How to Calculate Resistivity of a Non-Uniform Wire?

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Discussion Overview

The discussion revolves around calculating the resistivity of a non-uniform wire, particularly in geometries such as a bow-tie shape. Participants explore the implications of varying cross-sectional areas and the challenges in determining resistivity when multiple materials are involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Elionix questions how to calculate resistivity for a wire with a non-uniform cross-section, specifically in a bow-tie geometry.
  • One participant asserts that resistivity is a material property and is independent of shape, suggesting a method to calculate it using series resistors with varying cross-sectional areas.
  • Follow-up questions are posed regarding the extraction of resistivity when two materials are in series, with varying known and unknown resistivities.
  • Another participant mentions that resistivity can change with the shape of the material, particularly at sub-10nm scales, and proposes an integration approach for wedge-shaped configurations.
  • A participant emphasizes that the equation for resistance is an approximation and that more complex shapes require solving electrostatics problems with boundary conditions.
  • It is noted that for complex shapes, numerical solutions may be necessary.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between resistivity and geometry, with some asserting that shape is irrelevant while others argue that it can affect resistivity. The discussion remains unresolved regarding the best approach to calculate resistivity in non-uniform wires.

Contextual Notes

Participants highlight limitations in their approaches, such as the dependence on assumptions about material properties and the complexity of shapes involved in the calculations.

elionix
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Hello All,

I was curious to know if there is a way to calculate resistivity of a wire that did not have a uniform cross section? For example, what if the cable was in a bow-tie geometry? Is there anyway to quantify the electrical resistivity of the constriction (the area of the cable that has the pinch)?

Thanks!
Elionix
 
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Hello Elionix! :smile:
elionix said:
… is a way to calculate resistivity of a wire that did not have a uniform cross section?

Resistivity is a property of the material … its shape is irrelevant.

If you mean is it possible to calculate the resistivity if you know the resistance, then yes, just call the resistivity ρ, and treat the cable as a lot of resistors in series, each with length dx and cross-section area A(x). :wink:
 
Some follow up questions:

1. What if the resistance was known for, say, material A in series with material B, but both resistivities are unknown. Is there a way to extract the resistivity of material A?

2. What if we knew the resistivity of material A, but not of material B? Knowing the physical dimensions of both materials and also the resistance (A+B in series), is it possible to extract the resistivity of material B?
 
Also, resistivity does change with with the shape of the material, for example look in:
A. Naeemi et al., Proc. IEEE Int. Interconnect Technol. Conf., 183–185 (2008).

Especially in sub-10nm scales, there can be width-dependent resistivity.

Thinking out loud to a response for my questions:

R= ρ1 (A1/L1) + ρ2 (A2/L1)

It seems if I have a wedge shaped configuration, I could just integrate to get the area of interest. so: 1. it's not possible
2. Looks like it is possible to find ρ2 knowing ρ1
 
hi elionix! :smile:

(just got up :zzz:)
Some follow up questions:1. What if the resistance was known for, say, material A in series with material B, but both resistivities are unknown. Is there a way to extract the resistivity of material A?

2. What if we knew the resistivity of material A, but not of material B? Knowing the physical dimensions of both materials and also the resistance (A+B in series), is it possible to extract the resistivity of material B?
elionix said:
R= ρ1 (A1/L1) + ρ2 (A2/L1)

It seems if I have a wedge shaped configuration, I could just integrate to get the area of interest. so: 1. it's not possible
2. Looks like it is possible to find ρ2 knowing ρ1

that's right :smile:

to put it simply …
1. You have 2 unkowns and only one equation, so no you can't solve it.
2. You have 1 unkown and one equation, so yes you can solve it! :wink:
elionix said:
Also, resistivity does change with with the shape of the material, for example look in:
A. Naeemi et al., Proc. IEEE Int. Interconnect Technol. Conf., 183–185 (2008).

sorry, i can't comment on that :redface:
 
The above equation for R is just an approximation for cases where the length is long compared to sqrt(A) and A is constant. In general, you need to solve an electrostatics problem with boundary conditions. Look at this thread.
https://www.physicsforums.com/showthread.php?t=653837
 
Last edited:
If it's anything but a very simple shape, it's a very hairy problem and needs to be solved numerically.
 

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