- #1
Hannisch
- 116
- 0
Homework Statement
Two parallel plates with area A and charges Q and -Q are being pulled apart by force F, so that the distance between, x, slowly increases. Express F in terms of Q, A and x.
Homework Equations
[tex]-\Delta PE=W[/tex]
[tex]W=\int Fdx[/tex]
[tex]\Delta PE=q_{0}\Delta V[/tex]
[tex]Q=C\Delta V[/tex]
[tex]C=\frac{A\epsilon_{0}}{d}[/tex]
The Attempt at a Solution
[tex]Q=C\Delta V[/tex]
[tex]Q=\frac{A\epsilon_{0}}{x}\Delta V[/tex]
[tex]\Delta V=\frac{Qx}{A\epsilon_{0}}[/tex]
[tex]q_{0}\Delta V=\Delta PE[/tex] and in this case q0= -Q
[tex]\Delta PE=\frac{Qx}{A\epsilon_{0}} (-Q)[/tex]
[tex]W=-\Delta PE=-\frac{-QQx}{A\epsilon_{0}}[/tex]
[tex]W=\frac{Q^{2}x}{A\epsilon_{0}}[/tex]
[tex]W=\frac{Q^{2}x}{A\epsilon_{0}}=\int Fdx[/tex]
Force is then equal to the derivative of my expression with respect to x.
[tex]F=\frac{Q^{2}}{A\epsilon_{0}}[/tex]How can I get an answer independent of x? It.. just doesn't make sense to me. My friend and I both got this answer independently. Where have we gone wrong, or is it correct or.. something? I'm rather confused.
(Ohhh the pain, did you know that Ctrl+W closes a tab [at least in Firefox]? Yeah, I was going to write W and managed to hit Ctrl instead of Shift.. towards the end of my attempt at a solution.)