Electric Field Vector Resulting from Two Point Charges (not getting right answer)

  • Thread starter sugz
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Homework Statement


A +20nC point charge is placed on the x axis at x = 2.0 m, and a -25 nC point charge is placed on the y axis at y= -3.0 m. What is the angle that the direction of the electric field at the origin makes with the x-axis.

(a) 209 (deg)
(b) 61 (deg)
(c) 29 (deg)
(d) 241(deg)
(e) 119 (deg)

Homework Equations



E = ke(q/r^2)

The Attempt at a Solution



E= (8.99x10^9) [ (20x10^-9)/4 (i) - (25x10^-9)/9 (j)]
= (8.99x10^9) [ 5.0x10^-9 (i) - 2.78x10^-9 (j)]
= 44.95i - 24.97j
= 51.42

The angle between the x-axis is arccos(44.95/51.42) = 29 (deg)

But the correct answer is 209 (deg).
 

Answers and Replies

  • #2
berkeman
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E= (8.99x10^9) [ (20x10^-9)/4 (i) - (25x10^-9)/9 (j)]
Check the sign of your first term. Which way does the E field at the origin point when you have a positive charge at x=2m? :smile:
 
  • #3
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Oh makes sense, thank you !:)
 

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