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Archive for the ‘How Things work?’ Category

How Microwave Cooking Works

Posted by Ramesh Khati on May 12, 2009

How Microwave Cooking Works

Inside Microwave Oven

Inside Microwave Oven

A Microwave Oven

A Microwave Oven

You often hear that microwave ovens cook  food “From the inside out.” What does that mean? Here’s an explanation to help make sense of microwave cooking.

Let’s say you want to bake a cake in a conventional oven. Normally you would bake a cake at 350 degrees F or so, but let’s say you accidentally set the oven at 600 degrees instead of 350. What is going to happen is that the outside of the cake will burn before the inside even gets warm. In a conventional oven, the heat has to migrate by conduction from the outside of the food toward the middle. You also have dry, hot air on the outside of the food evaporating moisture. So the outside can be crispy and brown (for example, bread forms a crust) while the inside is moist.
In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There is heat everywhere all at once because the molecules are all excited together. There are limits, of course. Radio waves penetrate unevenly in thick pieces of food (they don’t make it all the way to the middle), and there are also “hot spots” caused by wave interference, but you get the idea. The whole heating process is different because you are “exciting atoms” rather than “conducting heat.”

In a microwave oven, the air in the oven is at room temperture, so there is no way to form a crust. That is why microwavable pastries sometimes come with a little sleeve made out of foil and cardboard. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.

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How does Photo Copiers work?

Posted by Ramesh Khati on April 16, 2009

Photocopier MachinePhotocopier Machine

working principle of photocopier

working principle of photocopier

The photocopier (commonly called Xerox® Machine) was a machine designed by an engineer working as a patent attorney. The principle is to spray electric charge upon the smooth surface of a semiconductor, and then flash an image of the material to be copied onto this plate. The light causes the plate to discharge in that local area, and when pigment powder is sprinkled over the plate, it sticks to the charged spots and falls off the discharged spots. The plate is then placed in contact with a blank sheet of paper to which the powder is transferred, then the paper is heated to stick the pigment to the paper. the fuser melts the toner into the paper permanently. This is why some Xerox copies, particularly those made at the end of a long printing run, can feel very warm to the touch.

After each run of copies has been completed, a Corona wire essentially clears off the photoreceptor drum or belt by running a new positive charge. Because many photoreceptor drums are smaller in diameter than the length of the copy paper, it is not unusual for the photocopying process to be duplicated several times over as the original document is scanned. The drum may have to be recharged positively to eliminated the older information and receive the new images several lines at a time. All of these actions are carefully synchronized in order to produce a photocopy in the time it takes for the lamp to scan across the original one time.

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