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Homework Help: Kirchoffs Law Parallel Circuit-with only I and R given.Solvable?

  1. Jun 14, 2014 #1
    Hi all
    I`m doing my first ever Physics course, and i`ve hit a brick wall with Kirchoffs Law.
    I understand what the laws are i just can`t seem to get the right answers on a particular question.
    I then decided to cheat(because i exhausted all my options in trying to solve it) which i really don`t like doing.My tutor then said I also need to show my workings out,and so this is where i`m stuck i`ve tried everything i could find to do with Kirchoffs Laws but still no luck.

    I`ve filled in the template the best i could, but if anymore information is needed please state.

    Thanks a ton to anyone who can and is willing to help.

    1. The problem statement, all variables and given/known data
    The particular Question is number 4 on Teaching Advanced Physics 117- 5: Questions on Kirchhoff's laws.(https://www.google.co.uk/search?q=kirchoffs+law+TAP&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:eek:fficial&client=firefox-a&channel=sb&gfe_rd=cr&ei=9ICcU___MajY8gfS6oCgDA)
    There are 6 total resistors, 1 on first reciprocal,3 on the second and 2 on the bottom.
    The question is "Ammeter A5 reads 3 A. All the resistors have the same value of 10 Ω. What are the readings on ammeters A1 to A8 and what is the terminal voltage of the battery?"

    2. Relevant equations

    Tried Ohms Law and Kirchoffs laws.

    3. The attempt at a solution

    I tried using Ohms law i.e V=IR, I=V/R etc i`ve also used some equations i saw that was saying about to find the total resistance use 1/R1+1/R2 etc also tried using Kirchoffs Law.
  2. jcsd
  3. Jun 14, 2014 #2


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    What was the result?

    The first thing you can calculate is the source voltage (or, to make smaller steps, the voltages at the two lowermost resistors). That allows to calculate everything else then.
  4. Jun 14, 2014 #3
    The result was different to what the correct answers were.
    Ok so if i`ve got this right what you`ve saying is the voltage is 60V?
    V=IR I=3A R=(10Ω+10Ω)=20Ω
    3A x 20Ω=60V?

    But then how do you work out separate reciprocals voltage? Because i don`t understand how this can work:
    "Kirchhoff's first law
    The total current into a junction = the total current out of the junction.

    Using the convention that currents leaving a junction are the opposite sign to currents entering the junction, the first law may be expressed as the following equation:
    I1 + I2 + I3 + … = 0 where I1, I2, I3 etc represent the currents in the branches connected to the junction."

    Because to me using that 3A goes everywhere.:cry: so :confused: :mad:

    Thank You for helping me:thumbs:
  5. Jun 14, 2014 #4


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    This law does not have any voltages in it?

    No. Why should it?

    You have three parallel lines, all those lines are connected at their ends, so they all see the same voltage - 60 V. This allows to determine their currents.
  6. Jun 15, 2014 #5


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    Hello Stragler, and welcome to PF.

    You have a tendency to make things difficult. For yourself (first ever Physics course = Teaching Advanced Physics ?) and for others (I had to drill to get down to the exercise you want to get assistance for). Personally I had to puzzle over your use of the word reciprocal (In my book that is an inverse).
    On top of that you seem to be in (lawful?) possession of the 'right' answers, but don't mention them, nor your own best guesses.

    Never mind, just because the title is Kirchhoff that doesn't mean you have to solve the problem with that law and with nothing else! It can be applied to conclude that A1 = A8 = A7 and that A3 = A4 (no branching), that A2 + A6 = A7 and that A4 + A5 = A6 and that's about all for Gustav Kirchhoff 1.

    The lines are (ideal) conductors, meaning voltage over the 1 resistor branch, the 2 resistor branch and over the three resistor branch are all equal to the battery voltage. Uncle Georg Simon's law helped you find the value. Same law helps you find A2 and I don't think finding A3 is that difficult with it either. Done !
  7. Jun 15, 2014 #6
    Hi Bvu
    I did a bit of physics in school but have forgotten it.
    The answers i got from the first link from the google link i provided.
    The reason i didn`t mention my own best guesses is because i had so many i`ve forgotten them all.
    I called them reciprocals because somewhere in my sheet given it mention about adding up the separate reciprocals,but i guess i should have called them brances.

    I have now since solved it (at 2am GMT)kinda felt like i had a eureka moment :surprised and no cigar to celebrate and now onto the next one.

    "You have a tendency to make things difficult. For yourself"
    Tell me about it, i`m always scared of getting the wrong answer on what i`m doing that, i feel i need to have a complete understanding of what i`m doing.But i always find it hard when one book explains something in one way and another source explains it different to a way i can understand it,so i end up coming to my own "understanding/conclusion" of it"See it`s like now i`ve solved it and my workings out are from what i can see right,i feel like i`ve done it wrong and need to do it a different way:shy:.

    Like in maths at school i used to suck at it so much, but then i had an american teacher (i`m british) who explained/taught the subject differently and i felt like a smart ***:biggrin:.

    You can probably tell by my name i ain`t very good at physics,but i have to pass it to go to uni, which is why is a higher level course.
  8. Jun 15, 2014 #7


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    No need to put yourself down (Quoting good old Bill: what's in a name). Im too prejudiced to even reconsider that physics is the one and only science, but I had fun long ago subtly pointing out the relevance of physics even to future medics and vets. And they had fun too!
    Apart from that, exercising the little grey cells is a good thing to do for everyone.

    And if you are stubborn enough to hold out even till 02:00 in a weeked you have the mettle to go far, no matter what !
  9. Jun 15, 2014 #8
    Thanks,lots of people keep telling me to not put myself down,but it`s one of those unhealthy habits that`s hard to break, especially since i got bullied all through my childhood, and still feel stupid.

    But anyway,i do kinda like physics,some of it i believe some i`m not so sure off.I do however sometimes have the urge to study it in my spare time,just annoying when i read up on a theory and it`ll just say "this is how it is" without explaining why it is,why it is.

    Haha call it stubbornness, worry, anxiety,frustration or just plain old being P***** 0**,but it`s strange how i seem to work best in the early hours, i guess it must be to do with my brain chemistry changing/my neurons that control my unsure thoughts/hindering thoughts going to sleep :rofl:

    Thank You:thumbs:
  10. Oct 1, 2016 #9
    what are the calculated answers to this question?
  11. Oct 1, 2016 #10


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    Hi chocolatePI,

    Problem solutions won't be handed out here. This thread is over two years old so it should probably be considered "retired". If you are working on the same problem and get stuck you can start a new thread and ask for help. You'll have to show what you've tried.
  12. Oct 1, 2016 #11


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    The essence of how to solve this is in the last paragraph of #4 so... oh.

    You realise chocolatepi the OP. has not been seen since June 2014? :H
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