Miscellaneous comments concerning electrostatic activities

 

Frank Peterson

1/18/2002

 

1.         It is helpful to have a reliable source of static charge, for charging objects, etc.  For this purpose, we use our locally-designed electrophorus.  With this, one can produce sign of either charge reliably.  When one discharges the electrophorus disk, one hears a noticeable spark.  For example, to charge an electroscope or sphere, the most convenience and reliable method is to charge the electrophorus plate (in the usual way), and then touch this plate to the electroscope or sphere.

 

2.         Our electrophorus plates and most heavily used friction rods are made of gray PVC (for negative charge) or clear acrylic (for positive charge).  To charge these, they are rubbed with dry facial tissue.  To clean finger prints, etc, they are wiped with facial tissue wet with methyl alcohol.

 

3.         One can make quantitative measurements of charge using only a capacitor (10 microfarads is a convenient size - large enough to keep the voltage with the lowest range of the meter) in parallel with a Soar model 540 in the DC autoranging mode.  One side of the capacitor and meter must be grounded.  (We call this setup the "crude coulombmeter".)  Often with this setup, however, one notices erratic behavior, which we now understand and which is entirely avoidable!  The problem is that as the user charges various objects, the body of the user can gain substantial charge and electric potential.  If the user than inadvertently holds or touches the insulated wire (say with ordinary PVC insulation) on the "hot" side of the capacitor (which is easy to do),  charge can flow from the user to the capacitor.  This charge can be of either sign, depending on the electrical state of the user!  Having the user wear a grounding wrist strap eliminates this problem.  We have tried various brands; they usually cost less than $10 and have varying degrees of durability in the face of heavy student use.  These straps typically incorporate a 1 mega-ohm resistor, through which the conducting strap is connected to ground.

 

4.         Using Teflon insulation, the ordinary apparatus of electrostatics functions reliably even in damp weather.  For example, the Braun electroscope (modified to have a Teflon insulator) works in all seasons, as does our electrophorus (with its Teflon handle).

 

5.         For conducting spheres, we mount ping pong balls on 1/4" Teflon rods.  The rod is inserted into the ball and epoxied to the inside of the ball. The ball is sprayed with a graphite paint (Newark #00Z725, EMI-RFI shield coating).  Disadvantages:  1. alcohol damages the coating; 2, the epoxy does not stick to the Teflon indefinitely - these things need repair or replacement periodically!

 

6.         If one uses the painted ping pong balls to measure and study the transfer of charge, often one wants to start with a ball that is totally uncharged.  However, after discharging the ball (say by touching the inside of the Faraday cup of the coulombmeter), one sometimes notices that the ball seems to still have a few nC of charge.  This problem results from charge that resides on the Teflon rod itself near to the ball;  to remove charge from the Teflon, one can "wash" the rod very gently with tissue wet with alcohol.  Usually if I give a rod a quick wash, this removes the charge to the degree necessary, and eliminates the problem.

 

7.         I did come across a nice fairly modern reference for electrostatic phenomena.  My notes list two - I found one particularly good, but I forget which one!  The two are:

            1.  Electrostatics: principles, problems and applications, Jean A. Cross (Hilger, 1987). 

            2.  Fundamentals of applied electrostatics, Joseph M. Crowley (Wiley, 1986)

 

file:  MISC.htm   printed 1/18/02   5:18 PM