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!

Kirchhoff's Law Problem -- Resistors in Series and Parallel and a Switch

  1. Apr 22, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-4-22_13-22-17.png

    2. Relevant equations
    Rparellel=1/R1+1/R2...
    IR1=R2/R1+R2 x I

    3. The attempt at a solution
    upload_2016-4-22_13-22-43.png

    I am unsure of how to answer d) and e) using KVL because I count 4 junctions?
    Where should I start?
     
  2. jcsd
  3. Apr 22, 2016 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Check your first equation ! It's not for Rpalrallel but for 1/=Rpalrallel !

    And the 20 and 25 Ohm resistors are NOT parallel to e.g. the 12 Ohm !
     
  4. Apr 22, 2016 #3
    Yes sorry that was a typo.
    The total resistance for the parallel resistors still equals 15.18ohms but yes I should have done them separately:
    (1/12+1/10+1/16)^-1 = 4.07ohms
    (1/20+1/25)^-1= 11.11ohms

    Do my values look correct?
     
  5. Apr 22, 2016 #4

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I only know $${1\over 12} + {1\over 10}+ {1\over 16}+ {1\over 20}+ {1\over 25} \ne 15.82$$
    Your values in brackets are probably the book answers ?
     
  6. Apr 22, 2016 #5
    (1/12+1/10+1/16+1/20+1/25)^-1=15.18 ohms

    Yes values are answers in the book
     
  7. Apr 22, 2016 #6

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Then why ask if the values are correct ?
     
  8. Apr 22, 2016 #7

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No. $${1\over 12} + {1\over 10}+ {1\over 16}+ {1\over 20}+ {1\over 25} = {1\over 2.98}$$However,$$
    {1\over {1\over 12} + {1\over 10}+ {1\over 16} } + {1 \over {1\over 20}+ {1\over 25} } = 4.07 + 11.11 = 15.18$$
     
    Last edited: Apr 22, 2016
  9. Apr 22, 2016 #8
  10. Apr 22, 2016 #9

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    What sort of device is R7 that its resistance changes depending on whether the switch is opened or closed
     
  11. Apr 22, 2016 #10

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The parts d and e are a bit weird, because basically you use KVL to calculate R7 in a and b.
    And it wouldn't be proof, just showing.
    I should think they want you to show that 1.2 A * (4.07 + 60 + 11.11 + 24.82) Ohm = 120 V
     
  12. Apr 22, 2016 #11

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    +1

    Although I prefer to make the voltages around the loop sum to zero.
     
  13. Apr 22, 2016 #12

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    @DevonZA ,

    Are you saying that this problem is from a textbook?

    What book? Who is the author ?
     
    Last edited: Apr 22, 2016
  14. Apr 23, 2016 #13
    Can you show this?
     
  15. Apr 23, 2016 #14
    Hi Sammy this is from our varsity study guide. The author is most likely the lecturer who doesn't respond to me hence why I must ask these questions on here.
     
  16. Apr 23, 2016 #15

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    With reference to the equivalent circuit below... KVL says that going around a loop the voltages sum to zero.
    KVL.png

    If I arbitrarily choose to start at the -ve terminal of the battery and go around clockwise we have to prove that..

    +120 + (-V1) + (-V2) + (-V3) + (-V4) = 0

    V1 = 1.2 * 4.07 = 4.884V
    V2 = 1.2 * 60 = 72V
    V3 = 1.2 * 11.11 = 13.332V
    V4 = 1.2 * 24.82 = 29.784V

    Substitute..

    +120 - 4.884 - 72 -13.332 - 29.784 = 0
     
  17. Apr 23, 2016 #16
    Thank you for the nice clear explanation CWatters
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Kirchhoff's Law Problem -- Resistors in Series and Parallel and a Switch
  1. Kirchhoff law problem (Replies: 11)

  2. Kirchhoff law problem (Replies: 10)

Loading...