Answer:
The electrostatic photocopier illustrates the basic properties of electric charges. The central device in the process is an aluminum drum covered with a fine layer, less than 50 mm thick, of the photoconductive metal selenium.
Photoconductors are materials that act as a conductor when exposed to light and as an insulator in the dark.When the copier is being set up to make a copy, an electrode, called a corotron, deposits a positive charge, in darkness, uniformly over the entire surface of the selenium (step 1).
The selenium will retain this charge unless exposed to light, in which case electrons from the underlying aluminum-an excellent conductor-roam through the selenium, neutralizing the positive charge. When the copier lamp comes on and the actual copying begins, light is reflected from the document through a series of lenses and mirrors onto the selenium (step 2).
In places where the document is white, light is strongly reflected onto the selenium drum surface, causing it to act as a conductor and lose its charge.Where the document is black, no light is reflected onto the drum, causing the charge to be retained.An electrical image of the document is thus created on the drum-neutral where the original is white, positively charged where the original is black. This image will persist as long as the drum is kept dark.
The electrical image on the drum is developed into a dry copy, using a dry black
powder called "toner." Toner particles, made of plastic, are first given a negative charge, and then spread over the rotating drum (step 3). The particles are attracted to the charged areas of the drum but not to the neutral areas. Powder that does not adhere to the drum falls into a collecting bin for reuse.
To create a copy of this image, the toner must be transferred to paper. To do this, a
second corotron gives a sheet of paper a positive charge greater than the charge on the selenium (step 4).
As the drum rolls across this paper, toner particles that a moment ago adhered to the drum are attracted to the paper, forming an image on it. If you were to rub your finger across the paper at this stage, the toner would smudge.
To "fix," or immobilize, the image, heat from pressure rollers melts the plastic toner particles, fusing them to the paper (step 5).