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