A charged oil droplet with mass $m$ falls, at first freely, in a vacuum between two horizontal metal plates separated by a distance $x$. A potential difference (p.d.) $V$ is then applied across the plates. As a result, the oil droplet keeps accelerating downwards, but now with a smaller acceleration $a$. The polarity of the applied p.d. is then reversed so that the direction of the electric force on the droplet is also reversed. This causes the downward acceleration of the oil droplet to increase to $3a$. Determine the magnitude of the charge on the oil droplet.
- A$\frac{ma x}{V}$
- B$\frac{2ma x}{V}$
- C$\frac{3ma x}{V}$
- D$\frac{4ma x}{V}$