1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook