Posted by Yuni 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
=3(40)
=120 J
what i want to ask is:
1why must we add  sign for v is not that if the v is against electric field is +?

physics  drwls, 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  Yuni, 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  bobpursley, 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