Lenz's law can be violated.

[neptune]

@EMdevices .Why can not the ferrite rod be biased by a permanent magnet???

[wayne49s]

I think there is an error in the math which makes the condition for violation more complex. The error is in the derivative of a product:

You have V=-NA d(uH)/dt
u=u₀ur
so V=-NAu₀d(u_rH)/dt

according to http://en.wikipedia.org/wiki/Product_rule
that breaks down to:
V=-NAu₀(u_rdH/dt + Hdu_r/dt)
so the condition you are looking for is:
u_r dH/dt + Hdu_r/dt <0

[wayne49s]

In analysing u_r dH/dt + Hdu_r/dt <0

I just realize it comes to the same interpretation. You want to operate in the negative slope of the u_r curve, and the saturation region where dH/dt is small. In that case, it simplifies to :

Hdu_r/dt <0 if dH/dt is ~0

[onthecuttingedge2010]

http://www.magnet.fsu.edu/education/tutorials/slideshows/eddycurrents/index.html

[EMdevices]

@EMdevices .Why can not the ferrite rod be biased by a permanent magnet???

the ferrite rod can be biased with a permanent magnet, not a problem.

@ wayne

In my derivations I usually do a few step at a time,  are you familiar with the chain rule?       du/dt  = (du/dH) * (dH/dt)     I use it right after the product rule, and then I factor out a dH/dt.

EM