. How do photoconductors work?
When the atoms and molecules in a solid join together, some of their electrons may become shared between them. These electrons can travel about the solid as waves. Because they travel as waves, they can only follow paths that bring them back perfectly in phase with how they started out, like steady ripples on a pond. As a result, they can only follow certain paths and can only have certain energies. For complex and fundamental reasons, only two electrons can adopt any particular path, so the electrons take turns filling up all of these paths or "levels" from the lowest energy ones up. The electrons fill up these levels until there are no more electrons seeking a path. The behavior of the solid depends on the nature of the levels remaining after all of the electrons have found a path. The last few levels filled with electrons are called "valence levels" and the first few empty levels are called "conduction levels". If there are no more empty levels at energies near the last one filled, the material will behave as an insulator. The conduction levels are far higher in energy than the valence levels. If there are empty levels at energies near the last one filled, the material will behave as a conductor. The conduction levels and valence levels are right nearby. A photoconductor is of the former type: there are no conduction energy levels near the last one filled valence level so it is an insulator. But it becomes a conductor when exposed to light because the light can move the valence level electrons into empty conduction levels at much higher energies.
. In the photocopying or xerographic process, what is the intensity, wavelength, and normal exposure time of the light that is emitted from lamps of these office machines? How does this light differ from sunlight?
The light sensing surface in a xerographic copier is a semiconductor or "photoconductor" film on a metal drum or belt. Light causes this film to convert from an insulator to a conductor of electricity, a change that is ultimately responsible for the formation of the copy image. However, the light particles ("photons") must each carry a certain amount of energy in order to cause that conversion. Since the photons of blue light carry more energy than those of red light, blue light tends to be more effective in the xerographic process than red light. In fact, far red and infrared light have no effect at all on the photoconductor film. However, considerable effort has been made over the years to make the photoconductor films used in xerographic copying very sensitive to all wavelengths of visible light. As a result, it doesn't take much light from even a normal lamp to produce a xerographic copy. Sophisticated copiers expose the original document to visible light from an incandescent lamp, a fluorescent lamp, or a xenon/krypton flashlamp and measure the light reflected by that document. They use this measurement to set the exposure time and/or the aperture of the lens that forms the image of the document on the photoconductor film. The light used in a copier doesn't contain as much ultraviolet light as sunlight, but otherwise the differences aren't very important to the xerographic process. As for the intensity and exposure times, you can see these for yourself when the machine operates. Just open the cover and watch the lamps or flashlamps in action.
. What is a photoconductor? — MN, Chicago, IL
A photoconductor is a material that behaves as an electric insulator in the dark but becomes an electric conductor when exposed to light. An insulator is unable to transport electric charges because its own electrons can't respond to modest electric forces. Because of quantum physics, electrons can only follow specific paths called "levels" as they move through a material and all of the easily accessible levels in an insulator are completely filled. For reasons of symmetry, there are always as many electrons traveling to the right in an insulator as are traveling to the left, so that on average, no electrons move anywhere, even when they are exposed to electric forces. But when light energy shifts some of the electrons from the filled levels to a collection of formerly unoccupied levels that previously weren't accessible, these shifted electrons can respond to electric forces and transport electric charge through the material. In the light, a photoconductor stops acting as an insulator and starts acting as a conductor. Such photoconductors are the basis for xerographic copiers and laser printers.
. How can an insulator carry a charge if it cannot conduct electricity? How can one charge an insulator? Can an insulator be charged by induction? — VV, Washington, DC
While charge can't move through an insulator, there is nothing to prevent charge from being placed on its surface or injected inside it. If you rub the surface of an insulator with a piece of silk, sliding friction will push electrons onto or off of its surface and leave its surface electrically charged. With no way for that charge to move about, the insulator's surface retains the charge indefinitely. A beam of fast moving electrons or other charged particles can be injected into an insulator and will become trapped inside it. Once again, the charges can't move around after the injection. Since charges can't flow in the insulator, you can't charge it by induction—a process in which proximity to a nearby charged object rearranges the charges in a conductor and allows you to trap those charges in a nonuniform arrangement.
. How is the xerographic copying process related to laser and led printers? - BG
The same basic printing process is used in both xerographic copiers and laser or led printers. In all cases, a charge image is formed on the surface of a photoconductor and this pattern of electric charge attracts a pattern of colored plastic powder. The powder is then transferred to paper and melted or pressed into the paper's surface to form a permanent print.
The main difference between a copier and a printer is in the source of light used to produce the charge image. In a copier, lenses and mirrors are used to form a real image of the original document on the surface of the photoconductor. Wherever light from the white portions of the document strikes the photoconductor, the photoconductor becomes an electric conductor and charge is able to move. The pattern of light then becomes a pattern of charge—a charge image.
In a printer, a laser or an array of light emitting diodes is used to form the pattern of light on the surface of the photoconductor. Wherever the light strikes the photoconductor, charge is again able to move about. Dot by dot or row by row, the charge image takes shape. The pattern of charge that's written on the surface of the photoconductor eventually becomes the printing itself.
