posted by Yuni on .
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
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 +?
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.
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?.
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