If you want a really nice representation of the exponential decay of
an RC circuit connect a square wave generator across the capacitor and
resistor and connect the probes of Vernier's MPLI voltage probe across the
resistor. Open an oscilloscope window in single sweep mode. It will act as
a storage scope. Record a few cycles of the square wave. By adjusting the
sweep rate of the scope and the frequency of the square wave generator you
can get virtually any capacitor-resistor combination to decay substantially
during one cycle from full voltage to almost zero.
This can, of course, be done with an ordinary analog scope too if one
adjusts trigger, sweep rate, and frequency of the square wave appropriately
but these things are much more easily done with the simulated scope. NOW
the versatility of the computer simulated scope comes in. Create a new
window, define a new column as ln(abs(V)), where V is the voltage input
across the resistor, and plot the new column as a function of time. The
result will be a series of straight lines whose slope is equal to -1/RC, one
for each half cycle of the square wave. The student will be able to observe
the exponential decay and the logarithm of the voltage plotted
simultaneously one directly above the other.
If the square wave introduces too many complications you can do the
same thing with a single sweep in which you trigger the simulated scope
properly. You will get a single decay extending across the width of the
screen if you adjust the sweep rate properly. A second window plotting the
ln(abs(V)) as before will yield the same results. There is a little bit of
hassle getting the scope to trigger when you want it and you may have to
choose your values of R and C more carefully but the results are much better
than using a voltmeter.
Jim Riley, Department of Physics
Drury College
Springfield, MO 65802
e-mail: jriley@lib.drury.edu
Phone: (407) 873 7233