Conservation of Electric Charge

The total electric charge of the universe is constant. No physical process can result in an increase or decrease in the total amount of electric charge in theuniverse.

When charge is transferred from one object to another, it is generally due to the movement of electrons. In a typical solid, the nuclei of the atoms are fixed in position. The outer electrons of these atoms, however, are often weakly bound and fairly easily separated. As a piece of fur rubs across amber, for example, some of the electrons that were originally part of the fur are separated from their atoms and deposited onto the amber. The atom that loses an electron is now a positive ion, and the atom that receives an extra electron becomes a negative ion. This is charging by separation.

In general when two materials are rubbed together, the magnitude and sign of the charge that each material acquires depend on how strongly it holds onto its electrons. For example, if silk is rubbed against glass, the glass acquires a positive charge, as was mentioned earlier in this section. It follows that electrons have moved from the glass to the silk, giving the silk a negative charge. If silk is rubbed against amber, however, the silk becomes positively charged, as electrons in this case pass from the silk to the amber.

These results can be understood by referring to Table 19-1, which presents the relative charging due to rubbing—also known as triboelectric charging—for a vari­ety of materials. The more plus signs associated with a material, the more readily it gives up electrons and becomes positively charged. Similarly, the more minus signs for a material, the more readily it acquires electrons. For example, we know that amber becomes negatively charged when rubbed against fur, but a greater negative charge is obtained if rubber, PVC, or Teflon is rubbed with fur instead. In general, when two materials in Table 19-1 arc rubbed together, the one higher in the list be­comes positively charged, and the one lower in the list becomes negatively charged. The greater the separation on the list, the greater the magnitude of the charge.

Charge separation occurs not only when one object is rubbed against another, but also when objects collide. For example, colliding crystals of ice in a rain cloud can cause charge separation that may ultimately result in bolts of lightning to bring the charges together again. Similarly, particles in the rings of Saturn are con­stantly undergoing collisions and becoming charged as a result. In fact, when the Voyager spacecraft examined the rings of Sit turn, it observed electrostatic dis­charges, similar to lightning bolts on Earth. In addition, ghostly radial "spokes" that extend across the rings of Saturn—which cannot be explained by gravita­tional forces alone—is also the result of electrostatic interactions.

Since electrons always have the charge —e, and protons always have the charge +e, it follows that all objects must have a net charge that is an integral mul­tiple of e This conclusion was confirmed early in the twentieth century by the American physicist Robert A. Millikan (1866-1953) in a classic series of experi­ments. He found that the charge on an object can be ±4e, ±2e, ±3e, and so on, but never 1.5e or -9.3847e, for example. We describe this restriction by saying that electric charge is quantized.

Polarization

We know that charges of opposite sign attract, but it is also possible for a charged rod to attract small objects that have zero net charge. The mechanism responsible for this attraction is called polarization.

To see how polarization works, consider Figure 19 -5. Here we shown a positively charged rod held close to an enlarged view of a neutral object. An atom near the surface of the neutral object will become elongated because the negative electrons in it are attracted to the rod while the positive protons are repelled. As a result, a net negative charge develops on the surface near the rud—the so-called polariza­tion charge. The attractive force between the rod and this induced polarization charge leads to a net attraction between the rod and the entire neutral object.

Of course, the same conclusion is reached if we consider a negative rod held near a neutral object—except in this case the polarization charge is positive. Thus, the effect of polarization is to give rise to an attractive force regardless of the sign of the charged object. It is for this reason that both charged amber and charged glass attract neutral objects—even though their charges are opposite,

A potentially dangerous, and initially unsuspected, medical application of po­larization occurs in endoscopic surgery. In these procedures, a tube carrying a small video camera is inserted into the body. The resulting video image is pro­duced by electrons striking the inside surface of a computer monitor's screen, which is kept positively charged to attract the electrons. Minute airborne particles in the operating room—including dust, lint, and skin cells—are polarized by the positive charge on the screen, and arc attracted to its exterior surface.

The problem comes when a surgeon touches the screen to point out an impor­tant feature to others in the medical staff. Even the slightest touch can transfer particles—many of which carry bacteria—from the screen to the surgeon's finger and from there to the patient. In fact, the surgeon's linger doesn't even have to touch the screen—as the finger approaches the screen, it too becomes polarized, and hence, it can attract particles from the screen, or directly from the air. Situa­tions like these have resulted in infections, and surgeons are now cautioned not to bring their lingers near the video monitor.

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