How to find missing resister with a given voltage value?

Discussion in 'Electronic Design' started by alishadevochka@gmail.com, Jun 19, 2007.

  1. Guest

    I have two circuits, where resister voltage are given, and I need to
    find resistor value.


    first circuit is -- both are serial circuits

    E= 24v, R1 = 2ohm, R2=4 ohm, and R3= 24W (watts).

    Find R. How do I do this, Also please explain the method.


    Second question is

    E = 20V, R1=2V, R2=40ohm, R3=32ohm. Find resistor R1 using voltage
    divider rule.

    This is what I have done so far for;

    formula; v1 = R1*E/R1

    2v=R1*20v/R1
    2V=20V
    =10ohm

    I use this method on another serial circuit with all the ohms,
    current, voltage, and I didn't get the right answer. There are no
    answer at the back of the book, and I don't know if I am right.
     
    , Jun 19, 2007
    #1
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  2. wrote:
    > I have two circuits, where resister voltage are given, and I need to
    > find resistor value.
    >
    >
    > first circuit is -- both are serial circuits
    >
    > E= 24v, R1 = 2ohm, R2=4 ohm, and R3= 24W (watts).
    >
    > Find R. How do I do this, Also please explain the method.


    You have one simple formula relating I to V for R3. I*V=24,
    or I=24/V or V=24/I.
    You need to come up with a second formula relating I and V
    to R value based on the other two resistors and total
    voltage. Then combine these two formulas to solve for R3.

    The voltage across R3 must be 24 - R1 drop - R2 drop. Each
    of these drops is pretty easy to describe in terms of the
    common current, which is also pretty easy to describe in
    terms of R3.

    > Second question is
    >
    > E = 20V, R1=2V, R2=40ohm, R3=32ohm. Find resistor R1 using voltage
    > divider rule.


    The divider rule is

    fraction of total voltage that is dropped across Rx
    = Rx/Rtotal

    So, in this case Vfraction = 2/20 = 0.1
    =R1/(R1+R2+R3)

    > This is what I have done so far for;
    >
    > formula; v1 = R1*E/R1
    >
    > 2v=R1*20v/R1
    > 2V=20V
    > =10ohm
    >
    > I use this method on another serial circuit with all the ohms,
    > current, voltage, and I didn't get the right answer. There are no
    > answer at the back of the book, and I don't know if I am right.
    >
     
    John Popelish, Jun 19, 2007
    #2
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  3. Eeyore Guest

    wrote:

    > I have two circuits, where resister voltage are given, and I need to
    > find resistor value.


    You can do this with Ohm's Law.

    Do you know it ?

    Graham
     
    Eeyore, Jun 19, 2007
    #3
  4. Fred Bloggs Guest


    > I have two circuits, where resister voltage are given, and I need to
    > find resistor value.
    >
    >
    > first circuit is -- both are serial circuits
    >
    > E= 24v, R1 = 2ohm, R2=4 ohm, and R3= 24W (watts).
    >
    > Find R. How do I do this, Also please explain the method.
    >
    >
    > Second question is
    >
    > E = 20V, R1=2V, R2=40ohm, R3=32ohm. Find resistor R1 using voltage
    > divider rule.
    >
    > This is what I have done so far for;
    >
    > formula; v1 = R1*E/R1
    >
    > 2v=R1*20v/R1
    > 2V=20V
    > =10ohm
    >
    > I use this method on another serial circuit with all the ohms,
    > current, voltage, and I didn't get the right answer. There are no
    > answer at the back of the book, and I don't know if I am right.
    >


    In both circuits E=I*(R1+R2+R3) and you have to solve for an unknown R,
    which means you must eliminate I from the equation.
    In the first circuit you are given the power in R3 is 24W which I^2*R3
    so that I=sqrt(24/R3) and the equation becomes:
    E=sqrt(24/R3)*(R1+R2+R3), or 24=sqrt(24/R3)*(6+R3), which you can now
    solve for R3.
    In the second circuit you are given the voltage drop across R1 is
    2V=I*R1 so that I=2/R1 and the equation becomes E=2/R1*(R1+R2+R3), or
    20=2/R1*(R1+72), which you can now solve for R1.
     
    Fred Bloggs, Jun 19, 2007
    #4
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