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Homework Help: Basic Ohm's Law Question

  1. Nov 28, 2017 #1
    1. The problem statement, all variables and given/known data

    So I'm just curious whether or not the following statement is correct for the circuit shown. It's part of a bigger problem involving OP-AMPS, the part of the circuit shown is the upper loop connecting from the inverting to the output.

    I was just having some doubts in my mind and wanted to confirm whether or not the equation i have written is correct.


    2. Relevant equations
    Ohms Law. V = IR.

    3. The attempt at a solution

  2. jcsd
  3. Nov 28, 2017 #2


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    Staff: Mentor

    For the given component values it looks okay. Presumably the rectangular component is meant to represent a 1 F capacitor?
  4. Nov 29, 2017 #3
    Interestingly gneill seems to understand your circuit and terminology, but I am struggling with it. However, any way I look at it I cant see the rectangular component as being a capacitor unless the OP AMP in question is a differentiating OP AMP and even then I would need to see a feedback resistor which is a key part of the the various voltage components. Since we are doing V = IR (or in this case I = V/R) I cannot understand this to be differentiating problem, so I would read that the rectangular component is a resistor with a value of 1/S but I am not able to determine what 'S' signifies. Also, if this is an integrating OP AMP, then there must be two resistor, R input and R feedback.
    I may be just having a dumb day, bit I would like to see a full circuit, with the components correctly defined before I could comment on the correctness of the value terminology used in the equation.
  5. Nov 29, 2017 #4


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    Staff: Mentor

    The 's' is the Laplace domain "frequency" variable/operator (yes, it's both). Reactive components such as inductors and capacitors have impedances in the Laplace domain of the forms:

    Inductance L: sL
    Capacitance C: 1/(sC)

    Laplace transforms are a very handy way to write and solve differential equations using simple algebra.
  6. Nov 29, 2017 #5
    It is a sad day when you don't learn something new:smile: Although vaguely aware of Laplace transformations I have never used them.
    For a differentiating OP AMP I would use the formula Iinput = C x dVinput/dt
  7. Dec 9, 2017 #6
    Yes! That's exactly what it was meant to be. Thank you
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