Only a semi-tricky quiz

Discussion in 'Radio Circuits, Repair & Performance' started by KL7AJ, Sep 10, 2017.

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  1. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    That's the answer I was looking for. You get a tin star!
  2. K8ERV

    K8ERV Ham Member QRZ Page

    Only till the government gets ahold of it, then it is whatever it wants it to be.

    TOM K8ERV Montrose Colo
    K9ASE and K0OKS like this.
  3. WR2E

    WR2E Ham Member QRZ Page

    Ahhh, so not using ONLY Ohm's Law then?

    Using Ohm's Law AND a multimeter AND a voltage source... Unh huh...
    N0TZU, K0OKS and WD3N like this.
  4. N2EY

    N2EY Ham Member QRZ Page

    You said using ONLY Ohm's Law. No mention of a voltage source nor a meter. FAIL.

    I call shenanigans.
    K8MHZ likes this.
  5. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page


    You definitely fail that one! The parameters were ONLY Ohm's Law and not with additional items. A voltage source, meters, etc., are things in addition to Ohm's Law.

    Using an ohmmeter, VOM, or DMM, would then meet the requirements and be much easier to calculate!

    The method described by ERV, the reciprocal of the sum of the 5 reciprocals, is a "paper" way of determining the results and does not require any test equipment, voltage sources, etc.

    Glen, K9STH
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  6. K7JEM

    K7JEM Ham Member QRZ Page

    No meter or voltage source is needed. All you have to do is assume a voltage, calculate current in each resistor based on that voltage, then add the currents together.

    From that total current and voltage you can then calculate actual resistance of the parallel combination.
    WR2E likes this.
  7. WR2E

    WR2E Ham Member QRZ Page

    The recip of the sum of recips is derived from ohm's law though.

    1/R = I/V

    1/Rtot = I1+I2+I3+I4+I5/V = I1/V +I2/V +I3/V +I4/V +I5/V

    1/Rtot = 1/R1 + etc...

    So isn't it in fact still Ohm's Law?
  8. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    Not true! You can use ANY arbitrary value of voltage and calculate the thing. 1 volt is nice and convenient. But you don't need a physical battery of any kind to do the calculation
  9. AG6QR

    AG6QR Subscriber QRZ Page

    Sure it is! Well, maybe it's Ohm's law plus a teeny bit of elementary algebra, plus the most basic knowledge about a parallel circuit.

    In order to solve the problem, you have to know that the current through the parallel combination is equal to the sum of the currents through each element. You also must know that, in a parallel circuit, the voltage across each element is the same, and is the same as the voltage across the parallel combination. And you have to know Ohms law.

    Ohms law tells you that R = V/I

    Knowing what a parallel combination is tells you that I of the parallel combination is I1 + I2 + I3 + I4 +I5, so you have

    R = V / (I1 + I2 +I3 +I4 +I5)

    Ohms law tells you that, for each individual resistor, I = V/R, so substituting that into the above, you now have:

    R = V / (V1/R1 + V2/R2 + V3/R3 + V4/R4 +V5/R5)

    Knowing that all the V's are equal in a parallel combination lets you cancel them out, so you've got

    R = 1 / (1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5)

    So it's the reciprocal of the sum of the reciprocals, using Ohms law, plus basic algebra, and a couple of facts from the definition of parallel circuits.

    If you want to do it using ONLY Ohms law, meaning without any understanding of what parallel circuits are, and no algebra whatsoever, then I guess it's going to be tougher.
  10. K8ERV

    K8ERV Ham Member QRZ Page

    No fair. Not using only the law. You also need a voltage source and current meter. Might as well just use an ohmmeter!

    TOM K8ERV Montrose Colo
    WR2E and KA9JLM like this.

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