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Confusion about combining resistors in series.

  1. Sep 30, 2012 #1
    Hello, everybody.

    I'm reading through my EE textbook and I'm having some confusion about when to combine resistors in series. The following figure is in my book:


    I see that they combine the parallel resistors of 6Ω and 3Ω to make the equivalent circuit shown in 2.42b. What I'm confused about is why they didn't then combine the 4Ω and 2Ω resistors. They do this throughout the entire chapter's examples and it's causing me confusion. Resistors are considered in series if they share the same current, correct? And the two resistors in 2.42b appear to share the same current, so I don't see why they don't combine them.

    Thanks for reading.

    (I apologize if this is considered homework help. It seems to be more of a general circuit question.)
  2. jcsd
  3. Sep 30, 2012 #2


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    If you only wanted to get the current from the battery, you would combine all the resistors.

    However, if you wanted to get the voltage across the parallel comination, then you would have to keep that resistance separate so that you could calculate the voltage.

    In fact you could do both. The combined resistance is 6 ohms (4 ohms + 2 ohms) so the current is 2 amps (12 volts / 6 ohms).

    So the voltage across (a) - (b) is 4 volts. (V = i * R = 2amps * 2 ohms = 4 volts)
  4. Sep 30, 2012 #3


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    The volts across ab will be 2/(4+2) of the total volts. It's a 'simple' potential divider, once you have manged to combine the 6 and 3 Ohm resistors correctly (which you have). The potential divider is one of those basic circuit elements that 'everyone' should get familiar with.
  5. Sep 30, 2012 #4
    So you can combine all of the resistors to find the current through the circuit, but you have to leave them separated if you want to find v0. That makes sense; thanks to the both of you.
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