- #1

KEØM

- 68

- 0

## Homework Statement

We consider the following configuration. A pile of N layers of sediments. Each layer has a conductivity [tex]\sigma_{i}[/tex] (i from 1 to N) and a thickness [tex]m_{i}[/tex]. The length of the system is L. In this case, the electrical conductivity is not a scalar but a second order symmetric tensor.

Using Ohm's Law and the idea of resistors working in parallel, demonstrate that the effective conductivity of the block along the x-direction is:

[tex]\sigma_{x} = \frac{\Sigma_{i=1}^{N} m_{i}\sigma_{i}}{\Sigma_{i=1}^{N} m_{i}}

[/tex]

## Homework Equations

[tex]\vec{j} = -\underline{\sigma} \vec{\nabla} V,

\underline{\sigma} = \begin{bmatrix}

\sigma_{x} & 0 \\

0 & \sigma_{z}

\end{bmatrix}}

[/tex]

## The Attempt at a Solution

I am not exactly sure if this qualifies as advanced physics. This is a geophysics course and I have never seen this before (not even in the course).

The first thing I did was I used the product over sum formula for two parallel resistors.

[tex] R_{eq} = \frac{R_{1} R_{2}}{R_{1} + R_{2}}[/tex]

Now using the formula [tex] R = \frac{\rho}{g}[/tex]

where [tex]\rho[/tex] is the resistivity of the specific layer and g is the geometrical factor (I don't know what this is in this case and I don't know how to use it).

Substituting this in for R and using [tex] \rho = \frac{1}{\sigma} [/tex] this parallel equation, after a lot of algebra, becomes:

[tex] R_{eq} = \frac{1}{g_{1} \sigma_{1} + g_{2}\sigma_{2}}[/tex]

Now, can I set [tex]R_{eq} = \frac{1}{\sigma_{x} g_{eq}} , g_{eq} = g_{1} + g_{2}?[/tex]

And then can I go as far as setting g equal to m?

If that is the case then I will have the correct answer because,

[tex]\frac{1}{\sigma_{x}(m_{1} + m_{2})} = \frac{1}{m_{1} \sigma_{1} + m_{2}\sigma_{2}}

\Rightarrow \sigma_{x} = \frac{m_{1} \sigma_{1} + m_{2}\sigma_{2}}{m_{1} + m_{2}}.

[/tex]

Which is the formula for the specific case of N =2.

Could someone please verify my approach or let me know how horribly wrong I am?

Thanks in advance,

KEØM

Last edited: