A copper ball of density `8.6 g cm^(-3)` and 1 cm in diameter is immersed in oil of density `0.8 g cm^(-3)`. What is the charge on the ball, if it remains just suspended in oil in electric field of intensity `3600 V//m` acting in the upward direction ?

Explanation

`E=36000Vcm^(-1)=3.6xx10^(6)N//C`
`=3.63xx10^(11)` dyne /C
`r=` radius of the ball `=0.5` cm
`V=` volume of the ball `U=` upthrust = weight of the displaced oil
`=Vdg=(pi)/6xx0.9xx980` dyne

m= mass of the ball `=Vd_(1)=(8.6)(pi)/6`
For equilibrium `U+qE=mg`
`(pi)/6xx0.8xx10^(-2)xx9.8+qxx3.6xx10^(11)`
`=(pi)/6xx8.6xx10^(-2)xx9.8`
Solving `q=1.11xx10^(-12)` coulmb.