V you mean ΔV not ΔU, right?
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  8. #8
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    Re: Work and Electric Potential

    Quote Originally Posted by joshuaa View Post
    Just want to clarify one thing -6V you mean ΔV not ΔU, right?
    yes
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  9. #9
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    Re: Work and Electric Potential

    Delta V is +6 V, why did you make negative?

    I did not understand this part Vo - (Vo + 6)

    Shoudn't it be V - Vo?
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  10. #10
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    Re: Work and Electric Potential

    Quote Originally Posted by joshuaa View Post
    Delta V is +6 V, why did you make negative?

    I did not understand this part Vo - (Vo + 6)

    Shoudn't it be V - Vo?
    you're right.. you should probably just ignore my post.
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  11. #11
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    Re: Work and Electric Potential

    alright

    then thanks
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