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3-1. What's the microwave? (1/2)
1. History of dielectric heating
In 1945, Dr. Spencer working for RAYTHEON Co. in the U.S.A found out a pieces of chocolate in his pocket melting suddenly by the
electric wave (ultra high frequency wave using for radar) on his experiment for radar. then began research in heating matters by the
electric wave.
So far, experiments for heating foods by the electric wave has been developed into the household telephone as well as microwave
oven, etc. The heating method for microwave is to give an electric wave to the food and make the food itself generate heat. It is
different from that of the past which was to give heat to the food and make the temperature rise.
So, since the microwave oven does not give any heat to the food, we do not need appliances like the gas oven.
Not only the microwave oven itself but utensil don't get hot either, when heat is provided, the utensil will gradually got hot by thermal
conduction, however, the temperature of the surface and the inside of the fond get hot simultaneously and the food is well-done rapidly.
2. The principle of dielectric heating
The microwave oven has a vacuum tube called magnetron (M.G.T), it injects the electric wave of 2450MHz (The vibrations of two
billion four hundred and fifty million per second are given to the oven) This electric wave is called the microwave. The wireless electric
wave (having not more than 1 m of the wave length) is using for the radio telephone and broad casting of T.V. and radar (a radio locator).
It is similar to an electric wave and its wave length is short. (The length of the electric wave having frequency of 2450MHz is 12.2cm)
The characteristics of this microwave is as follows:
1. When it touches the metal, it is reflected and changed direction.
2. It passes through ceramics, porcelain (glass, etc), plastics, and paper.
3. If it is touches the food or the wood having water in humidity, it is absorbed and generates heat.
The Microwave oven cook food using these characteristics. Generally, the food, whether it is large or small has humidity. So if you put
this food in the microwave oven and it gets the electric wave, so the energy of it is absorbed in the food since it set up a vibration of
the molecules with a frequency two billion four hundred and fifty million times per second. The friction among molecules of food occurs
repeatedly. It then causes the frictional heat, and the temperature of the food rises.
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3-1. What's the microwave? (2/2)
3. The safety of microwave oven
Since the microwave oven can't make the native fire like gas, you have no use for worrying about the fire.
Because it doesn't become hot like the ordinary electric range. It has been used in the dinner car of the trains etc.
Microwave oven has the shielded structure strictly with metal case not to leak electric wave at once not only using but on opening
the door.
Because it uses the electric wave, the x-ray is never emitted. The UHF wave generally called microwave is like the electric wave of
the TV or the radio and is used for the wireless communication, the radar and the UHF medical instruments (using 2450 MHz like
that of the microwave oven, etc).
4. Definition of electric wave
* An electric wave is the electron wave of the frequency from 10 KHz to 3,000,000 MHz.
* Wave length of 2450 MHZ.
c 300000 Km
0.122 m 12.2 cm
f 245000 KHz
: Wave Length
Where c: The Velocity of Light
f: Frequency
The division from the sound frequency to the light frequency.
20 Hz 3 Hz 30 Hz 300 KHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz
FREQ - - - - - - - - - -
20 KHz 30 KHz 300 KHz 3 MKHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz 1016 Hz
NAME SOUND VLF LF MF HF VHF ULF SHF EHF LIGHT
WAVE
USE MUSIC SHIP RADIO RADIO RADIO T.V MWO RADAR RADAR
T.V
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3-2. Conventional heating vs. dielectric heating
1. Conventional heating 2. Dielectric heating
FIRE!
FIRE! 1) The heating is uniform.
FIRE!
2) The temperature rise is fast (high).
FIRE! HEAT! FIRE!
3) If you break the electric power: The microwave oven stops
! HEAT!
HEAT heating at once, so you can't heat something with the heat
! HE
AT
HE WELL-DONE! AT
! of their surroundings.
4) Since it heats inside, the surface isn't damaged.
!
AT
HALF-DONE!
HE
FIRE! FIRE!
HE
AT
UNDER! WAVE
!
DONE! WAVE
!
AT
HE
WAVE
WAVE
HE
FIRE! FIRE!
AT
WAVE WAVE
!
HE !
AT AT
! HE
HEAT !
FIRE! ! HEAT! HEAT
FIRE! MOLECULES
FOOD!
WAVE
FIRE! FIRE!
FIRE!
TRANSMITTED!
BY CONDUCTION!
WAVE
WAVE
It shall be heated by the sources of the electric heater, the gas, and the briquet.!
WAVE
FOOD
* The heat energy from the sources of the heating power shall be transmitted WAVE WAVE
from the warm surface to the cold inside. WAVE
* Heating with the strong fire: The surface of the food shall be well done, but be
burnt to have a black side, and the center of the food shall not be well-done.
* Heating with the weak fire: The surface of the food shall be burnt and have a
little black side, but it shall be taken lots of time for the center to be well-done.