Simple degree help (tan-1 fy/fx)

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SUMMARY

The discussion focuses on calculating the net electric force on a 7.00 µC charge located at the corner of an equilateral triangle with two other charges of 3.00 µC. The calculations for the forces F_12 and F_13 are performed using Coulomb's law, yielding values of 1.18 N and 1.57 N, respectively. The user encounters difficulty in determining the correct angle using the arctangent function, initially obtaining -13.80 degrees and later considering the addition of 360 degrees to find the counterclockwise angle. The final consensus is that the correct approach involves adjusting the angle to reflect the proper orientation from the x-axis.

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  • Understanding of Coulomb's law for electric forces
  • Familiarity with vector decomposition in physics
  • Knowledge of trigonometric functions, specifically sine and cosine
  • Ability to use the arctangent function for angle calculation
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This discussion is beneficial for physics students, particularly those studying electrostatics, as well as educators looking for examples of electric force calculations and angle determination in vector analysis.

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Homework Statement



Three point charges are located at the corners of an equilateral triangle as shown in Figure P15.13 (q = 3.00 µC, L = 0.400 m). Calculate the net electric force on the 7.00 µC charge.

° (counterclockwise from the +x axis)

http://www.webassign.net/pse/p23-07alt.gif

The Attempt at a Solution



F_12 = 8.9875 x 10^9 (3e-6)(7e-6)/.4e2 = 1.18 N

F_13 = 8.9875e9 (7e-6)(4e-6)/.4e2 = 1.57 N

then

F_12x = 1.18cos60 = .59 N
F_12y = 1.18sin60 = 1.0219 N
F_13x = 1.57cos60 = .785 N
F_12y = -1.57sin60 = -1.3697 N

then i added them up

Fx = .59 + .785 = 1.375 N
Fy = -1.3597 + 1.0219 = - .3378 N

I know that to get the degree its arctan Fy/Fx but i get the incorrect answer. Is there something wrong in the equation or I am i using the wrong number?

I understand how to do the first part which is 1.4 N of electric force on 7uC charge.

Thanks for any help.
 
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What angle are you getting??
 
i get -13.80*

if i add 180 to that i get 166.2*

both are incorrect.
 
Wouldn't you want to add 360 to it? That would give you the angle from the x-axis going counterclockwise instead of clockwise. Just a guess.
 
Last edited:
You are correct. it is adding 360.

I thought it was 180 because i was thinking of a triangle which is 180*.

Thanks again.