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What is the resistance of a non-ohmic resistor in v-i graph

  1. Jun 8, 2010 #1
    What is the resistance of a non-ohmic resistor (lamp) in v-i graph given a voltage?
    I thought it would be the gradiant for specific voltages.
    But apparentlly it is the ratio of the specific voltage/current which makes sence too.
    So what is the correct out of the two?
     
  2. jcsd
  3. Jun 8, 2010 #2
    is voltage on the x axis and current on the y or the other way around .
    if voltage is on the x axis then the slope of that line would be 1/R
    if current is on the x axis then the slope of that line would be R
    using I=V/R
     
    Last edited: Jun 8, 2010
  4. Jun 9, 2010 #3
    For a non-ohmic resistor, the ratio V/I is not necessarily the same as the slope of the curve.

    If y = x^2, then x = 3 implies y = 9 so y/x = 3, but dy/dx = 6.

    Since resistance is defined via Ohm's law the resistance is the ratio, not the slope.
     
  5. Jun 9, 2010 #4

    berkeman

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    Staff: Mentor

    I'm not sure I'm understanding which one you are promoting, but the impedance is definitely the slope of the line:

    Z = dV/dI

    This is used all the time in circuit analysis.
     
  6. Jun 9, 2010 #5
    I was arguing on the opposite side. I'm not an expert on electronics, so I defer to you. Is there a difference between impedance and resistance? I was thinking in terms of DC voltage.
     
  7. Jun 9, 2010 #6

    berkeman

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    Staff: Mentor

    If all the components are resistors, then impedance = resistance.

    If some of the components are non-liinear (like diodes for example), then impedance = Z = dV/dI, and it is still real.

    If some of the coponents are reactive (inductors and capacitors), then you get a complex Z = dV/dI, with real and imaginary (in-phase and quadrature-phase) components.
     
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