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Re: [Phys-L] The Make-Believe World of Real World Physics



On 07/29/2013 07:13 PM, Craft, Peter wrote:
At that top of the page is a graph showing the output on a CRO for a
constantly rotating coil in a particular generator. The question
specifically states the emf is being measured. The graph is a uniform
sine curve with 4 or 5 complete cycles shown. Beneath the graph is a
statement saying that it is impossible to state with certainty that
the output is being produced by an AC generator, that it may be a DC
generator causing the output on the CRO. They are then asked to
justify this statement.

I am putting this up for comment, as my staff disagree about the
validity of the question.

First of all, as I see it, no matter whether you think it is
an AC or DC generator, the question is unrealistic.

In the real world, oscilloscopes are interactive. If you
ever need to know whether this a DC component to the signal,
there are lots of good ways of checking that.

Secondly, the notion of "DC generator" in the context of a
rotating coil is so underspecified as to be meaningless.
If it is a /simple/ system, with just a rotating coil and
a commutator or rectifier, then you get one answer ... but
if it is something more complicated, with perhaps LC filters
or whatnot, then you can get any answer you like. In the
real world, if you didn't know what kind of generator it
was, you would look at the label on the generator; no
oscilloscope needed.

I cannot imagine what the intent of the question might have
been. The rectified output of a rotating coil looks like
|sin(ω)| which looks a little bit similar to sin(2ω) ...
but not enough similar to fool anybody.

=====================

BTW you can't call it a CRO anymore. Oscilloscopes are digital
nowadays. Have been for years and years.
http://www.apogeekits.com/images/oscilloscope_pps10.jpg
http://www.tek.com/sites/tek.com/files/imagecache/related_products/media/image/Tek-tds2024c_02a_h_0.jpg