5A10.35 CONSERVATION OF CHARGE

PURPOSE: To show that the total amount of charge is conserved when charges are separated by friction.

DESCRIPTION: Electric charge is neither created nor destroyed when one performs charging by friction.  For example, rubbing fur and Teflon together will cause a movement of electrons from the fur onto the Teflon, making the Teflon negative and the fur positive.  Both the fur and the Teflon will hold equal amounts of charge, but with opposite sign because no new charges were created or destroyed by rubbing.  The photo above shows the demo in the class.

To perform this demo, use two aluminum cylinders that are each connected to separate electrometers.  One cylinder is coated on the inside with Teflon.  The other cylinder is uncoated aluminum.  A fur is pulled through the cylinders by a string.  The fur will rub the Teflon coated tube and produce a net excess of electrons on that tube.  The electrons are seen as a negative voltage reading on an electrometer.  The fur crosses a small gap between the cylinders and is pulled into the uncoated aluminum cylinder.  The transfer of positive charge from the fur is distributed to that cylinder, and the amount of charge is indicated as a positive voltage reading by a second electrometer. 

The two voltages are shown to be equal in magnitude but opposite in sign, indicating that the amount of negative charge equals the amount of positive charge, and the total net charge is conserved.  Another beautiful experiment is to ask what will happen if one of the now charged tubes is discharged to ground and then connected to the other, charged, tube.  Will it show zero?  Will it show equal and opposite?  Will it show half the charge with the same sign?

Answer: It shows half the charge with the same sign.  If the negatively charged tube is connected, it will show half the negative charge at both cylinders.  If the positively charged tube is connected it will show half of the positive charge across both cylinders.  A very nice demo, indeed.

Note: The conclusion is based on the premise that the capacitances of the two cylinders are identical.  The students are not expected to know what a capacitor is, but matched capacitors are installed in each cylinder.  The students are not expected to be informed about this matter nor do they need to understand why this fact is true.

REFERENCE: AJP 63(1) pp.90, apparatus and demonstration notes "Demonstration of Charge Conservation" by Wolfgang Rueckner. Jan. 1995.  A special note: This article uses two Teflon coated tubes.  We found that we could not easily obtain Teflon coated tubes.  So instead we constructed them using two identical aluminum tubes and a thin sheet of Teflon (available through McMaster-Carr).   We carefully adhered the Teflon to the inside of one of the aluminum tubes using double sided tape.

EQUIPMENT: Two cylinders with rabbit fur and string, ground plane, shorting prong, two electrometers connected as photographed below, Computer with science workshop.

SETUP TIME: 1 to 2 hours depending on the humidity.