Question

Two parallel plates (vertical) are given uniform charge +Q and -Q. A simple pendulum with a small metal bob is hung at the centre. Now, if the bob is displaced a little towards the positive plate and released, then the bob will

• A. be attracted towards the positive plate.
• B. be attracted towards the negative plate.
• C. oscillate as a simple pendulum.
• D. nothing can be predicted.

Answer 1

The correct option is C oscillate as a simple pendulum.

In this system the bob will be treated as a simple pendulum motion with time period , $T=2\pi \sqrt{\frac{l}{g}}$. The effective acceleration will change due to electric filed in between the plates.
Acceleration g becomes $g^{\prime }=\sqrt{g^2+\frac{q^2{E}^2}{m^2}}$ and $T^{\prime }=2\pi \sqrt{\frac{l}{\sqrt{g^2+\frac{q^2{E}^2}{m^2}}}}$