Electronics, Light, Magnetism, Temperature

How a micro wave oven works?

The micro wave oven is present in almost all homes. In this post will be explained how it works.

The micro wave

How the micro waves heats the food? The food has water molecules, which are polar, by that, have great electronegativity difference between the atoms.

This property allow the water molecule to direct itself with electric fields.

The micro waves are electromagnetic waves which vary constantly the electric and magnetic field in a frequency around 2.4 GHz. When these waves hit the water molecules, these try to change direction constantly and vibrate. Since temperature is the molecule’s motion, then the temperature increases and heat the food.

The magnetron valve

This component is the heart of micro wave oven and your function is to generate the micro waves. These are the main parts of valve.

When the magnetron is turned on with a direct voltage between 3000 and 5000 V, the cathode with a filament is heated and send electrons in direction to anode ring made of copper. However, has magnets which make the electrons follow a spiral trajectory due to Lorentz force, whose equation is below.

\mathbf{F_m}=\left | q \right |\cdot \mathbf{v}\times \mathbf{B}\cdot

Where v is the speed vector, q is the charge and B is the vector of magnetic field. Here shows the trajectory of electrons inside the magnetron.

For what serve the cavities in the anode ring? When the electron cloud circulate around the magnetron, emits a variable magnetic field which induces a current around the cavity. This induced current make the cavity behave like a LC parallel circuit tuned in resonance frequency.

The cavities emit waves in the resonance frequency of water which go by a waveguide to a beam divider, until reach the food.

The magnetron has a high power and it is dangerous, to have a idea of this component’s power, watch this video. It shows a magnetron valve turning up neon lamps and destroying a sound device.

Exist even weapon systems which use micro waves to burn people and destroy electronic devices in a distance. The military use of this radiation will be other post’s subject.

Other parts

How to obtain high voltage to magnetron? A step-up transformer increase the voltage from power plug to some thousand of volts and then a voltage multiplier with a capacitor and a high voltage diode to supply the magnetron with a pulsed DC voltage. This is the simplified circuit of magnetron supply.

The oven`s metallic internal cavity prevents the radiation to get out the oven. The door has a metallic grid to filter the microwave, but not the light due to a smaller wavelength. Why the food spins inside? Is to be heat the food equally.

About Pedro Ney Stroski

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