Coulombs Law Satement:

The magnitude of the electrostatic force of attraction between two point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them.

Working of a Xerox Machine:

1. Electrostatic charge is given to a cylindrical drum in a xerox machine by a high voltage wire called a corona wire . 
2. This drum is coated with a photoconductive material. 
3. A photoconductor is a semiconductor that is photo sensitive i.e which starts conducting when exposed to light.
4. When a bright lamp falls incident the original document, the white areas of the original document reflect the light onto the surface of the photoconductive drum. 
5. The areas of the drum that are exposed to light start conducting and therefore discharge towards the the ground. 
6. The area of the drum not exposed to light (black portions of the original document) remains negatively charged.
7. The toner of the xerox machine is positively charged. 
8. When it is applied to the drum to develop the image, it is attracted and sticks to the areas that are negatively charged (black areas), just as paper sticks to a balloon with a static charge.
9. The resulting toner image on the surface of the drum is transferred from the drum onto a piece of paper with a higher negative charge than the drum.
10. The toner is melted and bonded to the paper by heat and pressure rollers.
11. Therefore, a negative photocopy inverts the colors of the document when creating a photocopy, that results in letters that appear white on a black background instead of black on a white background.

This force of attraction between unlike charges and the force of repulsion between like charges and the transfer of charge due to them is used for copying of printed content in the xerox machine, this is how we conclude that the working mechanism of the xerox machine is based on Coulomb Law of Electrostatic Charges.

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