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Transcript

Chapter 18 Electric Forces and Electric Fields The equation for Coulombâs law is Kq1 q 2 FE = r2 where FE is the electric force, q1 and q2 are the charges, r is the distance between their centers, and K is a constant which equals 9 x 109 Nm2/C2. r F -q1 +q2 Sometimes the constant K is written as K = 1 4ÏÎµ o , where Îµo = 8.85 x 10-12 C2 / Nm2. Example 4 r +2 ÂµC -4 ÂµC Two point charges q1 = +2 ÂµC and q2 = - 4 ÂµC are separated by a distance r, as shown above. (a) If the force between the charges is 2 N, what is the value of r? (b) Where could you place a third charge q3 = +1 ÂµC on the horizontal axis so that there would be no net force acting on q3? Find an equation which could be solved for x, where x is the distance from the +2 ÂµC charge to q3. It is not necessary to solve this equation. Solution (a) Kq1 q 2 FE = r2 r= ï£« Nm 2 ï£¬ï£¬ 9 x10 9 C2 Kq1 q 2 ï£ = FE 2N ï£¶ ï£·ï£· ï£¸ = 0.19 m 205