Which voltage to use when calculating Current

Discussion in 'Homework Help' started by pprendeville, Jan 13, 2010.

  1. pprendeville

    Thread Starter New Member

    Nov 19, 2009
    15
    0
    Hi all,

    Just want to find out which voltage I should use when calculating the current through the 5 ohm resistor in the pic. Originally I thought it was the supply voltage, 20V but according to the notes it says to use the voltage drop across the resistor, i.e. 9.1V.

    Why is this so?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Well
    V=I*R;
    I=20V / (5Ω+ 12|Ω|18Ω||9Ω+4Ω||4Ω)=20V/11Ω=1.81A

    So voltage on 5 ohm resistor is equal
    V=1.81A*5Ω=9.09V
     
  3. mrmount

    Active Member

    Dec 5, 2007
    59
    7
    That is because the supply voltage 20 V is across the total resistor combination and not 5 ohm resistor alone. See Jony130's calculation.
     
  4. pprendeville

    Thread Starter New Member

    Nov 19, 2009
    15
    0
    cheers lads.
     
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