Static and Dynamic Resistance. (1 Viewer)

[Promight]

Hey everyone,

I don't know if this is the right place to post this, but I've been hung up on taking the gradient of the V against I graph as the resistance. Looking around I've found static and dynamic resistance as the gradient from the origin to any given point on the line and the differential of the line at any given point respectively. Can someone explain the difference between static and dynamic resistance and which one I should use, I can't find it anywhere.

 

[Promight]

I should add, this is with respect to a heating light bulb under DC current instead of a diode acting as the non ohmic resistor.

 

[anomalousdecay]

Static resistance refers to linear resistance. For a linear resistance, you want something with a constant gradient (a gradient which does not change at all). So basically you want a straight line which is increasing for the graph of voltage drop across the resistor (y-axis) vs current (x-axis).

With a light bulb, there is some inductance and also the resistance varies due to heat as well, so it won't necessarily have a static resistance. However, this depends on the light bulb and how it behaves. I've never tried it myself, but I'm pretty sure a light bulb will generally be non-ohmic.

Read through this http://www.researchgate.net/post/Wh..._nonlinear_resistor_a_dynamic_linear_resistor

 

[Promight]

Thanks, but I still don't quite grasp the difference between static and dynamic resistance. The site you provided even asked the question at the end (not word for word) "can you represent a non linear graph as a variable linear graph", i.e. can you take the ratio of V/I at any point and would that be your resistance (static resistance). This makes sense to me, although it is generally more accepted to take the tangent to any given point and find the gradient of the line to find out the resistance at that point which gives a completely different answer. We have to measure the resistance of a light globe in class at different voltages, which resistance should I use?

 

[anomalousdecay]

Use the non-ohmic case for class because it conveys the point you are making that the resistance of the light bulb is non-ohmic (was this the aim of the experiment?).

For exams, you usually want ohmic because you are relating it to a line of best fit for a specific set of values, unless it is stated otherwise to use some other value.

 

[Promight]

I understand what you're recommending and I'll take that into account in class, but my main concern is still unanswered. What is the difference between dynamic resistance and static resistance and which is more frequently accepted as the value for the resistance at any current/ voltage of a circuit.

 

[anomalousdecay]

Difference is ohmic and non-ohmic.

Ohmic refers to static (it is constant), non-ohmic is dynamic (as it is changing).

Accepted value; HSC, I'm pretty sure you disregard any non-ohmic unless stated otherwise or is a potential reason due to a non-linear V-I relationship.

Reason being is that in calculations, you want a constant value for given conditions.

For theory, generally resistance is assumed constant for learning purposes.

In practice, dynamic would be more accepted for engineering design purposes.

 

[Promight]

Ah okay, thanks know you so much.