Coaxial Electrostatic Field Collapse Apparatus

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Description

The apparatus shown in the photo above (with the tripod-mounted TESV) consists of a ~30 cm length of clean PVC pipe with an insulating end cap on one end and an insulating handle on the other end. An aluminum tube insert, somewhat less that half the length of the PVC pipe, fits inside the pipe and, held captive by the end cap and handle, slides freely within. In the sketch below, the PVC pipe is shown in half-sectional view so as to reveal the insert. Note that two holes, evident in the photograph, are cut in the PVC pipe so that electrical contact can be made with the insert when it is located at either end of the pipe.

The demonstration is performed by using a cloth to charge one end of the PVC pipe frictionally and then tipping it so that the insert slides down to the charged end. This apparatus, which can be operated with one hand, is used in conjunction with the TESV to demonstrate the basic principle of electrostatic field collapse.

The basic instructions provided below can be considerably expanded in order to explore other related electrostatic phenomena.

Basic instructions

1. Tip the apparatus to slide the insert all the way to the handle end of the PVC pipe, then discharge the PVC pipe using the ionizing air blower.
2. Now charge the PVC pipe by rubbing the far end of the tube vigorously with a clean cotton, wool, or Nylon^TM cloth.
3. Verify good charging of the PVC pipe using the TESV. It is very convenient to mount the TESV on a tripod so that the hands are free.
4. Tip the apparatus so that the conductive metal insert slides down inside to the charged end and then again measure the charge level. Usually, the TESV will indicate only a small reduction in the net charge outside the PVC pipe where the conduction insert is located.
5. Momentarily ground the conductive insert through the access hole in the PVC pipe, either with a ground wire or with your hand. The insert usually draws an audible spark and the TESV indicates that the external electric field has collapsed to near zero.
6. Now tip the pipe back so that the insert slides back toward the handle and then use the TESV to confirm that the tube has retained its negative charge. Though the conductor gains net positive charge by induction, this is difficult to confirm because surface charge on the PVC pipe surrounding it interferes with measurement.

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Practical examples

One of the most important examples of field collapse is provided by a charged, moving web when it passes over a grounded roller, as shown in the figure below. What happens here is that charge, of electrical sign opposite to the web, flows to the roller surface and draws lines of electric flux, thereby "collapsing" the electric field. As a consequence, any charge on the web is almost undetectable in this region. For this reason, non-contacting electrostatic voltmeters intended to measure the surface charge on a photoreceptor in a xerographic copy machine should not be positioned near rollers or any other conductor backing the web.

Another practical lesson for engineers is that a static neutralizer bar placed in this region will be ineffective [Electrostatics, Inc., 1997]. Neutralizer bars rely on the field of the web charge itself to guide ions to the surface. For some passive neutralizers, the field actually induces corona of the appropriate electrical sign. In either case, with no field present, web neutralizers become ineffective. Refer to the figure below which depicts a typical web system with static neutralizing bars located in both proper and ineffective places.

Library references

J.A. Cross, Electrostatics: principles, problems, and applications, (Adam-Hilger: Bristol, UK) 1987, pg. 370.

Electrostatics, Inc., "Before and after," Product newsletter, Harleysville, PA, June, 1997.