A neutral conductor (or dielectric) experiences a torque, but no net force, when placed in a uniform electric field. It does experience a net force in a non-uniform field.
What it shows:
When an electrically neutral object is suspended in a uniform electric field, it becomes polarized. The electric force on the separated charges produces a torque about the suspension point and the object rotates. There is no translational motion—the object simply aligns itself with the electric field.
When an electrically neutral object is suspended in a non-uniform electric field, it becomes polarized. The electric force on the separated charges produces a torque about the suspension point and the object rotates. However, now there is translational motion in addition to the rotation, and the object moves in the direction of the greater field strength.
Be forewarned that the object not only moves, but may very well come in contact with the H.V. source of the electric field. If it does, it will pick up charge and consequently be strongly repelled and collide with the opposite H.V. source, at which point it will pick up that charge and be repelled back again, and continue to bounce back-and-forth between the two.
How it works:
The electrically neutral object can be a dielectric material or a metallic conductor. We demonstrate both. The dielectric object is a short length of plastic tape (2"x¾"). The conducting object is a small piece of aluminum foil—each is suspended from a silk thread.
The uniform electric field is provided by two parallel plates, each measuring about 14" long x 6" wide. The separation between the plates is about 5." With a 10 kV potential difference between the two plates, the field strength is about 79 kV/m. A non-uniform field is secured by replacing one of the flat plates with a V-shaped plate. The neutral object moves toward the V-shaped plate.
The experiments are performed on an overhead projector and are made visible by shadow projection on a screen placed to either side of the blackboard.
Setting it up:
We use a dedicated cart, complete with overhead projector and power supply. Place the ½"-thick plate of glass on the overhead projector to provide sufficient electrical insulation between the H.V. plates and ground. The H.V. power supply is an Industrial Instruments 10 kV D.C. supply. For safety purposes, it's been modified so that each output (+ and –) is current-limited to 0.1 mA by a 30 MΩ resistor in series with the output. Note that the analog meter indicates the voltage between the (+) and (–) outputs, not the voltage between one of the outputs and ground. Furthermore, even though the maximum voltage reading on the meter is between 6 and 7 kV, the actual output is approximately 10 kV.
The "Coke Can Attraction" demo will complement these experiments.
Some of Prof Eric Mazur's concept questions inspired this demonstration: