The pointed conductor part B in Figure 1 on top in the large sphere picks up the charge. 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.
An ion source inside the sphere produces positive ions, which are accelerated away from the positive sphere to high velocities. A very large excess charge can be deposited on the sphere, because it moves quickly to the outer surface. 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.
Nevertheless, voltages of 15 million volts are well within practical limits. Rub a comb through your hair and use it to lift pieces of paper. It may help to tear the pieces of paper rather than cut them neatly. Repeat the exercise in your bathroom after you have had a long shower and the air in the bathroom is moist.
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. Most copy machines use an electrostatic process called xerography —a word coined from the Greek words xeros for dry and graphos for writing. The heart of the process is shown in simplified form in Figure 2. A selenium-coated aluminum drum is sprayed with positive charge from points on a device called a corotron. Selenium is a substance with an interesting property—it is a photoconductor.
That is, selenium is an insulator when in the dark and a conductor when exposed to light. 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. In the second stage, the surface of the drum is exposed to the image of whatever is to be copied.
Where the image is light, the selenium becomes conducting, and the positive charge is neutralized. In dark areas, the positive charge remains, and so the image has been transferred to 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. 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. 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.
Figure 2. Xerography is a dry copying process based on electrostatics. The major steps in the process are the charging of the photoconducting drum, transfer of an image creating a positive charge duplicate, attraction of toner to the charged parts of the drum, and transfer of toner to the paper.
Not shown are heat treatment of the paper and cleansing of the drum for the next copy. Laser printers use the xerographic process to make high-quality images on paper, employing a laser to produce an image on the photoconducting drum as shown in Figure 3. In its most common application, the laser printer receives output from a computer, and it can achieve high-quality output because of the precision with which laser light can be controlled.
Many laser printers do significant information processing, such as making sophisticated letters or fonts, and may contain a computer more powerful than the one giving them the raw data to be printed. Figure 3. In a laser printer, a laser beam is scanned across a photoconducting drum, leaving a positive charge image. The other steps for charging the drum and transferring the image to paper are the same as in xerography.
The laser assembly moves in only one plane, horizontally. After each horizontal scan, the printer moves the photoreceptor drum up a notch so the laser assembly can draw the next line. A small print-engine computer synchronizes all of this perfectly, even at dizzying speeds. Some laser printers use a strip of light emitting diodes LED to write the page image, instead of a single laser.
Each dot position has its own dedicated light, which means the printer has one set print resolution. These systems cost less to manufacture than true laser assemblies, but they produce inferior results. Typically, you'll only find them in less expensive printers. Toner Basics One of the most distinctive things about a laser printer is the toner. It's such a strange concept for the paper to grab the "ink" rather than the printer applying it.
And it's even stranger that the "ink" isn't really ink at all. So what is toner? The short answer is: It's an electrically charged powder with two main ingredients: pigment and plastic.
The role of the pigment is fairly obvious -- it provides the coloring black, in a monochrome printer that fills in the text and images. This pigment is blended into plastic particles, so the toner will melt when it passes through the heat of the fuser. This quality gives toner a number of advantages over liquid ink. Chiefly, it firmly binds to the fibers in almost any type of paper, which means the text won't smudge or bleed easily. Applying Toner So how does the printer apply this toner to the electrostatic image on the drum?
The powder is stored in the toner hopper , a small container built into a removable casing. The printer gathers the toner from the hopper with the developer unit. The "developer" is actually a collection of small, negatively charged magnetic beads. These beads are attached to a rotating metal roller, which moves them through the toner in the toner hopper. Because they are negatively charged, the developer beads collect the positive toner particles as they pass through. The roller then brushes the beads past the drum assembly.
The electrostatic image has a stronger negative charge than the developer beads, so the drum pulls the toner particles away. In a lot of printers, the toner hopper, developer and drum assembly are combined in one replaceable cartridge.
The drum then moves over the paper, which has an even stronger charge and so grabs the toner. After collecting the toner, the paper is immediately discharged by the negative electrons, which, are prepared by corona wire.
At this point, the only thing keeping the toner on the page is gravity if you were to blow on the page, you would completely lose the image. The page must pass through the fuser to affix the toner. Internal quartz tube lamps heat the fuser rollers, so the plastic in the toner melts as it passes through.
But what keeps the toner from collecting on the fuser rolls, rather than sticking to the page? Here are some examples from industry and everyday life. Many power stations burn fossil fuels such as coal and oil. Smoke is produced when these fuels burn. Smoke comprises tiny solid particles, such as unreacted carbon, which can damage buildings and cause breathing difficulties. 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.
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. In its most common application, the laser printer receives output from a computer, and it can achieve high-quality output because of the precision with which laser light can be controlled. Many laser printers do significant information processing, such as making sophisticated letters or fonts, and may contain a computer more powerful than the one giving them the raw data to be printed.
The ink jet printer , commonly used to print computer-generated text and graphics, also employs electrostatics.
Once charged, the droplets can be directed, using pairs of charged plates, with great precision to form letters and images on paper. 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 this is more difficult with xerography, requiring multiple drums and toners.
Electrostatic painting employs electrostatic charge to spray paint onto odd-shaped surfaces. Mutual repulsion of like charges causes the paint to fly away from its source.
Surface tension forms drops, which are then attracted by unlike charges to the surface to be painted. Electrostatic painting can reach those hard-to-get at places, applying an even coat in a controlled manner. If the object is a conductor, the electric field is perpendicular to the surface, tending to bring the drops in perpendicularly.
Corners and points on conductors will receive extra paint. Felt can similarly be applied. Another important application of electrostatics is found in air cleaners, both large and small. Home precipitators, often in conjunction with the home heating and air conditioning system, are very effective in removing polluting particles, irritants, and allergens.
To solve an integrated concept problem, we must first identify the physical principles involved and identify the chapters in which they are found. The following solutions to each part of the example illustrate how the specific problem-solving strategies are applied. These involve identifying knowns and unknowns, checking to see if the answer is reasonable, and so on.
The force an electric field exerts on a charge is given by rearranging the following equation:. Since the drop has a positive charge and the electric field is given to be upward, the electric force is upward.
This is an upward acceleration great enough to carry the drop to places where you might not wish to have gasoline. This worked example illustrates how to apply problem-solving strategies to situations that include topics in different chapters. The first step is to identify the physical principles involved in the problem.
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