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00:00It may amaze you to learn that the invention of the microwave oven was accidental.
00:06The scientist, Percy Spencer, was performing experiments on a device called a magnetron.
00:12Magnetrons generate powerful microwave radiation.
00:15During the experiment, he observed that the candy bar in his pocket was completely melted.
00:20That's when it occurred to him to explore the applications of microwaves in cooking
00:24food.
00:27From this experiment, it was observed that a high-powered traveling microwave has the
00:31capability of heating food.
00:33But of course, this raises the question of what was in the microwave that melted the candy
00:37bar.
00:39Microwaves are electromagnetic waves in a particular spectrum.
00:44Like any other electromagnetic waves, they have oscillating electric and magnetic fields.
00:49If you track amplitude of the wave in a specific area, you can observe this oscillation.
00:56In the chocolate melting accident case, the oscillating electric field component of the electromagnetic
01:01wave is responsible for cooking the food.
01:06The component responsible for producing microwaves is known as a magnetron.
01:11A magnetron emits microwaves in all directions.
01:15To confine the wave to propagate in one dimension, the magnetron is attached to the waveguide.
01:22From the waveguide, the waves come into the cooking chamber to heat food.
01:27Cavity magnetrons are also used in microwave ovens, where they are responsible for producing
01:31high-powered microwaves.
01:34In this video, we will explore the physics behind the cavity magnetron.
01:39Cavity magnetrons work on the principle of LC oscillation.
01:43LC oscillation occurs when a charged capacitor is placed along an inductor.
01:49This simple arrangement creates back-and-forth motion of electrons.
01:56When an antenna with an inductor attached to it is placed near to the inductor of an LC circuit,
02:01the antenna radiates electromagnetic waves.
02:04This is the theory behind the cavity magnetron.
02:07Obviously, the energy oscillation and associated radiation of this theoretical device will die
02:13out fast, since it loses energy in the form of radiation.
02:17How can this theoretical device be converted into a practical one?
02:22Let's look at this in the coming sessions.
02:25Consider this configuration, a cathode and a filament.
02:29The current flow through the filament will heat up the cathode, and due to this, electrons
02:34will be emitted from it.
02:36This phenomenon is known as thermionic emission.
02:40Interestingly, in this case, the electrons come back to the cathode.
02:46If we place an anode with positive potential, the emitted electrons accelerate and move towards
02:51the anode.
02:54As the theory of radiation states, the charges produce radiation when they accelerate.
02:59However, in this arrangement, the electrons radiate inefficiently as they spend very little
03:05time in the interaction space.
03:08In order to increase the time spent by the electrons in this space, a permanent magnet
03:13is introduced into the structure.
03:16The magnetic field forces the electrons to take a curved path.
03:20Since the path of the electrons is now curved, the time that the electrons spend in the interaction
03:25space is increased.
03:27The final structure thus formed is known as a Hull magnetron.
03:33Hull magnetrons are more efficient than the previously explained technology, however, its
03:37efficiency can be further improved with the help of the LC oscillations, which we saw in
03:42the beginning of this video.
03:44Let's see how we achieve oscillation in a magnetron.
03:50To achieve oscillation, the anode is designed with cavities.
03:54These cavities cause huge differences in the physics of magnetrons.
03:59To understand this, let's consider a simple case.
04:02Let's consider a metal bar with a cavity.
04:05Assume a negative charge is passing near to the metal.
04:08The negative charge will obviously repel the electrons near to it, as shown in this animation.
04:14Similarly, when the negative charge passes near to the cavity, the electrons around the
04:19cavity's surface are disturbed.
04:21You can see that an accumulation of positive and negative charges occurs across the cavity
04:26surfaces due to this disturbance.
04:28In short, the cavity surfaces acts like capacitor plates.
04:33If you connect an inductor across the cavity surface, the charges will start oscillating.
04:39This simple physics is the basis of the cavity magnetron.
04:44A magnetron has many such cavities.
04:49Many electrons are ejected from the cathode by thermionic emission.
04:53Let's track the effect of the very first electron ejected into these cavities.
05:00As explained above, this electron will induce positive and negative charges on the cavity's
05:04surfaces.
05:05Here, the cavities are arranged in a circular manner.
05:10This means the charged cavity surface pair cannot stay in isolation.
