Precipitators The smoke produced in industrial chimneys contains tiny particles suspended in the gas. One way of removing these particles is electrostatic precipitation. As the smoke rises in the chimney, it passes through a wire grid at a negative potential10 of about 50 kV. On passing through the grid, the dust particles become negatively charged. The charged dust particles then collect on positively charged plates attached to the sides of the chimney. At regular intervals, the plates are given a sharp mechanical blow that dislodges the ‘solid- ified’ dust into a receiver for collection and disposal. Air purifiers and smoke removers used in bars and restaurants often work on the same principle.
Flue gases and dust
Fig 15.19: Precipitator in a chimney
Xerography (photocopying) Photocopiers contain a drum coated in selenium, which is charged by a high voltage. In bright light, selenium loses its charge.When the photocopier is in use, the master sheet is brightly illuminated and its image is focused onto the drum. The white parts of the master copy reflect light onto the drum. Therefore the white parts of the image lose their charge but the black parts retain it. Fine carbon dust (called toner) is blown onto the drum, where it adheres to the charged parts of the drum. Positively charged paper is moved over the drum and the toner image is transferred onto it. Hot rollers fuse (melt) the image onto the paper. Laser printers work in the same manner.
Lens Mirror
Page being photocopied
Mirror Finished copies
Plain copier paper Heater Drum coated in selenium
Negatively charged image
Copy being made
Fig 15.20: Photocopier
10 Potential and voltage will be dealt with in more detail in Chapter 16.
