Question
The figure below shows two charged particles on an x axis, q = -3.20 10-19 C at x = -3.40 m and q = +3.20 10-19 C at x = +3.40 m.
(a) What is the magnitude of the net electric field produced at pointPaty=-4.53m?
(b) What is its direction?(counterclockwise from the positivexaxis)
Explanation / Answer
Part A Let the magnitude of negative charge be q. That of positive charge separated L cm away will be 9q The net electric field at any point x > 0 is given by Enet = (9*10^9)q*(10^4)*[9/(x+L)^2 - 1/x^2] ----------------------------- 1 To find where it will be maximum we differentiate [9/(x+L)^2 - 1/x^2] with respect to x and equate it to zero -18/(x+L)^3 + 2/x^3 = 0 or (x+L)^3 / x^3 = 9 = (x+L)/x = 9^(1/3) = 2.08 also d^2/dx^2[[9/(x+L)^2 - 1/x^2] = 54/(x+L)^4 - 6/x^4 = C say so C*(x+L)^4= 54 - 6*[(x+L)/x]^4 = 54 - 6* (2.08^4) = 54 - 112.3 for x = 2.08. So C < 0 for x = 2.08 So at x = 2.08 E net is maximum. It is given that at x = 20 cm 1 cm less than xs in the figure, the field is zero. That is possible only when 9/(20+L)^2 = 1/20^2 or (20+L)/20 = 3 or 1 + L/20 = 3 or L = 40 cm Substituting the appropriate values for q1 and q2 we get Enet(max) = (9*10^9)(3*10^-19)*(10^4)*[9/(2.08+40)^2 - 1/(2.08)^2] = (27*10^-6)*(-0.226) = -6.10*10^-6 N/C Something is wrong in the solution I cannot point out may be some one else points out. How can maximum value be negative wen it is also becoming zero at x = 20 cm This taking figure from other question creates problems may be the asker helps me in this.