Question

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.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (D)

The potential energy stored between the plates of capacitor,
$U=\frac{1}{2} \frac{Q^{2}}{C}=\frac{1}{2} \frac{Q^{2}}{\varepsilon_{0}A}x\quad\quad\quad\quad\left(\because \,\,C=\frac{\varepsilon_{0}A}{x}\right)$
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.