05:14To keep the electric field zero in the metal, all the cavity pairs have to be charged with
05:19the opposite polarity.
05:22One interesting thing to note here is that the curved surface of the cavity acts like an
05:26inductor.
05:28This means that the charges accumulated will go for a simultaneous LC oscillation.
05:35With the help of a metal loop and an antenna, this oscillation is extracted and converted
05:40into EM waves.
05:43These oscillations will be sustained in the magnetron since the electrons continually flow
05:48from cathode to anode and transfer their energy.
05:52Now let's see what happens to the remaining electrons in the interaction space.
05:56The very first electron that reached the cavity's surface has already created a charge pattern
06:01on the cavities.
06:03This means the remaining electrons will be attracted to the positive charge regions and
06:07they will form an interesting spoke wheel pattern like this.
06:11Since the charges on the cavities are oscillating, the spoke wheel has to spin as illustrated.
06:19This phenomenon could be related to the analogy of a donkey, a carrot and a stick.
06:23Here, no matter how many steps the donkey takes to reach the carrot, the carrot always remains
06:28out of its reach.
06:33As you must have noticed, the antenna is connected only to a single cavity, since the magnetic field
06:38lines generated in one cavity also link with the other cavities.
06:43This phenomenon is called mutual coupling.
06:47This means the extraction of magnetic energy from one cavity would be the same as the extraction
06:52from all of the cavities combined.
06:56The cavity magnetron was developed in the UK during World War II to enhance radar technology.
07:02Cavity magnetrons are able to produce high powered pulses at a shorter wavelength and this
07:07led to the detection of smaller objects being possible.
07:11The compact size of the cavity magnetron made the radar size smaller.
07:16This UK technology was transferred to the US during World War II.
07:26Now let's see how these oscillating electric fields cook food.
07:30Most of the food that we consume has water in it.
07:33Water is a polar molecule.
07:35The hydrogen atoms of the water molecule are placed at an angle of 104 degrees from each
07:40other and both the hydrogen and oxygen atoms have charges.
07:45This makes the water molecule behave like a dipole.
07:49When an electric field is applied to the water molecule, it starts to rotate due to the torque
07:53produced on the dipole.
07:56Since in electromagnetic waves the electric field oscillates continually, the water molecules
08:01will keep on oscillating.
08:03The microscopic collisions between atoms and molecules can transfer energy to the electrons
08:08in those atoms.
08:11Electron transitions.
08:12This energy transfer can excite electrons, causing them to jump to higher energy levels
08:18or even leave the atom entirely.
08:21Energy release.
08:23When these excited electrons return to their lower energy states, they release the excess
08:28energy as heat and electromagnetic radiation, contributing to the overall increase in temperature.
08:34Heat as thermal energy.
08:37This conversion of energy from motion, kinetic energy, to heat is here.
08:42Now, let's look at how to convert this heat generation concept into a workable product.
08:49To use the electromagnetic wave's energy efficiently, it must be reused several times.
08:55An efficient way of achieving this is to reflect it and keep it confined in a particular area.
09:00The best way of making this reflector is with the help of metal.
09:04The metallic surface causes the microwave to reflect from its surface and if you keep one
09:09more reflector at the source side, the reflection will keep on continuing.
09:13This way we will be able to trap the energy of electromagnetic radiation within a volume.
09:19However, the most efficient way of trapping electromagnetic wave energy is by use of a technique called
09:24resonance cavity.
09:26This method also increases the intensity of electromagnetic waves.
09:31Let's understand the concept of resonance cavity using a simplified approach of standing
09:36waves.
09:38A standing wave is a stationary wave that fluctuates in time but does not propagate in space.
09:44Just by observing these two wave animations, you can understand how a standing wave is
09:48different from a normal travelling wave.
09:53It is formed when two waves, having the same amplitude and the same frequency, moving in
09:58opposite directions, are superimposed on each other.
10:02Please have a look at these two electromagnetic waves, which are travelling in the opposite direction.
10:07Let's pause the animation here.
10:09You can see that the waves have 180 phase differences here.
10:13When you add both the electromagnetic waves, they will perfectly cancel.
10:18Now, let's pause at this instant.
10:21The resultant is a bigger sinusoidal curve.
10:24Let's pause at one more instance.
10:27Here you are getting an even bigger sinusoidal wave as the output.
