Capacitor and capacitance

[James2288]

Hi everyone,

1. Homework Statement

It takes 18 J of energy to move a 0.30-mC charge from
one plate of a capacitor to the other. How much
charge is on each plate?

Homework Equations

-We/q = ΔV

C = Q/V

The Attempt at a Solution

-18J/0.3.10^-3 C * 15.10^-6 F = Q

Q = -0.9 C
The problem is that in the book, the answer is 0.9 C but i found -0.9C. I had many problems with the formula

-We/q = ΔV

sometimes in the book they let the negative sign and sometimes not. If someone can explain me that..

 

[ramzerimar]

When you move a charge on a electric field, you do work against the electric field. So the work applied is equal to minus the work done by the electric field (assuming no changes in the kinetic energy).

 

[James2288]

Thank you ...

Can I say when the electric force is exerted in the same direction to the displacement that make a positive work ??