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The information below is intended to provide introductory material for elementary students and further material for high school students.

Please keep in mind that not all demonstrations are presented at each show, and each topic may not be covered.


ELECTROSTATICS

Introduction

  • There are two types of charges: positive and negative. Like charges repel and opposite charges attract.

    There exist in nature two types of charges: positive and negative. The force between them is such that like charges repel and opposite charges attract. For example, two positive charges brought near each other will be pushed away from each other. On the other hand, one positive charge and one negative charge brought near each other will be pulled towards each other. It is this principle on which all electrostatics is based.


  • In general, a material is either a conductor or an insulator. A conductor allows electric charge to travel through it easily; an insulator does not.

    Each type of material has a different arrangement of atoms, electrons, and protons. The particular arrangement of a material's atoms can be very advantageous for electric charge to travel through it. Metals, for instance, are very good conductors. Charges can flow through a metal with very little resistance. The particular arrangement of a material's atoms can by disadvantageous, however. Rubber, for instance, has its atoms arranged such that it is very difficult for electric charge to flow through it. Rubber is considered to be an insulator.


  • A person's body acts as a conductor.

    Surprisingly, a person's body allows electric charge to travel through it easily. It is for this reason that one must be careful not to plug in the radio while in the bathtub: electric charges from the outlet can run through your body, electrocuting you.


  • When certain types of materials are rubbed against other certain types, charge may be transferred from one to the other.

    Each object contains charge. For most objects, the number of negative charges equals the number of positive charges, giving a net charge of zero. These charges are free to move, however. When certain types of materials are rubbed against each other, the material's atoms are arranged such that some of the charges from one material will be transferred to the other material. This gives both objects a net charge. One object will be positively charged, the other will be negatively charged. For example, when a teflon rod is rubbed against silk, charge is transferred.


  • When an uncharged object is placed near a charged object its charges rearrange themselves. Those charges attracted to the charged object move towards the charged object and those charges repelled move away. This effect is known as polarization.

    As discussed before, most objects are uncharged. An uncharged object means that the total number of positive charges equals the total number of negative charges. The net charge on the object is zero. When an uncharged object is polarized as described above, its net charge remains zero. No additional charge is put on the object. What does happen, however, is the charges inside the object rearrange themselves. When a negatively charged object, for instance, is placed near an uncharged object, the negative charges move away from the charged object and the positive charges move towards the charged object. It is this effect that makes the uncharged object act as if it is charged.


  • Charges on a conductor tend to gather at sharp points.

    For several complex reasons, the charges on a conductor will gather at sharp points. A metal cone, for instance, which has been charged will have a lot of charge at the point, and much less charge elsewhere.


  • More Specifically

  • F = kq1q2/r2, where F = electrostatic force, k = Coloumb's constant, q1 = first charge, q2 = second charge, and r = the distance between the two charges.

    As can be seen from the formula, the force between two charged objects depends on the amount of charge and the distance between the charges. It should be noted that a negative force using the equation above indicates that the force is attractive, and a positive force indicates that it is repulsive.


  • Each type of material allows electric charge to flow through it to a certain extent. The measure of how receptive a material is to electric current is called the conductivity. Those materials with a high conductivity are called conductors. Those with a low cuductivity are called insulators.

    All materials have a certain conductivity. The conductivity of a material depends on the materials arrangement of free electrons. Below is a chart of some common materials and their conductivities.

  • Material Electrical Conductivity (1/(ohm*m))
    Silver 60 x 106
    Copper 60 x 106
    Lead 5 x 106
    Wood 10-8 - 10-14
    Glass 10-10 - 10-14
    Hard Rubber 10-13 - 10-16
    Values taken from Tipler, Paul A. Physics, Third Edition. 1991.


  • The human body acts as a conductor.

    The human body has a large enough conductivity so that it is considered to be a conductor. For this reason, it is ill-advised to plug in a radio while taking a bath. Dr. Thornton, do you know a ballpark figure for a person's conductivity?


  • When certain types of materials are rubbed against other certain types, charge may be transferred from one to the other. The amount and type of charge is determined by the Triboelectric Series.

    The Triboelectric Series is a list of materials which determines which materials will become positively charged and which will become negatively charged when rubbed together. A sample portion of the Series is shown below. To read the chart, those materials nearer the top will become positively charged and those nearer the bottom will become negatively charged. For instance, when Teflon is rubbed against silk, the Teflon becomes negatively charged and the silk becomes positively charged.

    Positive Charge

    • Human hair
    • Nylon
    • Wool
    • Silk
    • Paper
    • Cotton
    • Wood
    • Hard rubber
    • Teflon
    Negative Charge

    Values taken from http://www.screenweb.com/


  • An uncharged object placed near a charged object becomes polarized.

    As discussed before, most objects are uncharged. An uncharged object means that the total number of positive charges equals the total number of negative charges. The net charge on the object is zero. When an uncharged object is placed near a charged object, its net charge remains zero. No additional charge is put on the object. What does happen, however, is the charges inside the object rearrange themselves. When a negatively charged object, for instance, is placed near an uncharged object, the negative charges move away from the charged object and the positive charges move towards the charged object. It is this effect that makes the uncharged object act as if it is charged.


  • Charges on a conductor tend to gather at sharp points.

    Due to the properties of electric fields and conductors, the charges on a conductor will gather at sharp points. A metal cone, for instance, which has been charged will have a lot of charge at the point, and much less charge elsewhere.


  • Related Demos

    The following demonstrations illustrate this physics topic:

    Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia