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Homework Statement
Two charged balls are placed in point A and B and the distance between them is 9,54cm. Each of the balls are charged with 8,0 x 10^8 C. Find the scalar value and direction of the electric field in point C placed 5 cm from A and 6 cm from B.
Homework Equations
Cosine Rule: a^2 = b^2 + c^2  2ab x cosθ
E = K_{e} x (q/r^2)
The Attempt at a Solution
[/B]
I have calculated the electric field of A and B.
E_{A} = 287680 N/C
E_{B} = 199778 N/C
After that I used cosine rule to calculate E_{C} and got E_{C} = 255341 N/C. This is correct.
But when I try finding the direction I calculate the angle between E_{C} and E_{A} (β) like this:
β = cos^1 ((E_{A}^2 + E_{B}^2E_{C}^2)/(2E_{A}E_{B})) = 60 degrees
The solution says that I should calculate the angle between E_{C} and E_{A} (β), but that the equation looks like this:
β = cos^1 ((E_{C}^2 + E_{A}^2E_{B}^2)/(2E_{C}E_{A})) = 42,65 degrees
Why am I supposed to use that equation?
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