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: Calculating the current of a circuit- resistance and volatge known, see picture

  1. Jan 8, 2010 #1
    1. The problem statement, all variables and given/known data

    What current flows through the resistors R(1) = 1Ω, R(2) = 2 Ω and R(3) = 3 Ω, if the driving voltages are V(1) = 1V and V(2) = 2V?

    Picture:
    http://item.slide.com/r/1/151/i/lvtwIwH-0j8A8o-XAKDtPRQO1eHAb4Ik/ [Broken]

    2. Relevant equations

    V= RI → I= V/ R

    3. The attempt at a solution

    V(1)= I(2)R(2) + I(1)R(1)
    V(2)= I(3)R(3) – I(2)R(2)
    I(1)= I(2) + I(3)

    - From what I stated above, I conclude three equations:

    1) V(1) = I(2)R(2) + [(I(2) + I(3))*R(1)]
    2) V(2)= I(3)R(3) - I(2)R(2)
    3) V(1)= [I(2)*( R(1) + R(2))] + I(3)R(1)

    - I combined the three equations and simplify for the current that goes through ALL resistors, which in our case is I(3):

    I(2)= (I(3)R(3) - V(2))/ R(2)
    V(1)R(2)= [(I(3)R(3) – V(2))/ R(2))*(R(2) + R(1))] + I(3)R(1)R(2)
    (V(1)R(2))/ (R(2) + R(1))= (I(3)R(3)) + [I(3)R(1)R(2))/ (R(2) + R(1))]
    I(3)*[R(3) + (R(1)R(2)/ (R(2) + R(1))]= [V(1)R(2)/ (R(2) + R(1))] + V(2)

    I(3)= [(V(1)R(2)/ (R(2) + R(1)) + V(2)] / [R(3)* (R(1)R(2)/ (R(2) + R(1))]
    I(3)= 1.37 A

    Are my calculations correct?
    Thank you for helping!:smile:
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 8, 2010 #2

    diazona

    User Avatar
    Homework Helper


    Equation #3 here is the same as equation #1, just written slightly differently... so you really have only two independent equations. I'd just start from the original three you had above.

    I don't see where that last line comes from. The units are off, so something must be wrong in there.
     
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook