Question

a charge particle of mass 1 gram is suspended through a silk thread of length 40cm in a horizontal electric field 4*10^4 NC-1.if the particle stays at a distance of 224 cm from the wall.find the charge.

Answer 1

three forces act on mass here
1. Tension in the thread (along the thread)
2. mg weight of the mass particle (downward)
3. Electric force F=QE due to horizontal field
Let the angle between thread and vertical be $\alpha$  
Then use sine law
$T= \frac{mg}{sin(90+ \alpha )}= \frac{QE}{sin(180- \alpha )}$
$m=10^{-3} Kg$
$QE=mg(tan( \alpha ))$
$Q= \frac{mg(tan( \alpha ))}{E}$
the length of thread is 40 cm
But the distance from the vertical cannot be 224 cm please check ur question
any way this would help u