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## Homework Statement

Refer to the figure below. Find the magnitude of the electric field if the string with a 7 gram ball carrying a charge of 4 x 10-9 C forms an angle of 35o with the vertical.

http://www.montereyinstitute.org/courses/AP%20Physics%20B%20II/course%20files/assignments/lesson31selfcheckquiz_files/image001.gif

a. 1.41 x 106 N/C

b. 1.20 x 107 N/C

c. 2.30 x 107 N/C

d. 6.25 x 108 N/C

## Homework Equations

Fg=mg

E=F/q

## The Attempt at a Solution

Force due to electric field=Force due to gravity:

F=mgcos(θ)

F=7*10^-3kg*9.81m/s^2*cos(35)

F=Eq

E=F/q

E=(7*10^-3kg*9.81m/s^2*cos(35))/(4x10^-9)C

E=1.41x10^7 N/C

which is not an answer...the correct answer in the key is b, but I dont know why?