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Re: your mail



Power in equals power out.

On Wed, 14 Apr 1999, Digby Willard wrote:

To: PHYS-L@LISTS.NAU.EDU
Subject: Speaking of transformers...
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I've got some points of confusion on transformers, which I'm about to
teach.

Vp/Vs=Np/Ns The ratio of the emfs is equal to the ratio of the numbers of
turns. No real problem for me unless I think about it too much...generally
not a danger for my students at this time of year.

But then there's current to discuss. Most of the books I've seen just
kinda zip past it. My understanding is that the Ip/Is=Ns/Np, where Is is
the MAXIMUM current output of the secondary, and that the secondary current
could be much less than that predicted by this ratio, depending on the
resistance in the secondary circuit. But some texts work from the
assumption that Ip/Is = Ns/Np always, regardless of the resistance in the
secondary circuit. Which is it, and why?

Second question: When dealing with electromagnetic induction in general,
we speak of "induced emf" instead of "induced voltage" or "induced
potential difference". Why?

Digby Willard
IB/AP Physics Teacher
Central High School
St. Paul, MN