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Homework Help: Parallel circuit equation question

  1. Jul 13, 2008 #1
    I am starting a course in Sept in electrical installation. As it's been a while since I left full time education I thought I would buy the relevant text book to give me a head start and get my brain into gear.

    My problem relates to parallel circuit calculation!

    Example: three resistors of 16, 24, and 48 ohms are connected across a 240v supply find the the total resistance. I know that it's 8 ohms because it tells me so. What I don't understand is how that is arrived at.
    the equation is shown like this:
    1/Rt= 1/R1+1/R2+1/R3 = 1/16 + 1/24 + 1/48
    and therefore:
    1/Rt = 3+2+1/48 = 6/48
    I realise that 48/6 =8 what I don't get is what does 1 represent and where does 48 come from as in: 3+2+1/48

    There is no explination of how the calculation is done!

    2. Relevant equations

    3. The attempt at a solution
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 13, 2008 #2
    well the there are basically two ways u can add resistors

    one way is in series the general formula for that is

    Rt = R1+R2+R3 (considering there are 3 resistors)

    if the resistors are set up in parallel then the formula you use changes to

    1/Rt = 1/R1 + 1/R2 + 1/R3

    and so solve for RT.

    you have 1/Rt to find Rt you take the inverse.
  4. Jul 14, 2008 #3
    Thanks for that, that's much simpler than what's written in this book!

    So: 1/16=0.0625 1/24=0.041 1/48=0.020
    therefore: 0.0625+0.041+0.020=8

    Have I got it right?
  5. Jul 14, 2008 #4


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    Note that Rt=R1R2R3/(R1+R2+R3)
  6. Jul 14, 2008 #5


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    Yeah but you omitted the part where you take the reciprocal to get 8 ohms.
  7. Jul 14, 2008 #6


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    Sorry about that. This is not correct.
  8. Jul 15, 2008 #7
    Yes I forgot to inclucde 1/0.1235 = 8 totaly missed it out when I was typing!

    Thanks for everyone's help, I think my brain is finally engaging!
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