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Find current of resistors

  1. Dec 16, 2012 #1
    1. The problem statement, all variables and given/known data

    resistors.jpg

    2. Relevant equations

    I = E/(R sub_1 + R sub_2)

    3. The attempt at a solution

    So I tried using the same approach for part a and it did not produce the same result.

    here's what i did:

    I sub_2 = E sub_2/(R sub_2 + R sub_3)

    E sub_2 was given as 1.7V

    I sub_2 = (1.7V)/(5) = .34 A <- answer for part b

    I also noticed E sub_3 was not given in the problem.
     
  2. jcsd
  3. Dec 16, 2012 #2
    You got lucky if you got the right answer using this approach for (a) :)

    If you rearrange that equation, this is what you are saying:

    E2 = I2R2 + I2R3

    but this isn't true. The current through R2 is I2 but the current through R3 is not I2.
     
  4. Dec 16, 2012 #3
    for part a I used E sub_1 = I(R sub_1) + I(R sub_3)

    really? i got lucky. when i got the answer, I thought I knew how it works but I guess I do not. =(

    what should be going in my mind when I approach this type of problem. I was thinking in order to find the current I sub_2 I know it is across from I sub_3 and picked that one because it is closest. that's why I sub_2 was multiplied by R sub_3
     
  5. Dec 16, 2012 #4
    The current through a resistor results in a voltage drop and the total net voltage drop around a loop must be zero -- this is KVL.

    In the equation you wrote:
    E2 = I2R2 + I2R3

    the voltage drop across R3 depends on the current through R3, not the current through R2.

    So the correct equation would be:

    E2 = I2R2 + I3R3

    NOTE: BE CAREFUL THAT THE SIGNS ON THESE CURRENTS AGREE WITH YOUR ASSUMED CURRENT DIRECTIONS

    Similarly in (a) you wrote an equation for the left loop but you used the same current through R1 and R3, which is probably not right.

    The correct equation would use the currents through R1 (I1) and R2 (I2) to find the voltage drops across R1 and R2.

    After you've written those two equations, you'll notice you have three unknowns (I1, I2, I3) so one more equation is needed to solve the system.


    When I looked at the problem, I saw two loops (KVL) and would have used mesh analysis to solve it but the above is equivalent.
     
  6. Dec 16, 2012 #5
    thanks. i will use your input to finish this question.
     
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