How Everything Works
How Everything Works How Everything Works

Question 634

How does a magnetron work? — MM, Czech Republic
A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic "tuning fork"—electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity's resonant frequency. As electrons are "boiled" off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge "sloshing" back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities' positively charged walls than to their negatively charged walls.

However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.


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