The bob of a simple pendulum is positively charged. A negatively charged metal plate is placed just below the bob as the pendulum starts oscillating. How will it affect the time period of the pendulum?

Explanation

The electric field E of a negatively charged plate will be pointing downwards. The positive charge q on the bob will experience a force downwards due to this field. So, the effective weight of the pendulum increases, i.e. it becomes mg + qE and hence, effective acceleration becomes g + qE/m. The time period will be given by
`T = 2pi sqrt((l)/(g + qE//m))`
Thus, we can see that the time period of the pendulum will decrease.