PHYS 126/151 Electrostatics Lab Notes

by Jim Hetrick (last updated on Sept. 2, 2022).

Our electrostatics experiments are temperamental. This is a collection of tips:

Cabling

I find better results when there is no ground connection between the electroscope and the high voltage supply.  When the ground patch cord is connected, and the high voltage is on, I find that the electroscope reading slowly drifts upward.  I suspect there is a small leakage current somewhere that is causing charge to slowly build up.

Therefore, even though the lab handout shows the ground connection, I recommend not using it.

Grounding and Zeroing

The Faraday cage and electroscope should be zeroed and "grounded" regularly.  I recommend these steps:

A.        First, the user should make sure to be fully discharged by grabbing the metal end (alligator clip) of the short red patch cord connected to the COM (ground) terminal of the high voltage supply.  This will discharge the user if the user had picked up any static charge by moving about.  (This is the only purpose of the red patch cord. It is not to be connected to anything.  It simply provides a convenient way to ground the user.)  Like this:

B.        Then, immediately after being discharged, the user should grasp both "cages" to put them at the same potential, and while still grasping them, should press the zero button on the electroscope.

Holding the Paddles

The user should only hold (and touch) the three paddles (blue, white, silver) on the segment of the black plastic shaft that is between the two white segments. Like this:

The white segments are highly insulating spacers and should not be touched. If the white segments are touched, oils from the hand and fingers can be transferred to the surface.  This surface residue can create a conductive path that can discharge the paddles via a small surface current.  It is best to leave the white segments clean and untouched.

Neutralizing the Paddles

To neutralize a charged paddle, simply touch the back side of the paddle to the outer "cage," like this:

This works for all three types of paddles. The white, blue, and silver pads adhere to a conductive black plastic.  When this black plastic backing is touched to the outer cage (or the ground cable) it will neutralize the paddle.

Rejuvenating a Paddle

When the blue and white paddles are swiped lightly across each other, the blue paddle should become negatively charged, and the white paddle positively charged, by an equal amount.  I was consistently getting readings of plus-and-minus 8 V on the electroscopes (on the 10 V range), and sometimes more.

But often you don't get any charging at all, or some other strange result. In that case, the paddle surfaces probably need to be "rejuvenated."  Here are two techniques. One or the other, or both in combination, usually work.

A.        Use the hair dryer to heat the paddles for about 15 seconds on high heat. This should drive away any moisture on the surface. This helps (sometimes).

B.        Vigorously rub the white paddle against the "silver" (aluminum) paddle in a circular motion.  Then do the same for the blue paddle.  This helps (sometimes). (Not sure why. Surfaces are messy places.)

Charge Transfer

When the blue and white paddles are cooperating, it doesn't take much to transfer charge.  I found that even a single very light swipe, one against the other, can produce significant charge transfer. Vigorous rubbing should not be necessary.  If the user isn't getting good results with a light swipe, maybe one of the "rejuvenation" techniques should be attempted.