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

1. Jun 8, 2010

### chetan:)

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. Jun 8, 2010

### cragar

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
3. Jun 9, 2010

### dulrich

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.

4. Jun 9, 2010

### 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.

5. Jun 9, 2010

### dulrich

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.

6. Jun 9, 2010

### 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.