Coaxial Electrostatic Shielding Apparatus

Description

The apparatus shown above consists of a ~30 cm length of clean PVC pipe with an insulating end cap on one end and a removable insulating handle on the other end. A thin aluminum tube, somewhat less that half the length of the PVC pipe and held captive by the end cap and the handle, slides freely over the pipe. Refer to the sketch below for a side view of the apparatus. The handle can be removed to replace the aluminum tube with tubes make of other materials such as wire mesh. The demonstration is performed by using a wool or Nylon^TM cloth to charge the exposed end of the PVC pipe frictionally and then tipping it so that the aluminum tube slides down to cover the charged end of the pipe. This apparatus, which can be operated with one hand, is used in conjunction with the TESV to demonstrate basic principles of electrostatic shielding, charge induction, and grounding.

The list of instructions provided below is basic and can be changed or expanded to conduct a variety of experiments and demonstrations.

Basic instructions

1. Make sure your hands are dry, then grasp the apparatus by its handle and tip it so that the aluminum tube slides all the way to the handle.
2. After neutralizing any initial charge with the ionized air blower, charge the exposed end of the PVC pipe by rubbing it vigorously with a wool or Nylon^TM cloth.
3. Verify good charging of the PVC pipe using the TESV. PVC and most other plastics usually charges negatively against most materials.
4. While continuously monitoring the charge with the TESV, tip the apparatus so that the conductive tube slides down and covers the charged end. Note that the conductive tube has little effect on the externally observed field.
5. Momentarily ground the sliding aluminum tube with a ground wire (or touch it with your hand), and then use the TESV to measure the field again. The reading now drops close to zero.
6. Tip the apparatus back so that the aluminum tube slides back to the handle. Check the charge both at the charged end of the pipe and on the aluminum tube.

What this important exercise demonstrates is that the electrostatic charge shielding ability of a conducting vessel enclosing charged material depends on the vessel being connected to ground. If the vessel is not grounded, no shielding is achieved. The demonstration also teaches us that the shielding vessel itself will have a net charge.

CLICK HERE to view an interactive, animated version of this demonstration that reveals the movement of charge as the steps of the demonstration are followed. Please be patient while the Java script loads!

A very understandable explanation of the behavior of electric charges both outside and inside a Faraday pail is provided by Bill Beaty. By studying the sequence of diagrams on his page, you will gain a better understanding of how the induced surface charge in a conductor redistributes itself as a nearby charged object is moved about.

Further demonstrations

7. Replace the aluminum tube with a same-sized tube made of wire mesh. Repeat the previous steps to show that grounded, conductive wire screen or mesh is equally effective in shielding the charge as long as it is grounded. Practical and inexpensive electrostatic shielding is usually achieved using metallic screening in place of heavier and more expensive solid sheet.
8. Repeat the experiment with another tube made from a piece of fabric from a conductive flexible intermediate bulk container (FIBC). The same shielding effect is observed despite the fact that the conducting threads are widely spaced in the fabric.

Hint: The wire mesh tube must be free of all sharp points and burrs in order to avoid inducing a corona which can discharge the PVC pipe as the tube slides over it.

Electrostatic shielding can be demonstrated with the antistatic bags used to protect sensitive electronic chips and assembled boards. Click here to see this demonstration.

A dramatic example of electrostatic shielding is provided when lightning strikes an airplane in flight. Such lightning strikes are rather more common than might be thought. Usually, the plane is not damaged and the people inside do not feel any effect at all. To understand why, think more about electrostatic shielding.

Library references

H.A. Haus and J.R. Melcher, Electromagnetic fields and energy," (Prentice-Hall, Englewood Cliffs, NJ), 1989, pp. 27-28.