I am reading this book, the part in particular about Eddy-current separation starting at Page 246, in it there is an equation for calculating the "characteristic time with which the induced magnetic field decays in the particle".

$$ \tau = \mu_0\sigma_psb^2 $$


  • $\mu_0$ is the permeability of free space
  • $\sigma_p$ is the specific electric conductivity of a particle [S/m]
  • $s$ is the shape factor of the particle
  • $b$ is the particle radius

I am hoping someone can reflect some light on what $s$ is and how to calculate it, "shape factor" isn't very descriptive and I cannot find anywhere in the book describing it in further detail.

This shape factor is also used in the force equations that I am intersted in...

Another part of the equation I am unclear on is $b$, what if the particle is a cube or cuboid, what should the radius be then?

  • $\begingroup$ After reading through several papers it appears $b$ is the largest diameter that would fit in the projected shape. I am still unsure about $s$, from what I have read perhaps it is likening the shape to a sphere or cylinder. $\endgroup$ – Lerp Dec 16 '13 at 15:02

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