Electrostatics

Two Kinds of Electricity. - If a pith ball is hung from a support by a silk thread, and a rubber rod that has been electrified by stroking it with cat’s fur is brought near it, the pith ball is at first attracted to the rod. If the pith ball is allowed to come in contact with the rod, it is found that the ball is repelled by the rod. When a glass rod which has been rubbed with silk is brought near the same pith ball carrying the charge which is received from the rubber rod, the charged pith ball is attracted by the electrified glass rod. There are then two states of electrification, or, as it’s usually said, two kinds of electricity: that which appears on an ebonite rod when rubbed with cat’s fur and that which appears on a glass rod rubbed with silk. These charges differ in one important respect. Charges which are alike repel each other, and charges which are unlike attract each other. That kind of electricity which appears on a glass rod that has been rubbed with silk is called positive electricity and the kind which appears on an ebonite rod rubbed with cat’s fur is called negative electricity.

Electric Field of Force. - The region surrounding one or more charged bodies is known as the electrostatic field. It is frequently represented by drawing lines which represent the direction in which the force on a positive charge acts at different points in the neighborhood of the bodies. Such lines are called lines of force. They show the direction in which a positive charge would move if it were placed in the field of force.

The intensity of an electric field is determined by the force that unit positive charge experiences when placed in it. Suppose that A (Figure 45) is a charged sphere having an electrostatic field about it. The intensity of this field at P is the force that would be required to hold unit positive charge in position at P. The force on q units of electricity at P would be q times as great as the force on unit charge. If E denotes the intensity of the electric field at P and q the number of unit charges located at P, then the force F on this charge is

F = Eq dynes

An electrostatic field has an intensity of unity when it exerts a force of 1 dyne on unit charge at that point.

p
Figure 45 - Definition of intensity of an electric field

Positive and Negative Charges. - The only way to charge a body negatively is to add electrons to it, and the only way to charge it positively is to take electrons away from it, leaving an excess of positive electricity. When the rubber rod was charged negatively by rubbing with cat’s fur, some electrons passed from the cat’s fur to the rubber rod, leaving the cat’s fur charged and the rubber charged negatively. On the other hand, when the glass rod was charged positively by rubbing with silk, some electrons passed from the glass to the silk, leaving the glass rod charged positively and the silk charged negatively.

In the normal condition, the amount of positive electricity in the atom is just equal to the amount of negative electricity on its electrons. One or more of these elections may be detached from the atom, leaving it with an excess of one or more positive charges of electricity. In such a case, the residue that is left after detaching these electrons is what is called a positively charged ion. On the other hand, an atom may gain one or more electrons in excess of its quota. It has on it then one or more negative charges and becomes a negatively charged ion. For example, when hydrochloric acid dissociates to a form 3 hydrogen ions and chlorine ions, the chlorine takes one more electron than its normal quota, thus giving it one negative charge and making it a negatively charged ion. Since the molecule of hydrochloric acid is originally neutral, the hydrogen is left with one less electron than its quota and is thus charged positively and becomes a positive ion.

Conductors and Insulators. — In some cases the forces holding the electrons to the atoms are not very large. They may be detached temporarily and wander about in the vacant space between the atoms. Thus in copper and silver some of the electrons become detached from the atoms and wander about for longer or shorter times in the interstices between the atoms. Under the action of an electric force, these electrons migrate through the metal. Such substances in which there are free electrons which can be made to migrate through the substance under the action of an impressed electric force are called conductors. Metals belong to this class, of bodies.

If the electrons are rigidly bound to the atom so that they do not become free except under the action of very large forces, no free electrons will be found in the vacant spaces between the atoms. If an electric force is applied to such a substance, it cannot cause the electrons to migrate through the substance, and there is no flow of electrons from one part of the substance to the other. If an excess number of electrons be placed on one part of such a substance, they will remain there without wandering to other parts of the body. The most that an impressed electric force can do in such a case is to cause a limited displacement of the electrons within the atom without causing the electrons to migrate from atom to atom. Such substances are called nonconductors, or insulators. To this class of substances belong mica, porcelain, quartz, glass, wood, ebonite, etc.

The Electroscope.— One form of gold-leaf electroscope consists of a metal sphere (Figure 46) which is fastened by means of a metal rod to two thin gold leaves.

Brass sphere

Sulphur

Glass jar

Figure 46 - Cold-leaf electroscope. The leaves

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