Microwaves- electric field? magnetron?

[seashell3130]

I'm curious about how microwave ovens work. I know that there is a magnetic field in the magnetron which creates the microwaves, however, I'm curious if there is an electric field in the microwave oven box itself? I'm confused by the research I've been doing online. Also, how exactly does the magnetron create microwaves?

 

[John37309]

A microwave oven is really no different to any radio transmitter, except its just transmitting its waves inside the metal oven box to protect YOU from the waves.

The magnetron creates dipole electromagnetic waves, just like any radio transmitter. The waves are not an "electric field" as you suggested. An electric field is what surrounds charged particles. Its like static electricity. But like any radio transmitter, your microwave has a positive and negative electrode, or a cathode and an anode. So the waves are dipole electromagnetic waves. The waves are alternating sine waves at whatever frequency the microwave operates at. All radio's need one wire connected to the ground (Earth), and one wire for the Aeriel.

Read the Wikipedia page on the electric field to understand it better; http://en.wikipedia.org/wiki/Electric_field

John.

 

[seashell3130]

Oh, okay. That makes sense to me now. Thanks for your response!

 

[microwaveguru]

If you have a magnetic field, by definition, you must have an electric field. It is the electric field that interacts with foods to heat them. The magnetic field interacts with metals causing currents to flow. Since food is neither magnetic nor metallic, the magnetic field has no influence.

 

[John37309]

If you have a magnetic field, by definition, you must have an electric field. It is the electric field that interacts with foods to heat them. The magnetic field interacts with metals causing currents to flow. Since food is neither magnetic nor metallic, the magnetic field has no influence.

I would disagree with your interpretation of how the food gets heated. Its electromagnetic radiation that heats the food, not an "electric field". As an example, there is a very strong "electric field" surrounding a Van der Graaf Generator, but that electric field will not heat anything because its just a static electric field. And a microwave is not a variation of a Van der Graaf Generator. A microwave creates a dipole alternating "magnetic" field, not an "electric field". By creating the alternating dipole magnetic field, its emitting electromagnetic radiation, which in turn bounces around inside the metal microwave box until it hits food molecules that are capable of absorbing that frequency of electromagnetic radiation.

See; http://en.wikipedia.org/wiki/Van_der_Graaf_Generator

Also see; http://en.wikipedia.org/wiki/Electric_field

John.

 

[ckutlu]

A microwave creates a dipole alternating "magnetic" field, not an "electric field".

If there is an alternating magnetic field, so there "must" be an alternating electric field, both forming the electromagnetic waves, according to Maxwell. (?) Correct me if i am wrong.

 

[Antiphon]

Just for the record, it's not a dipole field. It a traveling wave in a waveguide that opens into a rectangular box.

The food is heated by the molecular resonance of water at 2.54 GHz. This is driven by the electric, not magnetic field in the box though both are present.

 

[Drakkith]

The creation of the microwaves involves varying electric and magnetic fields. The food is heated by these microwaves, however the exact mechanism of heating via EM radiation I am unfamiliar with.

 

[microwaveguru]

There are two major mechanisms of food heating in a microwave heating:

1.dipole rotation: because water is a molecular dipole, it is influenced by the alternating electric field component of the electromagnetic wave. The water molecule attempts to follow the alternating electric field, however, it lags behind the field, thereby gaining energy from the field that is stored as kinetic energy. As the field collapses to zero it gives up his energy in the form of random kinetic energy, which is a definition of heat. There may also be collisions between the water molecules that also will be manifested as heat.

2.ionic conduction: this heating mechanism is based upon the presence of ionic species such as sodium and chloride ions, present when salt disassociates in solution. In this case, each of the ions as an different charge, and as the electric field alternates, these ions are accelerated in opposite directions, colliding with unionized water molecules which, in turn, collide with other molecules. When the field collapses to zero and then builds up in the opposite polarity the ionic species move in the opposite direction again causing collisions, and again these are manifested as random kinetic energy, or the equivalent of heat.

What is strange about both of these mechanisms is that the ultimate healing effects are quite different. As water gets hotter it heats much more slowly: in other words, cold water heats faster than hot water. On the other hand, if their is salt present, the solution heats faster as it gets hotter causing something known as “thermal runaway”, which is extremely difficult to control.

 

[John37309]

microwaveguru,
Now that was a very reasonable explanation, it make's sense. You have earned your title of being "Guru"

I might possibly add to your theory, maybe it might help you build your knowledge. Microwave magnetron's all seem to operate in a given bandwidth, 2450 MHz. So you suggest that the cold water heats faster than the hotter water. That could be a result of the magnetron only transmitting its energy at one frequency, 2450 MHz, or 122 millimetres. Maybe, its a bit of a guess.

