![]() (This is more difficult with xerography, requiring multiple drums and toners.)Įlectrostatic painting employs electrostatic charge to spray paint onto odd-shaped surfaces. Ink jet printers can produce color images by using a black jet and three other jets with primary colors, usually cyan, magenta, and yellow, much as a color television produces color. Once charged, the droplets can be directed, using pairs of charged plates, with great precision to form letters and images on paper. A nozzle makes a fine spray of tiny ink droplets, which are then given an electrostatic charge. The ink jet printer, commonly used to print computer-generated text and graphics, also employs electrostatics. Various computer-driven devices are then used to direct the droplets to the correct positions on a page. ![]() The nozzle of an ink-jet printer produces small ink droplets, which are sprayed with electrostatic charge. Finally, the paper and electrostatically held toner are passed through heated pressure rollers, which melt and permanently adhere the toner within the fibers of the paper.įigure 4. Next, a blank piece of paper is given a greater positive charge than on the drum so that it will pull the toner from the drum. ![]() The third stage takes a dry black powder, called toner, and sprays it with a negative charge so that it will be attracted to the positive regions of the drum. In dark areas, the positive charge remains, and so the image has been transferred to the drum. Where the image is light, the selenium becomes conducting, and the positive charge is neutralized. In the second stage, the surface of the drum is exposed to the image of whatever is to be copied. In the first stage of the xerography process, the conducting aluminum drum is grounded so that a negative charge is induced under the thin layer of uniformly positively charged selenium. That is, selenium is an insulator when in the dark and a conductor when exposed to light. Selenium is a substance with an interesting property-it is a photoconductor. The heart of the process is shown in simplified form in Figure 2.Ī selenium-coated aluminum drum is sprayed with positive charge from points on a device called a corotron. Most copy machines use an electrostatic process called xerography-a word coined from the Greek words xeros for dry and graphos for writing. Is it easier to get electrostatic effects in dry or moist air? Why would torn paper be more attractive to the comb than cut paper? Explain your observations. ![]() Repeat the exercise in your bathroom after you have had a long shower and the air in the bathroom is moist. It may help to tear the pieces of paper rather than cut them neatly. Rub a comb through your hair and use it to lift pieces of paper. Take-Home Experiment: Electrostatics and Humidity Nevertheless, voltages of 15 million volts are well within practical limits. Practical limits arise because the large electric fields polarize and eventually ionize surrounding materials, creating free charges that neutralize excess charge or allow it to escape. An ion source inside the sphere produces positive ions, which are accelerated away from the positive sphere to high velocities.Ī very large excess charge can be deposited on the sphere, because it moves quickly to the outer surface. (The induced electric field at the points is so large that it removes the charge from the belt.) This can be done because the charge does not remain inside the conducting sphere but moves to its outside surface. The pointed conductor (part B in Figure 1) on top in the large sphere picks up the charge. Van de Graaffs utilize both smooth and pointed surfaces, and conductors and insulators to generate large static charges and, hence, large voltages.Ī battery (part A in Figure 1) supplies excess positive charge to a pointed conductor, the points of which spray the charge onto a moving insulating belt near the bottom. ![]() Figure 1 shows a schematic of a large research version. The first was built by Robert Van de Graaff in 1931 (based on original suggestions by Lord Kelvin) for use in nuclear physics research. Van de Graaff generators (or Van de Graaffs) are not only spectacular devices used to demonstrate high voltage due to static electricity-they are also used for serious research. ![]()
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