Question

A pendulum bob of mass 80mg and carrying a charge of `2xx 10^(-8)C` is at rest in a uniform, horizontal electric field of 20k `Vm^-1.` Find the tension in the thread.

Answer 1

Here, `m = 80 mg = 80xx10^(-6) kg.`
`q = 2xx10^(-8) C, E = 2xx10^(4) V//m`
Let T be the tension in the string and `theta` be the angle it makes with the vertical, Fig.

in equilibrium, `T sin theta = q E` ...(i)
and `T cos theta = mg`
Dividing we get
`tan theta = (qE)/(mg)`
`tan theta = (q E)/(mg)`
`tan theta = (2xx10^(-8)xx2xx10^(4))/(80xx10^(-6)xx9*8) = 0*5102`
`:. theta = 27^(@)`
From (i),
`T = (qE)/(sin theta) = (2xx10^(-8)xx2xx10^(4))/(sin 27^(@))`
`8*8xx10^(-4) N`