1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Resistancein a circuit ,,

  1. Mar 17, 2010 #1
    Resistance in a circuit ,,

    1. The problem statement, all variables and given/known data

    Hi guys ,, I have the following picture


    3. The attempt at a solution

    OK ,, I know it's easy but my answer's is not like my teacher's answer :

    for (a) I get 6.12 ohm and for (b) I get 2.4 ohm

    but my teacher's answer is :

    a. Rt=4+4+4=12 ohm
    b. Rt= (8//4)+4= 6.667 ohm

    what I don't get for :

    a. how can he just add them together ,, it's not like they are in series and 2 of them are in parallel .. and I don't know why adding only 3 resistors only ?? there are 5 ~.~ ,,

    b.how come 8//4 and there is a resistor between 4 & 4 ,, so don't we have to take a parallel (4//4) then add with 4 then parallel them with the upper resistor ??

    so can someone simplify circuits please -_- ,, (I usually use current direction to know whether they are in parallel or series)

    Attached Files:

    • 123.jpg
      File size:
      7 KB
    Last edited: Mar 17, 2010
  2. jcsd
  3. Mar 17, 2010 #2

    Filip Larsen

    User Avatar
    Gold Member

    Think about which wires in the circuit that would actually carry a current if you were to measure R_T by applying a voltage over the two left terminals. It may also help to redraw the diagram so you have the top-left terminal on the left and the bottom left terminal on the right.
  4. Mar 17, 2010 #3
    aha ,, now I get it ,, so the arrow (Rt) in the diagram means where the voltage terminals are ,, I get it for (a) and (b) too ,, thanks very much filiplarsen ,, but question , if both circuits where closed my answers were right ??
  5. Mar 17, 2010 #4

    Filip Larsen

    User Avatar
    Gold Member

    If you by closed mean that the two terminals to the right are short-circuited, then I get
    a) 4+(8//4)+4 = 10 2/3, and
    b) 4//(4+(4//4)) = 4//6 = 2 2/5
    So we have same result for b) but not a).
  6. Mar 17, 2010 #5
    Lol,, got it :)

    thanks very much m8
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook