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:

1-why 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

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