How does a microwave oven work?
How could these ovens heat our food in just a few seconds?
I’ll try to explain you all the theory step by step, the best that I can.
Remember that a few weeks ago I made a teardown of an old microwave oven and explained to
you how a magnetron works.
So in order to understand today's video, you first need to watch that other video and understand
how can we create microwaves.
So guys, let’s get started.
This is my new microwave oven, since my old one was torn down to pieces.
Now remember from the previous video, we apply 230V AC to this huge transformer, this will
apply more than 2000V to the magnetron, and this will output microwaves.
The magnetron output is inside of the oven, behind that gray plate called a wave guide.
But the question is, how can microwaves heat our food, and is this dangerous for us?
Well, let’s find out.
But first a quick word from the sponsor of
this video.
Microwaves are actually electromagnetic waves but in a particular range of the spectrum.
These waves oscillate between 1GHz and 1000 GHz.
If you analyze any electromagnetic wave, you will find that it has an electric field component
and a magnetic field component, that’s why they are called electromagnetic.
These fields are oscillating and perpendicular one to each other.
We are only interested in the electrical field which is responsible for heating up the food.
But the food must contain water, which pretty much any food has.
This is a water molecule.
It has one oxygen atom and two hydrogen atoms and for that reason is called a dipole.
So water is a polar molecule and we have 104.5 degrees between the hydrogen atoms.
But you see, both the oxygen and hydrogen are charged but the oxygen is more negative
and the hydrogen is more positive.
When we apply an electrical field, this will push the atoms creating torque and the water
molecule will align with the field orientation.
But remember, our microwave electrical field is oscillating, so it is going up and down,
so the water molecule will also oscillate at the same time, always following the electrical
field.
So, a vibrating molecule in physics, represents heat.
Basically, heat is just the average kinetic energy of the particles of a system, so vibrating
molecules, equals heat.
That’s how the water is heating up under an electric field that is oscillating.
And since food is made out of a lot of water, the food is also getting hot.
That’s why if you place something without water inside of a microwave, it won’t get
hot, such as an empty glass container, ceramic, plastic and so on.
So by now we know that the electrical field oscillation of the microwaves are heating
the food.
We also know that the magnetron is creating the microwaves.
All is there left to know about is the metal resonating cage of the oven.
You see, the magnetron uses a lot of power and we need to make it as efficient as possible.
We also need to confine the microwaves only inside of the oven, otherwise they would harm
us.
To increase efficiency, we use reflection.
The microwave is exiting the magnetron.
In front of it we place a metal sheet which reflects the microwave.
On the other side, we place another metal sheet with a hole for the magnetron output,
and this will reflect back the microwave once again.
Like that, we create a back and forth reflection of the microwaves.
At the same time, the energy of the microwave is now trapped inside of this confined space
which is what we also want.
But this is not yet the most efficient way of doing this.
What we are looking for is called a resonance cavity.
To understand that term, we first need to see the difference between a traveling wave
and a standing wave.
The microwave that is coming out of the Magnetron is something like this.
It is traveling forward, so if we track this node for example, we can see how it is advancing
forward as well.
On the other side, a stationary wave is something like this.
Its amplitude oscillates but the nodes are always in the same place.
I guess that with this simple animation you can see the difference.
And this is what we want to achieve, a standing wave.
Waves can be superposed and the resulting wave is the sum of the other two waves.
So imagine a wave going to the right and another going to the left.
The sum of these waves would result into a stationary wave that would increase and decrease
its amplitude.
Even more, the maximum peaks are now double the amplitude because it will sum the maximum
peaks of two waves.
For example, when both waves re at the same angle, we get the maximum amplitude.
And when the waves are opposed 180 degrees, we get them minimum amplitude.
That's how we get a more powerful standing oscillating wave instead of traveling waves.
To achieve this wave reflection, the oven is using the metal walls.
Like that the wave will bounce back and forth creating the superposed waves.
But there is one more important thing.
The distance between the reflectors.
You see, without controlling the distance between reflectors, the wave will reflect
again and again and again creating a lot of unsynchronized waves.
What we want is to have only two waves, that are superposed.
So the reflected wave needs to be exactly over the original wave.
So by moving the reflector back and forth, we can find the exact distance where the reflector
wall would be at the intersection of the first two waves.
So now, the third wave would be exactly on top of the first wave and the forth wave would
be on top of the second wave and so on.
So basically, we would only have two waves going back and forth.
And that’s exactly what we need to create the stationary wave of double the amplitude.
And this is the wave that will heat our food and is a lot more efficient.
So the distance between the walls of the oven metal case must be proportional to half of
the wavelength of the wave.
Typically, consumer microwaves ovens are using a wave of 2.45Ghz so the wavelength would
be 12.2 centimeters.
So the distance between the walls could be 6.1cm, 12.2, 18.3, 24.4cm and so on.
But one more thing.
Why do we have a rotating plate on the bottom of the oven?
Well, by looking at the wave, that’s easy to understand.
As you can see, the oscillating wave will have all the power on these peaks of the wave
and almost zero power where the node is.
Basically, the food would get hot here but it would stay cold here.
So in order to apply uniform heat, we rotate our food so all the parts of the food will
pass through the wave peaks.
If we didn't do that, we would only have hot spots on our food and the rest would be cold.
By rotating the plate, we ensure uniform temperature all around.
So by having a metal case we get the stationary wave but we also keep the dangerous waves
inside.
But wait a minute, the door of the microwave is made out of glass and not metal.
How is it that the microwave can’t get out and hurt us?
Well, because behind the glass wall, we have this metal mesh.
This metal mesh is specially designed so the wave couldn’t pass.
Basically, the wavelength of the microwaves is too big compared with the holes of the
metal mesh, so the waves would still reflect and stay inside so they are not a danger for
us, even with the glass door.
And one more thing you should know.
Why use microwaves and not any other type of waves?
Well, because of the energy absorption.
You see, if you were to use shorter wavelength such as the UV light, the energy would be
absorbed on the exterior of the food and the interior would still be cold.
UV light simply can’t penetrate too deep.
On the other side, if we use larger electromagnetic waves, they would have such a big wavelength
that they would pass over the food.
Also, imagine how big the oven should be in order to achieve superposed waves and create
the stationary wave.
So guys, that’s how we use the magnetron from the previous video, together with the
resonating case of the oven to heat our food.
Is quite an interesting phenomenon that was found by mistake during World War II by Percy
Spencer while working on improving radar technology.
I hope you have learned something new, and if so, consider giving me a like or comment
below.
Thanks again and see
you later guys.