Magnetron

Magnetron

One of the most important components of the microwave oven is the magnetron, which produces the microwaves.

Physics

Keywords

magnetron, electromagnetic spectrum, microwave, electromagnetic induction, Lorentz force, spektrum, X-radiation, gamma radiation, X-ray, ultraviolet, infrared, wavelength, frequency, visible light, radio wave, anode, cathode, electric current, electromagnet, electricity, physics, wave

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Electromagnetic waves

  • Electromagnetic spectrum
  • Wavelength
  • Frequency
  • Warning light - Certain electromagnetic waves are harmful to living beings.

Microwaves are a type of electromagnetic radiation with frequencies ranging from about 0.3 GHz to 300 GHz, and correspondingly, with wavelengths between 1 metre and 1 millimetre. They are most commonly used in microwave ovens, radars, mobile phones, Wi-Fi and Bluetooth connections, or in terrestrial television broadcasting.

Electromagnetic radiation is constantly present around us. Besides sounds, we gather most of our information about our environment through waves of this type. The various types of electromagnetic waves differ only in their wavelength and therefore, their frequency; the way they are produced and propagate is basically the same.

Electromagnetic waves are produced by sudden changes taking place in an electric field. When an electric field changes, a magnetic field is generated; and when the magnetic field changes, an electric field is induced and this process goes on repeatedly. This is how electromagnetic waves propagate.

Electromagnetic waves of different frequencies form under different circumstances and have a different impact on their environment. This is the reason why they are considered to be different waves.

The electromagnetic spectrum includes the following wave types (listed in descending order according to their wavelength): radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and Gamma rays.

These categories can be further divided into subcategories. For example, there are long, medium, short and ultra-short radio waves; visible light is categorised into the well-known colours: red, orange, yellow, green, blue, violet; and in the case of ultraviolet radiation, we can talk about UV-A and UV-B radiation.

Generally speaking, the shorter the wavelength of the electromagnetic wave, the higher its frequency and, accordingly, its energy, which means it can cause more damage.

Microwave oven

  • door with protective mesh - It prevents radiation from escaping from the cooking cavity.
  • cooking cavity
  • casing
  • control panel
  • turntable

Microwaves are electromagnetic waves, just like light, but their wavelength is larger: it ranges from 1 mm to 1 m. In microwave ovens, the wavelength is approximately 12 cm.

The heating effect of microwaves is based on the electric dipole property of water molecules, meaning that they are partially positively charged at the hydrogen side and partially negatively charged at the oxygen side. The water molecules try to align themselves to the periodically changing electric field of the microwaves, which causes them to vibrate, leading to an increase in their kinetic energy. As a result, the temperature of the water-rich food placed in the microwave oven increases.

The magnetron generates microwaves using electric current, which are directed into the cooking cavity by a waveguide. There they are scattered by the blades of a fan. Reflected by the inner metal walls of the cooking cavity, the waves enter food and heat it.

The door of the microwave oven is fitted with a protective mesh, which prevents microwaves from escaping from the cooking cavity. Without this protective layer, our own body tissues would also heat up while standing near the microwave oven, which could lead to burns.

Microwave oven construction

  • magnetron - It generates microwaves using electric current.
  • transformer - It converts the voltage of AC power to the value necessary for the magnetron.

Magnetron construction

  • Casing
  • Magnet
  • Heat sink
  • Plug

In high-performance microwave sources, such as microwave ovens or radars, a magnetron is usually the source of radiation.
A magnetron is a special electron tube in which the electrons flow at a high speed from the negatively charged cathode towards the positively charged anode. However, contrary to traditional electron tubes, the electrons follow a more complicated path inside the magnetron, and as a result of their zigzag motion, microwaves are produced.

The cathode and the anode

  • Cathode - In a heated state, it releases electrons.
  • Anode - The positively charged, ring-shaped anode attracts electrons.
  • Electron trajectory - Without the magnetic field, electrons would move in a straight line.

In the centre of the magnetron, there is a heated cathode, surrounded by a positively charged, ring-shaped anode. The cathode releases electrons that start to flow outwards, towards the anode.
Since strong magnets are placed at the top and bottom of the magnetron, a magnetic field is produced, in which a Lorentz force arises and the path of the electrons becomes curved, spiral-like, with occasional loops before reaching the anode.

In practical magnetrons, the path of the electrons is also influenced by the cavities found inside the anode. These cavities function as electronic oscillators, that is, they force electrons to oscillate at a specific frequency.

These complicated paths have certain hotspots where electrons accumulate, forming a spoke-like structure that rotates at a specific rhythm. This rotating electric field produces the microwaves.

Magnetic Lorentz force

  • Magnetic induction lines
  • Magnet
  • Electron trajectory - Without cavities, the path of the electrons would be a spiral because of the magnets.

The magnitude of the Lorentz force can be calculated from the following formula:


F = q * B * v * sin α

where q is the charge of the particle, B is the magnitude of magnetic induction, v is the particle velocity, and α (alpha) is the angle between particle velocity and the magnetic induction lines (vectors v and B).
There is, therefore, no force when v and B are parallel, and the maximum force arises when v and B are perpendicular.

The role of the cavities – the resonant circuit

  • Cavity - It functions as an electronic oscillator.
  • Electric field - It changes periodically in the cavities.
  • Spoke - Its shape and size affect the frequency of microwaves.
  • Antenna - Microwaves escape from the magnetron through it.
  • Resonant circuit - It changes the electric field periodically, at a specific frequency.

The cavities inside the anode function as electronic resonant circuits. An oscillator circuit is an electronic circuit in which the charges flow back and forth at a specific frequency. It is similar to a swing because if pushing the swing once, it will also move back and forth at a certain frequency, without any additional external impact.

A resonant circuit consists of a capacitor and a coil. However, in the case of magnetrons, the opening of the cavity is what fulfils the role of the capacitor and the coil is substituted with the material of the cavity itself, in which the electric current flows.

Oscillating current is formed when, due to an external impact, the charges start to flow along the wall of the ring-shaped cavity in a circular path and the electric current generates a magnetic field. Charges accumulate at the opening of the cavity, causing the electric current to become weaker and as a result, the magnetic field weakens as well. But, because of the change of the magnetic field, self-induction occurs, generating an electric field which can keep pushing the electrons in the same direction for a short instance, eventually causing even more charges to accumulate at the opening of the cavity.

When the process is about to stop completely, the accumulated charges start to flow back towards the opposite charges, that is, the direction of the current becomes opposite and the whole process will start again.
Therefore, the electric current will change periodically, at a specific frequency until the system runs out of energy. The frequency of the magnetron depends on the physical dimensions of the cavity, therefore it can be adjusted by modifying the dimensions of the cavities. The oscillating current flowing in the cavities affects the electron flow around the cathode, generating pulsating current instead of a steady flow. This is how microwaves are produced.

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