Conservation of Charge

charge conservation - triboelectric effect - static electricity

What it shows:
Electricity is never created or destroyed, but only transferred. Rubbing fur and Teflon™ together transfers charge (electrons) from the fur to the Teflon, making the Teflon negatively charged. Conservation of charge requires the fur to become equally and oppositely charged as is demonstrated in this experiment to an accuracy of ≤1%.

How it works:
The difficulty in demonstrating charge conservation quantitatively lies in catching all the charge before it leaks away, the fur being the main problem. This is overcome by enclosing both the Teflon and the fur in metal tubes and simply measuring the voltage (with respect to ground) developed on each of the tubes. Since V = Q/C, care has been taken to ensure that the capacitances of the two tubes are identical (31 pF to ground) so that comparing voltages is indeed equivalent to comparing charges.

The experiment is illustrated above. The metal tubes are 18" (45.7 cm) long, 4" (10.2 cm) in diameter, and are supported by a plastic holder 2" (5.1 cm) above the ground plane (a sheet of 1/16" x 24" x 44" aluminum). The inside of one tube is coated with a thin layer of Teflon 1   while the other tube is all metal -- the two tubes are separated by 2 to 3 cm. A piece of rabbit's fur 2   is rolled up and tied so that it loosely fits inside the tube(s) and can be pulled from one tube to another by a piece of monofilament (fish line). The voltage of each of the tubes is monitored by high impedance electrometers. 3   One starts with the fur in the Teflon tube. Both tubes are momentarily grounded so that their respective voltages are zero. The fur is then slowly pulled from the Teflon tube into the plain tube. The frictional triboelectric effect causes the fur to lose electrons and negatively charge up the Teflon tube. As the fur is pulled, only a couple of centimeters are momentarily exposed to the air as it emerges from the Teflon tube and enters the plain tube. Ordinarily, the fur would rapidly lose its charge due to the large surface area and intense electric fields around each of the hairs. The sea of electrons from the surrounding air quickly neutralizes the positive charge. This loss of charge is prevented by pulling the fur into the plain metal tube which acts as a shield as well as a sea of electrons. Unlike the air, this reservoir of electrons is an insulated system and the change in the overall charge can be measured. The metal tube supplies the electrons lost by the fur and becomes positively charged.

The amount of charge transferred is quite considerable (about 5x10-5 C) and will almost immediately overload the electrometers whose maximum input voltage is 200. This problem is overcome by adding an external capacitance. Identical capacitors are connected between each of the tubes and ground. This not only solves the 200 volt maximum problem, but the additional capacitance makes the experiment much more reliable -- any small leakage of charge from either of the tubes becomes negligible with the increase in capacitance. A 0.047 µF capacitor charges up to about 10 volts. Without the external capacitors in place, the voltages generated would be 1500 times greater. 4   Smaller than 0.047 µF capacitors can of course be used if higher (but ≤ 200) voltages are desirable.

The following measurements can be performed in quick succession. (1) Having produced equal and opposite charges (voltages) on the two tubes, although redundant, one can short the two together to demonstrate that no net charge has been produced -- the voltage will be zero. (2) Perform the experiment again, but this time short either one of the two tubes to ground to show that the charge on the other tube is not affected by this (to dispel any ideas that the equal and opposite charges are somehow being induced by the presence of the other tube). (3) With zero charge on one of the tubes now, short the two tubes together to show that the charge becomes equally distributed between the two -- that is to say, both tubes will read exactly 1/2 the voltage of the charged one and the same polarity. The point here is to prove that they indeed have identical capacitances.

Setting it up:
The experiment can be set up on one of the lecture benches (not being too tall, it won't block the blackboard) or on any one of the larger demo carts. Although the experiment itself is large enough to be easily seen, the voltage readings on the electrometers are not. Position the two electrometers next to each other so that their digital readouts can be displayed simultaneously on a video monitor.

Benjamin Franklin introduced the idea of the conservation of charge. He devised many clever experiments along those lines of thought, but they are (as with most electrostatic experiments) temperamental and success is usually weather dependent. This high-tech version is much more reliable. Rating ****

1 We happen to have found an aluminum tube coated with Teflon on the inside (coating similar to a frying pan). However, Teflon is available in very thin sheet stock which can be secured or glued to the inside of any metal tube.
2 An inexpensive source of rabbit's fur is an old used fur coat. We bought ours for $5.
3 Keithley models 616 and 617 have input impedances >2x1014Ω
4 The voltage can be measured with a calibrated electroscope and indeed is several thousand volts.