Assertion : The force between the plates of a parallel plate...

Assertion : The force between the plates of a parallel plate capacitor is proportional to charge on it. Reason : Electric force is equal to charge per unit area.

Options

AIf both assertion and reason are true and reason is the correct explanation of assertion.
BIf both assertion and reason are true but reason is not the correct explanation of assertion.
CIf assertion is true but reason is false.
DIf both assertion and reason are false.

Correct answer

D. If both assertion and reason are false.

Step-by-step solution

The potential energy stored between the plates of capacitor,

U=\frac{1}{2} \frac{Q^2}{C}=\frac{1}{2} \frac{Q^2}{\varepsilon_0 A} x \quad\left(\because C=\frac{\varepsilon_0 A}{x}\right)

Force

F=\frac{-\partial U}{\partial x}=-\frac{\partial}{\partial x}\left(\frac{1}{2} \frac{Q^2 x}{\varepsilon_0 A}\right)=-\frac{1}{2} \frac{Q^2}{\varepsilon_0 A}

Negative sign shows force is attractive.

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