Question

A version of this problem appears on some web-based learning sites as a homework problem for this material.} A uniform electric field E runs horizontally from left to right in a region of space (sec figure). In this space, a sphere of mass m with charge q hangs from a cord; the electric force pulls the sphere so it hangs with an angle theta from vertical. If m = 0.3 g, theta = 20 degree, and |q| = 0.5 nC, what is the magnitude of the electric field E, and what sign is the charge?

Answer 1

equillibrium condition grav force = Electrical force

mg sin theta = E q cos theta

E = mg tan theta/q

E = (0.3*10^-3 * 9.81 * tan 20)/(0.5* 10^-9)

E = 2.14*10^6 N/C (answer)

-------------------------------------

since electric force is in right wards and charge is pulled (attracted)

this must hve opposite charge as that E

E towards right is +ve charge

so as the charge q must be +ve charge