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[Phys-l] inductance measurement



Pasco makes some coils of various number of turns for the intro lab.
I have a bunch that are 3200 turns. I measured each of their
inductances (using a handheld multimeter) and find they are all 0.17
H (with air core).

If we estimate L using mu0*n^2*volume we get an answer about a factor
of 2 larger. That is not surprising because the coils are roughly
cubical, about 4 cm on a side. We know that the field at the end of
an ideal solenoid is half that in the middle. So measuring an L
smaller than what is predicted for an ideal solenoid is reasonable.

So I thought why not put a bunch of them end-to-end in series to
decrease flux loss at the ends? But it doesn't work out the way I
expected.

For just two of them, if I connect them in series when they are
widely separated, I measure 0.31 H for the pair. WHY DIDN'T I GET
DOUBLE ONE OF THEM, NAMELY 0.34 H?

If I now push them close together, I get 0.34 H. Okay, I expected L
to go up, since I'm now capturing some of the end flux. Still, they
only went up to double one of them, whereas I expected to now get
more than double one of them alone (so that they would closer to the
ideal prediction).

Can you help me understand the answer to the question in caps above?
Maybe if I understand that, I'll understand why my original thought
does not work.

Thanks, Carl
--
Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/
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