FEM induced by a moving magnet... so easy?

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I am trying the following experiment:

I would like to estimate the fem produced by falling magnet (at
high=d=70mm) through a tube with a coil at its end.

I use cylinder neodium magnet with remanence of 12T (Bo), radius=6mm,
h=10mm.
I made a coil (area=A=7.5mm, N number of coils=250). Could you please
check if what I am doing is correct?


t0=sqr(2d/g)=0.12s time when the magnet reaches the coil.

B=Bo t/t0 [T] Equation of the magnetic field. B is 0 when the
magnet is outside the coil and it is equal to Bo when it is inside the
coil (at t0)

Phi=NAB=0.053 [Wb] flux of the magnetic field inside the coil

Fem=-dPhi/dt= -NABo/to=-0.43V

Using a scope, the max fem voltage results of -1.2V instead of -0.43.
Am I doing something wrong?

I am not confident about the strength of the magnetic field, since I
am using a small cylinder, I don't know if the B equation is correct.

I looked at the table of the neodium and I found the remanence value
=1.2T. Can I assume this value equal to the magnetic field B of the
magnet?



Could you please help me out?
Thanks in advance
Nick

 

[Rich Grise]

.

Two fun things to try:

1. Drop a supermagnet down a vertical section of copper water pipe.

2. Drop a supermagnet through a coil, a couple hundred turns maybe,
connected to an LED.

I once dropped a quarter down the vertical bore of a megabuck
superconductive magnet, just to see how long it would take to come out
the bottom. It didn't come out... it got stuck on a flange inside.
There was some consternation.

Quarters these days have a bunch of nickel in them. You should have
dropped a penny or a little chunk of Al.

Cheers!
Rich