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Force to open a capacitor

  • Thread starter Karol
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  • #1
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Homework Statement


A parallel plates capacitor has charge Q and the plates are at distance x. they are separated incremental distance dx. what is the force F to separate them.

Homework Equations


The capacity: ##C=\varepsilon\frac{A}{x}##
The work done to charge a capacitor: ##W=\frac{1}{2}\frac{Q^2}{C}##

The Attempt at a Solution


The differential of the capacity:
$$dC=-\frac{\varepsilon A}{x^2}dx$$
The differential of the work:
$$dW=-\frac{Q^2}{2C^2}dC=\frac{\varepsilon A Q^2}{2C^2}\frac{1}{x^2}dx$$
$$F\cdot dx=dW=\frac{\varepsilon A Q^2}{2C^2}\frac{1}{x^2}dx$$
$$F=\frac{\varepsilon A Q^2}{2C^2}\frac{1}{x^2}$$
It should be ##F=\frac{Q^2}{2\varepsilon A}##
Is it reasonable that the force depends on the distance x, as in my result?
 

Answers and Replies

  • #2
ehild
Homework Helper
15,426
1,823

Homework Statement


A parallel plates capacitor has charge Q and the plates are at distance x. they are separated incremental distance dx. what is the force F to separate them.

Homework Equations


The capacity: ##C=\varepsilon\frac{A}{x}##
The work done to charge a capacitor: ##W=\frac{1}{2}\frac{Q^2}{C}##

The Attempt at a Solution


The differential of the capacity:
$$dC=-\frac{\varepsilon A}{x^2}dx$$
The differential of the work:
$$dW=-\frac{Q^2}{2C^2}dC=\frac{\varepsilon A Q^2}{2C^2}\frac{1}{x^2}dx$$
$$F\cdot dx=dW=\frac{\varepsilon A Q^2}{2C^2}\frac{1}{x^2}dx$$
$$F=\frac{\varepsilon A Q^2}{2C^2}\frac{1}{x^2}$$
It should be ##F=\frac{Q^2}{2\varepsilon A}##
Is it reasonable that the force depends on the distance x, as in my result?
It is correct for an ideal capacitor.
The derivation could have been a simpler by using that the force between the plate is the negative gradient of the potential energy.
potential energy of the capacitor with charge Q: ##U=-\frac{Q^2}{2C}=-\frac{Q^2x}{2 \varepsilon_0 A}##
##F=-\frac{dU}{dx}=\frac{Q^2}{2\varepsilon_0 A}##.
 
  • #3
1,380
22
Thanks, it's correct, but i was instructed this way by the book and i want to know why my way isn't correct.
Not only x is in my result, it's totally different with the constant's placement
 
  • #4
ehild
Homework Helper
15,426
1,823
Thanks, it's correct, but i was instructed this way by the book and i want to know why my way isn't correct.
Not only x is in my result, it's totally different with the constant's placement
It is correct, but substitute the expression for C. x will cancel.
 
  • #5
1,380
22
Thanks
 

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