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Homework Help: Charge and excess electrons in a rod

  1. Jan 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A charged nonconducting rod, with a length of 3.00 m and a cross-sectional area of 5.37 cm2, lies along the positive side of an x axis with one end at the origin. The volume charge density ρ is charge per unit volume in coulombs per cubic meter. How many excess electrons are on the rod if ρ is (a) uniform, with a value of -2.54 µC/m3, and (b) nonuniform, with a value given by ρ = bx2, where b = -1.36 µC/m5?

    2. Relevant equations
    q=volume charge density*area of circle*length

    3. The attempt at a solution
    q/e=-4.09e-9/-1.6e-19=2.6e10e<--this was correct

    part b: p=bx^2=-1.36e-6x^2
    dq=5.37e-4*-1.36e-6x^2 dx
    did the integral procedure and it may be wrong:
    integral(dq)=7.3032e-10 *integral (x^2) from 0 to 2 =-1.94e-9
    q/e=-1.94e-9/-1.6e-19 = 1.22e10<-- marked incorrect

    I dont know how to fix this.
  2. jcsd
  3. Jan 17, 2010 #2


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    Homework Helper

    Why did to integrate from 0 to 2 if the length of the rod is 3 m?
  4. Jan 17, 2010 #3
    I was practicing the example from the book and mistakenly used that value haha... Thanks!
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