Electrophorus

Introduction

The electrophorus device was invented by Johannes Wilcke and perfected by Alessandro Volta over two hundred years ago. This device was quickly adopted by physical scientists because it filled the need for a reliable and easy-to-use source of charge and voltage for experimental research in electrostatics.

Concepts

• Static charge
• Induction
• Electrons
• Insulator
• Conduction

Materials

Safety Precautions

Be alert to the potential dangers related to electrostatic shocks. Be sure to practice electrostatic demonstrations prior to performing demonstrations for students. Follow all other normal laboratory safety rules.

Prelab Preparation

1. Screw the insulated handle into the threaded nut on the top of the electrophorus disk as shown in Figure 1.
   {13924_Preparation_Figure_1_Assemble electrophorus disk}
2. Place one self-adhesive foot pad in each corner of the insulating acrylic sheet. Rest the insulating sheet on its foot pads on a flat surface to use as a charging base for the electroscope.
3. Obtain charging materials and other electrostatic equipment necessary for your anticipated electrostatic demonstrations.

Procedure

1. To begin, a charge must be generated on the insulating plate. This is done by using friction and rubbing the plate with materials such as wool, silk, fur or rubber. Note: Depending upon the material used, the resulting charge can be either positive or negative. See Table 1 (in the Discussion section) for a list of common materials and their typical charges.
2. To charge by conduction, the neutral electrophorus disk is brought in contact with the charged insulating plate. The disk will then become charged with the same charge as the plate.
3. To charge by induction, bring the electrophorus near the charged plate, but not touching. The charges in the disk will separate on either side of the disk as illustrated in Figure 2. Opposites attract so the disk charge aligns opposite to the plate charge.
   {13924_Procedure_Figure_2}
4. Ground the disk by touching the top of the disk with a finger while lifting the disk by the insulated handle (see Figure 3). Touching the disk causes the electrons to travel to the ground, leaving the disk with a positive net charge.
   {13924_Procedure_Figure_3}
5. The electrophorus disk is now charged with the opposite charge as the plate and can be used as an effective transfer wand in your electrostatic demonstrations.
6. Once charged, the insulating plate will stay charged for several additional charges of the disk as needed.

Teacher Tips

• Try charging the electrophorus with different materials to obtain different charges and charge intensities. The disk may be charged either by induction or conduction.
• The electrophorus disk is especially effective with a condensing electroscope (one with a flat disk mounted on top for receiving charges). When the two flat disks are brought into close proximity, the increased surface area can cause dramatic results in the electroscope.
• Static electricity experiments always work best on a dry day. Since water is a polar molecule, it can easily drain off an excess of charge making lower humidity days better than high humidity days. Air-conditoned air or heated winter air tends to be drier and more conducive for electrostatic demonstrations.
• Be sure to rub the insulating plate rapidly for at least 15 seconds in order to obtain a good charge on the plate. After continuous use, items may become permanently charged. It may be necessary to ground them occasionally to return them to their neutral state.
• Recommended negatively charged test objects: plastic Beral-type pipets, plastic straws, rubber balloons, and PVC pipes make excellent negatively charged items when rubbed with wool, flannel or fur.
• Recommended positively charged test objects: Lucite^® friction rods, glass friction rods, glass stirring rods and curled-up overhead transparency sheets (acetate) create positively charged items when rubbed with wool or silk.

Discussion

Static electricity is different than current electricity. Static electricity is produced by physically pushing electrons from one place to another. This causes a temporary, uneven distribution of electrons over an object. The tendency is for an object to return to its neutral state and thus electrons tend to rearrange back to a neutral state. This may occur rapidly (as in a spark) or by a slow leaking of electrons from the object to the air or to other nearby objects. In contrast, current electricity is created by using a magnetic field to continuously force electrons to flow over a conductor.

Static electricity is created by mechanically moving electrons from one place to another. If some material has mobile electrons, they will, in general, be evenly distributed on the surface along with positive charges in such a way that the overall charge of the object is zero (the object is neutral). By rubbing two such materials together, however, it is possible to mechanically redistribute the electrons in an uneven way. At the location where the electrons are concentrated the object will have a negative charge. If the two objects have different amounts of free electrons, some of the electrons from the object with more electrons will be transferred to the other object. As long as the two objects are in contact, the overall charge will remain neutral. If, however, the two objects are separated, one will have more electrons than it started with and the other will have less. Thus, one object will have an overall negative charge (more electrons) and the other will have a positive charge (less electrons). It is this “charging” phenomenon with different materials that serves as the basis for electrostatic demonstrations.

Static electricity is a stationary electric charge. Atoms are composed of electrically charged particles: positively charged protons, negatively charged electrons, and neutrons which carry no charge. The positive and negative charges of protons and electrons, respectively, are equal in magnitude, so the combination of one proton and one electron results in an electrically neutral atom (a hydrogen atom). Generally speaking, most objects have an equal number of protons and electrons and are therefore considered electrically neutral. Since protons form the dense inner core of atoms, they are not able to move about freely within an object. Therefore, the positive charge in an object remains reasonably constant. Electrons, on the other hand, are not held in place by rigid bonds. The electrostatic attraction between electrons and protons keep the electrons moving closely around the nucleus, but the electrons are generally not “locked” into position. Electrons have the ability to migrate throughout a material, and therefore are referred to as being delocalized. Electrons can also be removed from an object leaving the object positively charged, or added to an object to give the object an excess negative charge. The ease with which the electrons in a material can do this depends on the atomic composition of the material.

A substance may acquire static-electric charge through contact with a different type of substance. When two different substances are rubbed across each other, frictional energy may be enough to remove a few electrons from an “electron-releasing” material and transfer them to an “electron-holding” material. When this happens, both substances become static-electrically charged. The material that loses electrons becomes positively charged and the material that gains electrons becomes negatively charged. The ability of one substance to hold onto electrons better than another when two different objects are rubbed together reflects differences in the atomic composition of different materials. Certain atoms give up electrons easily, while other substances hold on to electrons tightly. Typically, in the electrostatic sense, metals tend to hold on to their electrons more tightly than nonmetals. A list of the relative electron “holding” and “releasing” abilities of different common materials is shown in Table 1.

If any two substances in Table 1 are rubbed together, the substance that is higher in the table will become negatively charged, while the material lower in the table will become positively charged. As an example, when rubber-soled shoes (Ebonite—a form of hard rubber) are rubbed along the carpet (wool), the rubber-soled shoes will retain and collect excess electrons from the carpet. As a result, the shoes (and you) become negatively charged and the carpet becomes positively charged. The electric shock you then receive when you grab a doorknob is the result of the surplus of electrons that have accumulated and redistributed throughout your body “jumping” toward the positively grounded doorknob in order to reestablish a charge balance.