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Re: Alternating Current



Hi all-
As others have pointed out, David is sampling the power,
so that his "expected result" is 120 HZ, not 60 Hz. Now let's understand
why he got 124 Hz instead of 120, given that the power company makes
clocks run on time. Following Brian's comment, let's estimate the
uncertainty in David's result.
David used a sampling interval of 10^-4 s, and took 100 data points.
He therefore sampled over a time interval of 10^-2 s. What was the precision
of his result? Aha, the uncertainty principle rears her pretty head:
(delta t)*(delta f) = 1
But delta t = 10^-2 s (according to David's message) so delta f can be
no better than 100 Hz. How can this be if David sees 124 Hz with good
precision?
It can't. Either David's precision is much worse than he thinks
or there was a typo in his report of a sampling interval. If the sampling
interval was 10^-3 instead of 10^-4, then he sampled over a time interval
of 0.1 s, and recorded about 8 cycles. His uncertainty was then ~10 Hz,
and 124 Hz is a reasonable estimate of the "expected value" of 120 Hz.
Regards,
Jack
********************************************************************
I just did a lab today with my Pre Calculus class. Since we have
introduced sinusoidal variation I had them use a CBL to look at the
light intensity from an incandescent bulb sampling every .0001 seconds
for 100 data points.

When one does this, the calculator shows a beautiful sinusoidal graph
and the regression analysis shows an excellent fit with supporting
residuals.

However, I would have expected to see a graph more like |sint| which
should not look like a sin but should have the frequency of double the
line voltage frequency since each half of the voltage cycle produces a
full cycle of light intensity. The frequency came out consistently at
124Hz by the way and I wonder if the power company is typically this far
off their standard value of 2*60Hz?

Can anyone help me with this? Am I really seeing |sint| and it just
looks like sint? Has anyone else tried this?

Thanks for the help.
--
David Abineri dabineri@choice.net
******************************************

"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography