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Homework Help: Finding currents in resistors

  1. Sep 13, 2006 #1
    A 23 V battery is connected to terminals A and B below.
    (a) If R = 95 Ω, find the current in each resistor (/12, /55, /95).
    (b) Suppose the value of R is increased. For the 12 Ω resistor, does the current flowing through it increase or decrease? What about the 55 Ω resistor? What about the 95 Ω resistor?

    What I did for the /12 resistor is I = V/R = 23 V / 12 Ω = 1.9167, but got it wrong. Is this problem not as simple as I originally thought it is?
  2. jcsd
  3. Sep 13, 2006 #2
    since the 12 ohm resistor and the 2 resistors in parallel are in series, the potential difference across the 12 ohm and the 2 parallel resistors is not the same so you cant simply do V= iR where V = 23 for the 12 volt resistor

    you're going to have to find equivalent resistances for hte paralell ones first and then proceed

    for b - would it make a difference?? Why?
  4. Sep 13, 2006 #3
    How do I find equivalent resistances? Do I solve I = 23/35 + 23/95 = 0.8992?
    Last edited: Sep 13, 2006
  5. Sep 13, 2006 #4
    your textbook should show how to combine 2 resistors whether they may be in series or parallel

    in any case
    for resistors in series; Req = R1+ R2+...

    for resistors in parallel: [tex] \frac{1}{R_{eq}}= \frac{1}{R_{1}} + \frac{1}{R_{2}} + ... [/tex]
  6. Sep 13, 2006 #5
    So is I = 23/35 + 23/95 = 0.8992? correct, in regards to the parallel resistors?
  7. Sep 13, 2006 #6
    how did you arrive at that??
    sorry im too lazy to do it myself
  8. Sep 13, 2006 #7
    I already showed my work in that. I = V/R1 + V/R2 = 23/35 + 23/95 = 0.8992
  9. Sep 13, 2006 #8

    wheres the 35 coming from?? I ll assume you meant tosay 55...

    when two resistors are in series (the 12 and the 55/95 parllel) is the potential diffrerence across the 12 and the 55/95 the same??? Note that the 55/95 are in series, the current may be the same, but the potential difference is NOT. Although if the 12 ohm resistor was not present you would be right but here the 12 ohm resistor makes thing s a little morei nteresting

    try the equivalent resistor route.
  10. Sep 13, 2006 #9
    I'm sorry, you lost me there. Would you be able to help set up the equation for me?
  11. Sep 13, 2006 #10
    find theq equivlanet resistance of ALL the resistors first
  12. Sep 13, 2006 #11
    Okay, 1/Req = 1/R1 + 1/R2 + 1/R3 = 0.11204 Ω.

    I don't know if this is moving in the right direction, but I also did:

    (0.11204 Ω)^-1 = 8.9253 Ω.
    I = E/Req = 23 V / 8.9253 Ω = 2.57695 A.

    I hope this helped speed things up a bit.
    Last edited: Sep 13, 2006
  13. Sep 13, 2006 #12
    is 12 in parallel with the 55 and 95??

    also when you complete the sum you have to invert your answer to get Req otherwise you have 1/Req
  14. Sep 13, 2006 #13
    12 is not in parallel with 55 and 95, so does this mean I don't include it when solving 1/Req?
  15. Sep 13, 2006 #14
    thts correct
  16. Sep 13, 2006 #15
    So Req = 1/55 + 1/95 = 0.0287 Ω^-1 = 34.833 Ω. Do I simply add 12 to this number?
  17. Sep 13, 2006 #16
    yes that is the total equivlaent resistnace

    now you can find the current through that resistor

    now since the 12 and the 34 are in series what can you say about the current passing through each? Also can you find the potential diffrence across each?

    after that you know the current passing through the 12 and the 34. Since the 34 is 2 resisotrs(55/95) in parallel what is equal between them?? Can you find the current across each using that thing which is equal??

    dont try to rush it... take it slow
  18. Sep 13, 2006 #17
    Eek...I still do not understand what you are asking. I looked through the book and lecture notes, and could not find anything that seemed relevant to help me answer your questions :(.
  19. Sep 13, 2006 #18
    do them step by step

    find the current through that resistor (the 12 + 38) which you just found

    and then follow those steps only once you have completed each

    when resistors are in parallel th potential diff across them is same but the current is not
    total current = sum of the current across each

    when resistors are in series the potential difference is not the same but the current is the same
    total potential difference = sum of potentei8al diff across each
  20. Sep 13, 2006 #19
    Okay, so I = E/Req = 0.4911 A. Mathematically, what is the next step? I understand what you are saying with the resistors being in parallel and in series, but I do not know how to mathematically use that information.
  21. Sep 13, 2006 #20
    dont go to the next step without completing each step

    ok so this means taht the current across the 2 resistors in series (the 12 and 38) have the same current passing through because they are in series

    now we know that the total current passing thru the 55 and 95 is 0.4911 but that doesnt help us becase the current passing thru each is different

    so find the voltage across the 12 and the 38

    now since resistors are in parallel have the sam potenetial difference across them the potential across the 38 is the smae as taht across the 55 and the 95

    now u can find the current across each resistor
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