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Homework Help: Electricity questions

  1. Feb 14, 2009 #1
    1. The problem statement, all variables and given/known data
    f a voltage of 40.7 V is applied between points a and b, find the current in each resistor.

    The drawing looks like this (idk how to add a picture here) A-[ ]-/\/-[ ]-B In between brackets (space at top and bottom are resistors)... top of the first (A) is 12, bottom 6, middle there is one (designated by /\/) at 5. On top and bottom of B are 4 and 8 respectively.

    2. Relevant equations

    3. The attempt at a solution First question is Find the equivalent resistance between point a and b. So I tried to break them down and do Rab (hypothetically) as 1/r1 + 1/r2 so 1/12 + 1/6 = .25 then I made a Rcd (hypothetically) and did 1/r4 + 1/r5 which is 1/4 + 1/8 = .375 then I added R3 to get the total R which would be +5 so answer would be 5.625 This was marked incorrect. Then it says if a voltage 40.7 is applied b/t a and B find current in each resistor. There I am completely lost but I attempted something random and got of course a wrong answer.
  2. jcsd
  3. Feb 14, 2009 #2


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    Homework Helper

    I can't follow your description of the diagram. If you can scan one in, you can attach it to your message. Or post it to a site like photobucket.com and give us a link. Some are putting (IMG) and (/IMG), with square brackets instead of round, around the link so it shows up as an image in the post.

    Perhaps you could just describe it better. The current goes through the 12 ohm resister . . .
  4. Feb 14, 2009 #3
    Last edited by a moderator: Apr 24, 2017
  5. Feb 14, 2009 #4
    Ehhh fail wrong one sorry... 1 second.
  6. Feb 14, 2009 #5
  7. Feb 14, 2009 #6

    you have to calculate for the 12 and 6 Ohm the following:

    1/12 + 1/6 = 1/r_1 so r_1 = 6*12/(6 + 12) = 4 = 1/0.25.

    So 0.25 is wrong. Do the same for 1/4 + 1/8 = 1/r_3 -> r_3.
    r_2 = 5 Ohm.

    So R = r_1 + r_2 + r_3.
    Now you have I = U/R (between a and b) with U = 40.7 V.
    Try to figure out the other currents by yourself.
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