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Posted by on Saturday, December 31, 2011 at 9:36pm.

potential difference=40v b2n plate A & B.
find work done if q=+3C from
a) B to A
b) A to B

*i already know the ans but i need the explanation for the formula.

-this is my work
a)w=-qv b)-120J
=120 J

what i want to ask is:
1-why must we add - sign for v is not that if the v is against electric field is +?

  • physics - , Sunday, January 1, 2012 at 5:55am

    You don't say whether A or B has the higher potential. You only say that the difference "between" them is 40 V.

    If a positive charge moves to a higher potential, the potential energy increases. The E field and force is opposite to the direction of motion in that situation.

  • phy - , Sunday, January 1, 2012 at 6:24am

    i'm sorry. the electric field are from A to B.

    dear drwls,you didn't answer my question.
    i know about the concept but why the v is negative? or how can it be proven mathematically by the formula?.

  • phy - , Sunday, January 1, 2012 at 9:14am

    Start with the definition of work
    Work=Force dot distance
    = Eq dot distance
    but E=-dV/dx by definition of direction of E field. E goes from + V to -V (the direction of a pos test charge)
    Work= q*-dV/dx dot distance
    work= -q dV/dx dot dx

    Now, if the distance is in the direction of the gradient, then
    work=-q DeltaV

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