Question
Mon October 17, 2011 By: Shefali Khurana

why the toner in photocopies machines have positive charge means from where positive charge come to it?

Expert Reply
Tue October 18, 2011

The surface of the drum is charged. An intense beam of light moves across the paper that you have placed on the copier's glass surface. Light is reflected from white areas of the paper and strikes the drum below. Wherever a photon of light hits, electrons are emitted from the photoconductive atoms in the drum and neutralize the positive charges above. Dark areas on the original (such as pictures or text) do not reflect light onto the drum, leaving regions of positive charges on the photocopier drum's surface. Negatively charged, dry, black pigment called toner is then spread over the surface of the drum, and the pigment particles adhere to the positive charges that remain. A positively charged sheet of paper then passes over the surface of the drum, attracting the beads of toner away from it. The paper is then heated and pressed to fuse the image formed by the toner to the paper's surface. When the copier illuminates the sheet of paper on the glass surface of a copier, a pattern of the image is projected onto the positively charged photoreceptive drum below. Light reflected from blank areas on the page hits the drum and causes the charged particles coating the drum's surface to be neutralized. This leaves positive charges only where there are dark areas on the paper that did not reflect light. These positive charges attract negatively charged toner. The toner is then transferred and fused to a positively charged sheet of paper.

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