How does shielding work? Is it a two-way street and work both ways? Can electric fields not penetrate metals? What's going on? This sequence of demonstrations addresses these questions.
What it shows:
(1) Shielding the inside from the outside. It is well known that no electric fields exist inside a hollow conductor, even if there are charges present outside. The conductor acts like an electrostatic shield. This is only true if the conductor is kept at a constant potential. Indeed, assuming electrostatic equilibrium and the concept of equipotential surface, one can argue by contradiction that there cannot be an electric field inside. Even though Gauss' law proves that it must be so, the nuances prevent many students from appreciating what's going on. Using a "segmented shield," one can demonstrate that electrostatic shielding doesn't work when the potential is not constant. When it is constant, the shielding effect arises from superposition of the field from the outside charge distribution and the opposing "back-field" of the hollow conductor. The "back-field" is directly observable in this demonstration.
(2) Shielding the outside from the inside. A charge inside a hollow conductor produces a charge distribution on the outer surface of the conductor, and this induced charge distribution creates an electric field outside the closed conductor. Again, Gauss' law tells us it must be so. A test charge (probe) positioned outside the conductor seems to be repelled by the charge inside the conductor no differently whether or not the conducting shield is there. Thus, electrostatic shielding does not work both ways. (Note that the field outside will depend on the shape of the shield and not in any way reflect the internal charge distribution. For example, if the shield is a metal sphere, the field outside will be Coulomb-like and radial. Not so if it's a box.) However, shielding the outside can be accomplished by grounding the conductor. This allows charges to flow (from ground) onto the conductor, producing an electric field opposite to that of the charge inside the hollow conductor. The conductor then acts like an electrostatic shield as a result of the superposition of the two fields.
(3) Shielding with non-metallic enclosures. One can secure electrostatic shielding effects even with insulating materials such as paper and/or cardboard. Given time, surface charges will migrate and rearrange themselves under the influence of outside charges. The superposition of the fields due to the surface charges and the outside charges leads to zero field inside a cardboard enclosure. Again, the "back-field" is directly observable in this demonstration. Unlike metallic enclosures, where the shielding effect is essentially instantaneous, the time scale for cardboard enclosures is in the 15 to 30 second range. For comparison purposes, rubbing the cardboard surface with a graphite pencil dramatically shortens the time scale.
How it works:
The electrostatic detection probe in all of these experiments is either a tiny charged pith ball or an electrostatic "compass," both of which are suspended from a silk thread. The compass is an electric dipole shaped like an arrow to indicate the direction of the E-field.
The experiments are performed on an overhead projector and are made visible by shadow projection on a screen. Alternatively, video projection could be used if desired.
(1) Shielding the inside from the outside:
A cylindrical metal can (without top and bottom, for viewing purposes) serves as the shield. An electrostatic compass hanging in the middle of the cylinder from a silk thread serves as the E-field detector. When a charged object is brought near the can, the compass does not respond and will point in random directions. This behavior is the same regardless of whether or not the metal shield is grounded; the shield is an equipotential surface with or without a ground. A cylindrical wire mesh also works as well. The cylindrical shields are approximately 5" tall and 5" in diameter.
The "segmented shield" consists of twelve 1"-wide by 6"-long copper strips. These are electrically insulated from each other and arranged to create a 4"-diameter cylinder. An electrostatic compass hanging in the middle of the cylinder from a silk thread serves as the E-field detector. If one brings a charged object near the cylinder, the electrostatic compass inside the cylinder is not shielded from this charged object and points toward it. This is because the copper strips that make up the cylinder are not at the same potential and simply get polarized in the presence of the charged object. However, if one grounds all of the copper strips (either by touching them all with your hands, or with a wire), then the electrostatic compass becomes shielded and no longer points at the charged object outside the cylinder. The shielding effect arises from superposition of the field from the outside charge distribution and the opposing charge distribution that was induced on the copper strips. This induced charge distribution, or "back-field," can be "frozen in place" by removing the wire interconnecting the strips. When this is done, the field inside is still the combination of the outside charged object and the charge distribution on the copper strips, and is equal to zero. Now the "back field" can be made directly observable by simply removing the charged object, whereupon the electrostatic compass once again "sees" an electric field and points towards it. But now it is in the opposite direction from where the charged object used to be! In other words, inside the metal cylinder, the field due to the charges on the cylinder is the negative of the field due to the charged object outside the cylinder. It is the vector sum of these two fields that is equal to zero inside the cylinder.
The above discussion is summarized in the following video:
Setting it up:
We use a dedicated cart complete with overhead projector. The image is shadow projected onto a screen set up on either side of the front blackboard.
This sequence of demonstrations was conceived by Prof Peter Heller at Brandeis University and developed in collaboration with him.