Question

A ball of 80mg mass and a 2*10^-8 charge is hung with a thread in a uniform horizontal electric field of 2*10^4V/m. What is the angle made by the thread with vertical?

(a) 27 degree

(b) 30 degree

(c) 45 degree

(d) 0 degree

This question was posed to me in semester exam.

This intriguing question originated from Electric Field topic in section Charges and Fields of Physics – Class 12

Answer 1

Correct answer is (a) 27 degree

To elaborate: According to the diagram, Tsin⁡θ = Eq; Tcos⁡θ = mg. Where T is the tension in the thread, m is the mass of the ball, E is the electric field and q is the charge of the ball. Dividing these two, we get tanθ = \(\frac {Eq}{mg}\). Now substituting the value of E=2*10^4V/m, m=80mg, q=2*10^-8C, g=9.8m/s^2, we get tanθ = \(\frac {25}{49}\). Therefore θ=27 degree.