# Homework Help: Find an expression for the attenuation from the eqn given

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1. Nov 6, 2016

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Having a big issue working through this problem and was wondering if somebody, much smarter than I, could give me a few pointers. Anyway here's the problem:

Given that k2=εμω2-iωμσ

By writing the wave number as k=kr-iki.
Show that ki, which determines attenuation, can be expressed by:
ki=ω(εμ/2)1/2[(1+(σ22ε2))1/2)-1]1/2

I've started by equating real and imaginary parts however I find an impasse where I have a quartic equation.

Just as a quick note, I followed the following process but reached an impasse (may be completely off base here):
k2=εμω2-iωμσ
Substitute k=kr-iki. into the equation which gives;
kr2-2ikrki-ki2=εμω2-iωμσ
Then split into two equations by equating real and imaginary parts;
(1) kr2-ki2=εμω2
(2) εμω2=ωμσ/2ki
Now (2) into (1) gives;
(ωμσ/2ki)2-ki2=εμω2
Then rearranging gives;
ki4+εμω2ki22μ2σ2/4

[EDIT: Realised i'd been a bit of an idiot and missed what should have been quite obvious, thank you for pointing it out!]

Last edited: Nov 6, 2016
2. Nov 6, 2016

### Dr Transport

it is a quadratic... set $k_{i} = \alpha$ and us the quadratic equation for $\alpha$ and take the square root.....

3. Nov 6, 2016