IIT JEE , AIEEE Forum - Physics , Electricity

elessar_iitkgp

Let the time taken for n( = 20) when the electric field is off be T and T' when its on.
Then,
T/n = 2

(L/g)
(T/n)² = 4

² (L/g) ----------(1)
When the electric field is switched on,
T'/n = 2

(L/(g-qE/m))
(T'/n)² = 4

² (L/(g-qE/m)) ---------------(2)
Dividing (2) by (1)
(T'/T)² = (g/(g-qE/m))
T' = T

(g/(g-qE/m)) =

Substitute the values and get the answer.