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Calculating voltages help

Discussion in 'Electronics Homework Help' started by Sam Clay, Oct 29, 2014.

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  1. Sam Clay

    Sam Clay

    Oct 29, 2014

    So I just started a course at uni and there is a fair amount of electronics which I have no background in really. I did a lab the other day where we built the circuit shown in the diagram here.

    I can't work out why the voltage across where the volt meter is attached is 3.33, and how to prove it mathematically.

    Any help would be greatly appreciated.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Have you covered KVL and KCL? If so, you can use them to determine the voltages at the nodes where the voltmeter is connected (compared to some reference node). What is the difference between them?

    If you've not covered that, but understand ohms law and formulas for resistors in series and parallel, you can achieve the same result.
  3. Sam Clay

    Sam Clay

    Oct 29, 2014
    Hi, I haven't covered that yet I'm afraid, but understand Ohm's law and calculating resistance in series and parallel.
  4. Laplace


    Apr 4, 2010
    Sometimes it helps to redraw the circuit in a more familiar orientation.....

    If you have learned about voltage dividers, now may be the time to divide some voltage!
    chopnhack and Arouse1973 like this.
  5. chopnhack


    Apr 28, 2014
    @Laplace - That was clever!! - shall we call it commutative property of voltage in parallel :)

    Sam, I am still struggling with the math, much to everyone's chagrin around here, but this site might help you understand the calculations behind this circuit.
  6. jbelectric777


    Nov 29, 2012
    Sam, Series/parallel resistance isn't tough at all, maybe this will help: Series just add the sum which in your diagram series are 90Ω then 60Ω now we did series now we parallel those two:
    1/90 =.011 and 1/60 = .016 the sum is .011111 + .0166666 = .0277778 then then sum of all divided by 1 to get closest resistance value or 1/35.99997Ω (Ohms law: E/R=V or 10/3Ω = 3.33) It looks like on the last stage they rounded 5 or less to nearest tenth and used 3Ω
    or maybe so the student could understand the digit 3 rounded is better than your calculators finishing with all these decimals and just showed the close as possible end product. Ask the moderators if that seems to be the case. Jim B:(
    Last edited by a moderator: Nov 2, 2014
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The answer to this question doesn't actually require any resistance in parallel calculation. :)
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I don't follow this at all. I can see how you get a total equivalent resistance of 36 ohms, but I'm not sure that this helps you at all in answering the question.

    Then for the last step... where did 3 ohms come from?

    The solution involves finding the voltage at the points labelled + and - by Laplace, and then finding the difference between them (because that's what is being measured).

    However, I think at this point @Sam Clay has probably just found another forum that has given him the answer and thus won't be returning here to try to solve it himself. :(
    davenn and KrisBlueNZ like this.
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