Re: Alternating Current

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

I have done this experiment before and I think what you are getting is the
"AC-variation" on the basic clight output of the bulb. Once the bulb heats
up to its average temperature, then as the ac-current varies, the
temperature varies around that average by a small amount. I would guess
that wht you saw was a variation of only a few percent (less than 5%) in
the amount of light emitted over time, and since the light output is
proportional to the power consumed in the bulb, which is proportional to
the square of the current, what you are seeing is not |sin wt|, but (sin
wt)^2.

As to the frequency you measured, I think something is wrong. I doubt the
power company would be that far off, at least for very long. I think that
they work very hard to make sure that they get very close to 60*86400
cycles of their current per day. That way synchronous motors operating
clocks or other timing devices (and record-players, tape drives and
CD-players) will be accurate, at least on he average over a day's time. My
guess is that the clock in the CBL isn't that accurate (after all, that is
only about 3.3% off). I read somewhere that someone was trying to construct
at stopwatch mode for the CBL, but that it was running into problems with
accuracy. This may be the same thing.

Hugh

> 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

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Hugh Haskell
<mailto://haskell@odie.ncssm.edu>

"The box said 'requires Windows 95 or better,' so I bought a Macintosh."
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