Re: [Phys-l] Ugh - RLC circuit issues

["Josh Gates-fac" <Josh_Gates-fac@nobles.edu>]

Determining the frequency is mostly a pretext for getting them to do a
little creative experimentation.  It doesn't really matter to me if their
prediction isn't accurate, but I want them to be able to test for the
resonant frequency with some success.
jg

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
writes:


> Josh,
>
> Maybe the best way to determine the frequency of the station the students
> are picking up is to listen to the broadcast for their station ID/call
> letters.  Then if they don't say what the frequency is themselves you can
> look it up yourself.  My guess as to what the station is, is that it is
> the one with the strongest signal strength in the vicinity of your
> school.  Try looking outside the school for the nearest AM broadcast
> tower & track down the station using it.
>
> Any attempt to calculate the frequency the set is tuned to from scratch
> will probably not work.  Besides the problem of accurately finding all
> the necessary physical parameters is the problem that the station being
> received is quite possibly not on the set's resonance frequency anyway. 
> Often homemade crystal radio sets have very poor off-resonance signal
> rejection properties, and a strong station with that is off resonance
> response can swamp the signal of a weaker signal that is broadcasting
> closer to the set's supposedly tuned resonance frequency.
>
> David Bowman
>
> ________________________________
>
> From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Josh
> Gates-fac
> Sent: Sat 5/19/2007 11:41 AM
> To: Forum for Physics Educators
> Subject: [Phys-l] Ugh - RLC circuit issues
>
>
>
>  I'm on my last nerve with a couple of student independent lab project
> that won't behave - perhaps the combined wisdom of phys-l can guide me to
> the light at the end of the tunnel!
>
>  #1- they have a crystal radio kit, and they're trying to identify the
> frequency of the radio station that they're picking up.  I suggested that
> they'd get a good answer by determining the inductance of the coil and the
> capacitance of the variable cap at the correct tuning.  For this, they're
> trying to find the L by pairing it with a known C and finding the resonant
> freq., and vice-versa for the C.  The problem is in the measurement; I'm
> getting some inconsistent data.  Just by number of coils, area, and
> length, the L should be in the 300 uH range, but I haven't been able to
> get an experimental value for the inductance anywhere near that.  I'm
> driving the unknown coil, a known electrolytic cap (100-1000 uF range)
> with a ScienceWorkshop freq. generator, and continuously measuring the
> voltage across a resistor in series with the cap and coil, looking for the
> highest voltage difference to find the highest current.  Any ideas?
>
>  #2- another circuits project, a la Mythbusters this time.  They're
> looking for the "break even" time for a fluorescent light bulb
> to make up its high energy start-up cost in low steady-state consumption.
> We trying to measure the current drawn with a PasPort current probe.  The
> max current for these is listed at 1.1 A, and the steady state current for
> the 20W tube should be .167 A, but the start-up current is apparently high
> enough to spark, fry, and let the smoke out of the probe...  I have analog
> meters and a scope as well, but nothing that can capture the I vs t curve
> to integrate. Ideas?
>
> Thanks for the help!
> Joshua Gates
> Stoneleigh-Burnham School
>
>
>
>
>
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