The bob of a simple pendulum has a mass of 40 g and a positive charge of `4.0 xx 10 ^(-5) C` . It makes 20 oscillations in 45 s. A vertical electric field pointing upward and of magnitude `2.5 xx 10^ 4 NC^(-1)` is switched on. How much time will it now take to complete 20 oscillations?

Explanation

`T_1=2pisqrt(l/g )`
` a=(qE)/m `
` = (4xx10^-6xx2.5xx10^4)/(40xx10^-3) `
` =2.5m//s^2 `
` When no electric field is applied `
` T_2=2pisqrt(l/(g-a))`
` where a= (qE)/m upword acceleration of mass due to electric field. `
` T_2/T_1=sqrt(g/(g-a))+sqrt(g/(g-(qE)/m)) `
` =sqrt(9.8/(9.8-2.5)) `
` T_2=45xxsqrt(9.8/7.3)= 52sec.`