Experiment of The Month

The MU Physics Department does not claim to have invented these labs. The origin of these labs is currently unknown to us. Our labs do not have written instructions. In keeping with this spirit, the description given here will be brief and general. The intent is that each performance of the lab will be unique; in each nature will reveal a slightly different face to the observer.

This is actually done at MU as a demonstration of a capacitor made by slipping aluminum foil sheets between the pages of a book. If it were done as a laboratory exercise, we would compare the measured and calculated values for the capacitance. In the end, we might calculate the dielectric constant of the paper in the book.

Two sheets of aluminum foil are placed in the physics textbook, with one sheet of paper between them. The book is closed, and a signal generator and inductor are series-connected as shown. Alligator clips make contact to the foil.

The capacitance is calculated from the parallel plate formula, assuming a dielectric constant of about 1 for the paper. The resonant frequency is calculated for the capacitor in series with the 1 Henry inductor. Starting at low frequency, the signal generator frequency is increased towards the resonant frequency. The signal generator output is approximately 5 volts.

Students have studied resonant circuits, phase and have observed the high voltages that can occur at resonance in an RLC series circuit. Their voltage and current measurements are not repeated in the demonstration. Instead, a more direct indication of the resonant high voltage is used.

The resistance of the inductor we use is about 50 Ohms, and at resonance this implies a maximum potential across the capacitor of about 1000 Volts. This turns out to be enough to force the two "plates" closer together when the potential is maximum. The periodic flexing of the plates makes an audible, if unpleasant, tone near resonance. The tone seems enhanced if some of the foil is allowed to hang out of the book, as in the sketch.

The high voltage may be verified, Benjamin Franklin style, by gently touching a knuckle to the foil.