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 Resistance In Circuit

  1. Mar 17, 2008 #1
    1. The problem statement, all variables and given/known data

    A loop circuit has a resistance of R and a current of 1.8A. The current is reduced to 1.3A when an additional 2.7 ohm resistor is added in series with R.

    What is the value of R?

    2. Relevant equations

    I is constant throughout a series circuit
    V is shared in a series
    Total resistance=the sum of the resistors in series

    V=I x R

    3. The attempt at a solution
    I substituted R (in the equation V=IR) with 4.7+R because this would represent the total resistance in the series. I knew that I was 1.3A. I calculated the Voltage on the 4.7 resistor (using V=IR) and got 6V. So then I substitued V total with 6 + V.

    I ended up with the following equation. 6+V=1.3(4.7+R). I then tried substituting V (of the unknown resistor) with 1.3R, but then I realized that couldn't possibly work, because the R's would cancel. Then I tried 1.8R instead so I would end up with the equation .5R=.11, R=.22 but that doesn't sound right.
    Last edited: Mar 17, 2008
  2. jcsd
  3. Mar 17, 2008 #2
    You don't have to figure out V. In the first case, you have [tex]\frac{V}{R}=1.8[/tex]

    And in the second case, you have [tex]\frac{V}{2.7+R}=1.3[/tex]

    Two unknowns and two equations.
    Last edited: Mar 17, 2008
  4. Mar 17, 2008 #3
    Okay. I got an answer of 7.02 using those equations, but it was incorrect.
  5. Mar 17, 2008 #4
    Nevermind, I see where the mistake was. I used 2.7, it's supposed to be 4.7
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook