A mass m carrying a charge `q` is suspended from a string and placed in a uniform horizontal electric field of intensity `E`.The angel made by the string with the vertical in the equilibrium position is

Explanation

Let the angle made by the string with the vertical be `theta` with the vertical . Refer the figure.
Electric field is horizontal , so force (qE) experienced by the positively charged ball has to be i the same direction as the applied electric field . Tension T acts along the string as shown in the figure. Weight (mg) of the body is acting vertically downwards. Vertical and horizontal components of tension are shown in the figure.
Since the ball is equilibrium , we can write the following equations for vertical and horizontal equilibrium :
`T cos theta =mg` …(i)
`T sin theta =qE` …(ii)
Dividing equation (ii) by (i) , we get the following
`tan theta =(qE)/(mg)`
`rArr theta = tan^(-1)((qE)/(mg))`