1. Not finding help here? Sign up for a free 30min 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!

Integration Problems

  1. Mar 11, 2008 #1
    I am not sure exactly what is happening between the first part and second part. What happens to the 2?

    Also, what technique do you use to integrate after that? I am a little unsure as to what slick moves are being used here. Any help is appreciated.

  2. jcsd
  3. Mar 11, 2008 #2
    I don't understand the context this is in but it looks like the 2r(dr) is being replaced by the substitution d(r^2) = 2r(dr) (this resembles usub, but i've never encountered this before). The denominator in the first step is just brought up top with a negative exponent. From step two to three, it looks like polynomial integration.

    Hmm are you learning electromagnetism right now?
  4. Mar 11, 2008 #3
    Yep electromagnetism. It is out of the first chapter on the subject in Serway and Jewett. The example is for finding the electric field on a disk.

    I am really stuck on that two, it seems like it has to be a mistake. There is probably some perspective I am missing here, that dr*2r=dr^2, but I am not seeing it now.

    I can work it out fine with a substitution, but I really like to understand everything I encounter, and this doesn't mesh well.
    Last edited: Mar 11, 2008
  5. Mar 11, 2008 #4
    If you let u = r^2, then you would replace 2rdr with du, wouldn't you?
    Here, they have just not bothered renaming r^2.
  6. Mar 11, 2008 #5
    Perfect, yep that is the angle I was missing. Of course I made the substitution u=x^2 +r^2 so du=2rdr. So thinking this way, keep going du=d(x^2+r^2)=d(r^2) etc.

    Thank you for the advice.
    Last edited: Mar 11, 2008
  7. Mar 11, 2008 #6
    It's kind of an abuse of notation. What's d(r^2)? It's the differential of r^2, which equals 2r dr.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Integration Problems
  1. Integration problem (Replies: 3)

  2. Integration problem (Replies: 4)

  3. Integration problems (Replies: 3)

  4. Integral problem (Replies: 3)