It is common to find establishments that make copies of personal documents, school work, book fragments, etc. Inside these copiers we find huge machines that make hundreds of copies per minute. But how can we associate its functioning with something we study at school?
We can say that the “soul” of these machines, both copiers and laser printers, is nothing more than a metal cylinder covered with a thin layer of photoconductive material, which can be electrically charged. In this way, we see that the physics we study is practically everywhere around us.
When turning on a copier, the cylinder is positively charged through a wire that removes electrons. The surface of the material that is in contact with the metal acquires a negative charge and remains charged while in the dark. If a part is illuminated, it loses its surface charge, becoming electrically neutral in that area.
Copiers illuminate the page to be copied with a very intense light. The page image is projected onto the cylinder surface through a lens system. Thus, the most illuminated parts lose electrical charge while the less illuminated parts remain charged with negative charges, achieving an electrostatic image of the page.
Continuing the copying process, the cylinder receives a thin layer of a very fine negatively charged powder. In the parts of the cylinder that have been charged, the fine powder settles due to the forces of electrical attraction. The powder is called toner and forms on the surface of the cylinder an image equal to the one initially projected by the lens system.
As a last step, it remains to transfer the image to paper. This is done by charging the paper with positive charges before it comes into contact with the cylinder. Finally, the paper is pressed against the cylinder, causing the toner to be transferred to its surface. And finally, in order to fix the toner on the paper, the paper is heated to the toner’s fusing temperature, when the paper is again pressed by two metal rollers. Thus, the image is permanent on the paper.
We can say that laser printers are based on the same working principle, the only difference between them is that the laser printer does not need the original. The image to be printed is transferred from the computer to the printer’s internal memory. The light from a small laser device travels across the entire surface of the cylinder, being momentarily turned off when it passes through the points that must remain charged.