Example – 1 A positively charged oil droplet remains stationary in the electric field between two horizontal plates separated by a distance of 1cm. If the charge on the drop is 9.6 × 10-10 esu and the mass of droplet is 10-11 g, what is the potential difference between two plates ? Now if the polarity of the plates is reversed what is the instantaneous acceleration of the droplet? [g = 9.8 m/s2]
Solution : As the droplet is at rest, its weight W = mg will be balanced by electric force F = qE
Now if the polarity of the plates is reversed, both electrical and gravitational force will act downward
so, F = mg + qE = 2mg [as mg = qE]
And hence instantaneous acceleration of drop : a = (F/m) = (2mg/m) = 2g = 19.6 m/s2
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