Can someone explain the negative sign in the capacitance formula?

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SUMMARY

The discussion centers on the calculation of charge in a capacitor, specifically addressing the confusion surrounding the negative sign in the capacitance formula. The participant calculated the charge as -0.9 C using the equation -We/q = ΔV and expressed concern over the discrepancy with the textbook answer of 0.9 C. The negative sign arises because work is done against the electric field when moving a charge, which is a fundamental concept in electrostatics.

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  • Understanding of electrostatics and electric fields
  • Familiarity with capacitor equations, specifically C = Q/V
  • Knowledge of energy concepts in physics, particularly work-energy principles
  • Basic algebra skills for manipulating equations
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James2288
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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..
 
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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).
 
Thank you ...

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

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