John.

 

[microwaveguru]

John

Actually, magnetrons operate over a large bandwidth - +/- 50 MHz around the fundamental frequency. No, the reason for the slower heating of water as it becomes warmer is for a different reason: The resonant frequency of water at room temperature is approximately 22 GHz, or nearly 10 times higher than the operational frequency of the microwave oven magnetron. This means that the efficiency of energy capture is rather poor, but sufficient to cause heating. Now, as water gets warmer, it's resonant frequency keeps increasing, while the operational frequency of 2450 MHz remains the same. Therefore, the efficiency of energy capture keeps getting poorer as the water becomes warmer.You can see this also by looking at the change in dielectric loss factor of water at various temperatures:
1.5° C equals approximately 25
25° C approximately 12
35° C approximately 9.4
45° C approximately 7.5
etc.

A fun experiment you can run in your home microwave oven is to take three beakers, 500 or 1000 mL, and carefully fill each one with exactly the same amount of water, but in one case the water temperature is 10° C, another 20° C, another 30° C. Now heat each one of them, separately, for exactly 60 seconds and measure the temperature rise. You'll have to stir like a maniac because the water temperature will be variable throughout the water sample.

 

[John37309]

microwaveguru,
You have done your homework on this topic. Can i ask you something? The name microwaveguru. Does it refer to expertise in microwave ovens, or do you have a broader knowledge of the subject of electrical microwave transmitters, receiver's, radio astronomy, stuff like that? Have you studied this as a profession or a pastime?

John.

 

[microwaveguru]

I am a scientist who has been working with microwave ovens, microwave heating, industrial microwave heating systems, etc. for 50 years. I do research relating to microwave heating, teach courses relating to this, and more. But, I don't have anything to do with microwaves for communications, antenna design, etc. I am a physical chemist, not an electrical or electronics engineer. I came up with "microwaveguru" when I needed a name when answering high school kids questions regarding microwave ovens, and the name stuck.

Thanks for asking.

 

[John37309]

I asked cos I'm doing a bit of research in the field of microwave transmission.

Here is a question, don't know if you will have an answer;
Lets say I'm transmitting microwaves between 1Ghz and 2.84Ghz, and i build the transmitter inside a building with concrete walls. Any idea how much of the radiation will be absorbed by the solid concrete walls, let's say the wall is 30cm thick (1 foot)?

Or to phrase that a better way. In theory, if the walls were made of rounded stones, wouldn't less of the microwaves be "absorbed" by the walls? Isn't there a connection between the wavelength of the radiation and the ability of stone or concrete to absorb the radiation at those wavelengths?

In other words, if I'm correct, i could theoretically encase my microwave transmitter in a walled building, and if the "rounded stones" in the walls are the correct size to match the wavelength of the radiation, then the microwaves would pass through the walls almost unimpeded? Am i correct in what I'm suggesting? Can i build a wall to match the wavelength of the microwave radiation so the round stones won't absorb the microwaves?

John.

 

[microwaveguru]

John
Concrete has a loss factor of approximately 12 at the frequencies that you mention. That means that the wavelength that 1 GHz will be approximately 9 cm, and a 2.45 GHz approximately 3.5 cm. Penetration depths will be somewhat smaller than that. So it is likely that there will be very little, if any, transmission, outside of the room. Your comment about round stones is somewhat confusing. If the stones are packed without any concrete surrounding them then of course there will be transmission through the air in between the stones. However, if the stones are packed in concrete, my previous analysis holds and there will be very little transmission.

 

[John37309]

Yea, its a theoretical question. No, no concrete in the wall any more.

Pretend the walls are held in place by plastic sheets or glass that won't interfere with the waves. The sheets are just to keep the walls standing and stop the round stones rolling away. Let's say the transmission wavelength is exactly 10cm to make it a nice even number. If the walls of my building are made of round stones that are all 5cm in diameter, half the wavelength, two round stones together matching the 10cm wavelength. There is no concrete filling the gaps, just air, wouldn't the 10cm waves pass through unimpeded? In theory, my walls could be 20 meters thick with the round stones and the 10cm waves would just pass straight through like as if there is no wall.

Is that theory correct?

I want to build a theoretical wall that won't interfere with the waves in any way. A thick wall that is invisible to certain wavelengths of microwave radiation. The wall might end up being built for real at some stage. Currently its still in the design stage.

John.

 

[microwaveguru]

No, I believe you don't understand the wavelength - if it is 10 cm in air, that's :the "free space wavelength". But the wavelength thru the stones is the "material wavelength" which is

material wavelength = free space wavelength/ Square root of the dielectric constant,

or, for stones (this is approximate) 10/1.5 = 6.7 cm.