10:31By comparing the results of these three instances, it is clear that the resultant electromagnetic
10:35wave just oscillates in its position without travelling.
10:40Let's examine how to produce two oppositely travelling waves practically.
10:47We will get a clear solution for this if we understand how electromagnetic waves get reflected
10:52on a metal surface.
10:54We know that when a wave meets a reflector, it returns to its source.
10:58Can you spot any connection between this reflected and incident wave?
11:02The reflected wave is, in fact, the wave that would have travelled forward if there were
11:06no reflector.
11:07First, of course, you have to fold this imaginary part 180 degrees, as shown.
11:15Now let's add one more reflector, this time at the side of the source.
11:20This will reflect the same way again and produce a third wave, and this process will repeat.
11:26However, if you keep the second reflector at the intersection point of the first and second
11:32waves, the third wave produced after the reflection will be the same as the first wave.
11:37This is a clever arrangement.
11:39When you arrange the second reflector this way, we will see only two waves travelling in
11:43opposite directions, instead of many reflected waves and chaos.
11:48If you find out the resultant of them, it will be a standing wave.
11:53This is a well-known fact.
11:54The standing waves get produced when the distance between the source and reflector is an integer
11:59multiple of half wavelength.
12:01Thus, the dimensions of the closed structure are determined by the wavelength of these waves.
12:07Now, a fun fact.
12:09Just measure the cavity length of the microwave oven in your kitchen.
12:12It will be an integer multiple of this wavelength.
12:16It is clear from this visual that some points of the standing wave are at high energy intensity,
12:22and some other points are at zero intensity.
12:25Due to this, there would be many spots in a microwave, some cold and others hot.
12:31Using cheese, you can demonstrate these cold and hot spots of your kitchen's microwave oven.
12:36Just keep the shredded cheese inside your microwave oven for one minute.
12:50What you see after one minute is the cheese surface with a few hot spots.
12:55The presence of such hot spots causes a microwave to cook food unevenly.
13:00In short, the cavity resonance technique we use to trap the microwaves more efficiently has
13:05led to creation of cold and hot spots.
13:08To overcome this problem nowadays, a microwave consists of a rotating plate, which helps the
13:13food cook evenly.
13:15This problem nowadays, a microwave consists of a rotating plate, which helps the food cook
13:20evenly.
13:23The component responsible for producing microwaves is known as a magnetron.
13:28A magnetron emits microwaves in all directions.
13:31To confine the wave to propagate in one dimension, the magnetron is attached to the waveguide.
13:39From the waveguide, the waves come into the cooking chamber to heat food.
13:44Another question that has to be answered is whether microwaves are the only electromagnetic
13:48waves capable of heating food, or if there are any other waves that could accomplish the
13:53same result.
13:54Any electromagnetic waves have the capability to heat food, but they come with certain limitations.
14:01Waves with long wavelengths can easily pass through our food so that they won't be able to transfer
14:05much energy to it.
14:07Additionally, to get standing wave, large devices would be required.
14:15Shorter wavelength waves are absorbed more rapidly on the outer surface of the food, so they do
14:20not penetrate far enough down to cook it evenly.
14:24If we want to cook deeply, we have to switch to a very high power source that would be unfeasible.
14:29From the microwave range, the suitable frequency for all practical purposes and which did not
14:34require a license was 2.45 GHz.
14:39The powerful microwaves produced by an oven can be hazardous to humans if we come in direct
14:44contact with them.
14:45But don't worry.
14:46The electromagnetic radiation produced by a microwave oven is always confined within
14:51it.
14:52Never leave the chamber.
14:54So there is no point in worrying about the health hazards caused by the electromagnetic
14:58radiation of microwave ovens.
15:01Now the most interesting question.
15:03Why is heating with a microwave oven superior to conventional heating methods?
15:08Since microwaves can penetrate the food, the food is cooked from the inside, slowly cooking
15:13at the surface.
15:14Moreover, it cooks food faster than other conventional methods.
15:19The convection method cooks food from the outside in as opposed to the inside out.
15:26This is because the heat energy has to travel from outside to inside.
15:30But this method can be useful on some occasions.
15:33When you need food with a crisp surface and a soft interior or baking, the convection heating
15:38method is preferred.
15:40Due to this reason, modern microwave ovens come with a convection option for baking purposes.
15:45And this method can access the light on these two different levels.
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