Results 1 to 2 of 2

Thread: changing in time problem

  1. #1
    Super Member
    Oct 2007

    changing in time problem

    Ok two resistors R1 and R2 are connected parallel.

    Total resistance is given by $\displaystyle R^-1 = R1^-1 + R2^-1$
    R1 is 80 ohms & increases at 0.3 ohms per sec
    R2 is 100 ohms & increases at 0.2 ohms per sec
    so the question is now, "how fast is R changing at that time?"
    not sure how to go about this question. any help much appreciated
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    skeeter's Avatar
    Jun 2008
    North Texas
    looks familiar ...

    $\displaystyle R^{-1} = R_1^{-1} + R_2^{-1}$

    take the time derivative ...

    $\displaystyle -R^{-2} \cdot \frac{dR}{dt} = -R_1^{-2} \cdot \frac{dR_1}{dt} - R_2^{-2} \cdot \frac{dR_2}{dt}$

    get rid of the negative exponents and isolate $\displaystyle \frac{dR}{dt}$ ...

    $\displaystyle \frac{dR}{dt} = \frac{R^2}{R_1^2} \cdot \frac{dR_1}{dt} + \frac{R^2}{R_2^2} \cdot \frac{dR_2}{dt}$

    you have all the info for R1 and R2 ... calculate R from the original equation and determine dR/dt.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Problem with changing of base log question
    Posted in the Algebra Forum
    Replies: 6
    Last Post: May 10th 2010, 11:02 AM
  2. Replies: 4
    Last Post: Aug 28th 2009, 02:43 PM
  3. matlab changing time scale on a waveform
    Posted in the Math Software Forum
    Replies: 3
    Last Post: Jul 27th 2009, 09:06 AM
  4. Replies: 5
    Last Post: Jul 3rd 2009, 06:48 PM
  5. Replies: 12
    Last Post: Jul 9th 2008, 04:35 AM

Search Tags

/mathhelpforum @mathhelpforum