. How does a laser printer work? What is the toner made of and how does the printer place it on the paper? — PC, Brussels, Belgium
A laser printer uses the light beam from a laser to control the placement of electric charges on a photoconductor surface. A photoconductor is a material that only conducts electricity when exposed to light, so that charges can move through the photoconductor only when the laser beam hits it. The printer uses a corona discharge to place charges on the darkened photoconductor and then uses the laser beam to remove charges from certain places. The end result is a pattern of electric charges that's an image of the final print. The toner particles, which are made of black plastic, are given an electric charge so that they cling to the charge image on the photoconductor. This pattern of toner particles is then transferred to electrically charged paper and fused to that paper with heat and pressure.
. What is an electric field and how does it affect us? — MT, Brampton, Ontario
Electrically charged particles exert forces on one another. For example, a negatively charged particle attracts a nearby positively charged particle and repels another negatively charged one. These attractions and repulsions are mediated by electric fields that are created by those charges. By this statement, I mean that the negatively charged particle creates an electric field around itself and this electric field is what ultimately exerts forces on the other two charges—attracting the nearby positively charged particle and repelling the negatively charged one. Whenever an electrically charged particle finds itself in an electric field, it experiences a force. The direction of that force depends on its electric charge (either positive or negative) and on the direction of the electric field (which may have somewhat different directions at different points in space). The strength of that force depends on the amount of electric charge on the particle and on the strength of the electric field (which can vary from nothing at all to extremely strong).
But while electric fields always exist around charged objects and exert forces on any other charged objects that enter them, electric fields can also exist far away from charges. Electromagnetic waves contain electric and magnetic fields (the magnetic equivalents of electric fields) and these two fields sustain one another as the wave travels. Although electromagnetic waves are created and destroyed with the help of charged particles, they can travel alone and without any nearby charged particles to assist them.
While electric fields exert forces on the charged particles in our bodies, the response of those charges isn't likely to injure us. When you are exposed to an electric field, there is a subtle rearrangement of electric charges on the surface of your body that then creates its own electric field. The result is that there is essentially no electric field inside you. Only when you are exposed to extremely strong electric fields, and spark and currents begin to flow through you, is there any significant effect to you.
. If the Fermi level is the highest energy level used by an electron, how can electrons shift to conduction levels that are at energies above the Fermi level? — PH
The Fermi level is the highest energy level occupied when all the electrons have as little energy as possible. That situation occurs only when all the electrons are paired two to a level and the levels are filled all the way from the lowest energy level up to the Fermi level. At any reasonable temperature and in the presence of light or other energy sources, some of the electrons will have been shifted out of their normal levels and into levels above the Fermi level. The Fermi level doesn't change when these shifts occur—it's defined before the electrons shift.
. Why can we see through glass and some minerals? — KH, Newport Beach, CA
Light consists of electromagnetic waves—fluctuating electric and magnetic fields that travel through space at enormous speeds. As light passes through an insulator such as glass, diamond, quartz, or salt, the light's fluctuating electric and magnetic fields cause electric charges in the insulator to vibrate back and forth. This interaction between light and the charged particles in a material is the first step in absorption—the material is "trying" to absorb the light. But light carries energy with it and any material that absorbs light must be prepared to accept the light's energy. The charged particles in insulators generally have no quantum states that allow them to accept that light energy. As a result, the insulator's charged particles respond to the light as it passes, but they can't actually absorb the light. The light simply passes through the insulator. However, the light is delayed by its interaction with the charged particles in the insulator (the speed of light in a material is less than the speed of light in vacuum) and the light may be redirected (reflected or scattered) by encounters with inhomogeneities. So glass and the other insulators don't absorb light and are often transparent. Those that aren't transparent are usually white—they scatter light in all directions.
. Is wearing rubber-soled shoes more dangerous or less dangerous if you are hit by lightning? — JH, Santa Fe, New Mexico
Once lightning strikes you, whether or not you are wearing rubber-soled shoes will make little difference. The voltages involved in lightning are so enormous (hundreds of millions of volts) that the insulating character of rubber soles will be completely overwhelmed. If the electric current can't pass through your rubber soles, it will simply form an electric arc around them or through them.
However, I would guess that rubber-soled shoes provide some slight protection against being hit by lightning in the first place. Lightning tends to strike objects that have acquired an electric charge that is opposite that of the cloud overhead. This opposite charge naturally appears on grounded conducting objects because the cloud's charge pulls opposite charges up from the ground and onto the objects. Once this charging has taken place, the object is a prime target for a lightning strike.
If you are standing alone and barefoot on the top of a mountain during a thunderstorm, the cloud will draw opposite charge up from the ground through your feet and you will become very highly charged. There are even photographs of people on mountaintops with their hair standing up because of this charging effect. Unfortunately, some of these people were struck by lightning shortly after experiencing this effect. If you ever experience it, run for your life down the mountain! It's possible that wearing rubber soles shoes will prevent or delay this charging effect, and it might keep you from being struck by lightning. But I sure wouldn't count on it.