# Help With Magnitude-Angle Notation

1. Jan 26, 2010

### whydanwhy

1. The problem statement, all variables and given/known data
The figure below shows an uneven arrangement of electrons (e) and protons (p) on a circular arc of radius r = 2.24 cm, with angles θ1 = 31.0°, θ2 = 40.0°, θ3 = 41.0°, and θ4 = 27.0°. (a) What is the magnitude of the net electric field produced at the center of the arc? (b) What is the angle between the positive direction of the x axis and the electric field vector?

http://edugen.wiley.com/edugen/courses/crs1650/art/qb/qu/c22/pict_22_14.gif

2. Relevant equations
$$E=\frac{kq}{r^2}=8.99 \times 10^9\ast\frac{1.602 \times 10^{-19}C}{(2.24 \times 10^{-2}m)^2}=2.86671 \times 10^{-6} N/C$$

3. The attempt at a solution
I've solved for the magnitude simply by adding together for $$E_{net}$$, however I'm running into a problem with finding the angle of the electric field vector. I would think to start from the x-axis and go counterclockwise so that the first particle has an angle measure of 0°, then 31°, 71°, 112°, and lastly 153° for the final particle (most left). But adding these up in my Ti-89 (using M-A notation) gives me a wrong answer.

I also various other combinations such as (starting from most left particle and not including the one on the x-axis): -27°, 112° -109° and -149°. Or -27°, -68°, 71°, 31°. Along with a few others.

So I'm getting the right magnitude measurement, but not angle.
Any help would be appreciated.