Download The applied field potential (E, volts) of electromagnetic radiation is

concern about phenomena of food in the microwave
About micro wave oven
oven. First, we put the food - just neither boiled nor
0341058 김성연 0340580 이재훈 0340104 송승관
heating - in the microwave oven. Next we click button on
the microwave oven, then food will boil or heat. But you
Introduction
take care of operation. When you put the food, you are
Microwave oven was fisrt invented during World War
2. At first, it was not used to cook or heat food. Two
scientists invented the magnetron, a tube that produces
microwaves. It was installed in a British radar to install
careful about food is not sealing up. If you do that, the
food of sealing up may explode. Because, the air in the
food of sealing up also heating with food, but the air
expand quickly. Then, how the air heating? The solution
is microwave!!. This is curiously thing. Now we discuss
Nazi airplanes.
of source of microwave oven, that is electromagnetic
Few years later Percy LeBaron Spencer of the
Raytheon
Company
accidently
discovered
wave.
that
microwaves also could cook food. He found that radar
waves had melted a candy bar in his pocket. And
Experiements showed that micorwave heating could
raise the internal temperature of many foods much faster
than conventional oven. So he invented the first
we've already know how moving charges produce
magnetic fields. A constant current produces a constant
magnetic field, while a changing current produces a
changing field. We can go the other way, and use a
magnetic field to produce a current, as long as the
magnetic field is changing. This is what induced emf is
microwave oven, 'Radar Range', in 1945.
Since then, because of it's convience, the concept of
quick microwave cooking had arrived. Other companies
had started to join the microwave oven market. So price
of micro ovens started to decrease while its capabilities
were expanded. Microwave oven had become one of
all about. A steadily-changing magnetic field can induce
a constant voltage, while an oscillating magnetic field
can induce an oscillating voltage.
Focus on these two facts. One is an oscillating electric
field generates an oscillating magnetic field. And an
oscillating magnetic field generates an oscillating electric
"prior-home-electro-machines".
Now, although microwave ovens are in every houses,
not many of us know exactly how they work. So we
begin to discuss what happens inside the oven and
field
Those
two
points
are
key
to
understanding
electromagnetic waves.
An electromagnetic wave propagates outwards from
analyze the function of divices in the microwave oven.
the source at the speed of light. What this means in
Discussion of Phenomena
practice is that the source has created oscillating electric
In our cooking time, we used to microwave oven for
convenience.
Comparison
the
heating
oven,
the
and magnetic fields, perpendicular to each other, that
travel away from the source. The E and B fields, along
microwave oven is more convenient. For example, we
with
being
perpendicular
to
each
other,
are
used to microwave oven for heating food. Then we just
perpendicular to the direction the wave travels, meaning
click button on the microwave oven. Now we have to
that an electromagnetic wave is a transverse wave. The
energy of the wave is stored in the electric and magnetic
mechanical relay or an electronic switch called a triac as
fields.
shown in Figure 2 . Sensing that all systems are "go,"
Analysis and Explanation
the control circuit generates a signal that causes the
relay or triac to activate, thereby producing a voltage
As shown in Figure 1, electricity from the wall outlet
path to the high-voltage transformer . By adjusting the
travels through the power cord and enters the microwave
on-off ratio of this activation signal, the control system
oven through a series of fuse and safety protection
can govern the application of voltage to the high-voltage
circuits. These circuits include various fuses and thermal
transformer, thereby controlling the on-off ratio of the
protectors that are designed to deactivate the oven in the
magnetron tube and therefore the output power of the
event of an electrical short or if an overheating condition
microwave oven. Some models use a fast-acting power-
occurs.
control relay in the high-voltage circuit to control the
output power.
In the
high-voltage section
If all systems are normal, the electricity passes through
Figure3, the high-voltage transformer along with a
to the interlock and timer circuits. When then oven door
special diode and capacitor arrangement serve to
is closed, an electrical path is also established through a
increase the typical household voltage, of about 115
series of safety interlock switches . Setting the oven
volts, to the shockingly high amount of approximately
timer and starting a cook operation extends this voltage
3000 volts!!! While this powerful voltage would be quite
path to the control circuits .
unhealthy -even deadly- for humans, it is just what the
magnetron tube needs to do its job -that is, to
dynamically convert the high voltage in to undulating
waves of electromagnetic cooking energy.
The microwave energy is transmitted into a metal
channel called a waveguide , which feeds the energy
into the cooking area where it encounters the slowly
revolving metal blades of the stirrer blade . Some models
use a type of rotating antenna while others rotate the
food through the waves of energy on a revolving
Generally, the control system includes either an electro-
carousel. In any case, the effect is to evenly disperse the
Hence the current (I, amperes) varies as
microwave energy throughout all areas of the cooking
compartment. Some waves go directly toward the food,
I = (dP/dt) = -Pmax.sin()
others bounce off the metal walls and flooring; and,
The power (P, watts) given out as heat is the average
thanks to special metal screen, microwaves also reflect
value of (current x potential). This is zero if there is no
off the door. So, the microwave energy reaches all
lag (i.e. if  = 0), otherwise
surfaces of the food from every direction.
All microwave energy remains inside the cooking cavity.
P = 0.5 PmaxEmax.sin()
When the door is opened, or the timer reaches zero, the
microwave energy stops, just as turning off a light switch
stops the glow of the lamp
It is convenient to express the dielectric constant in
terms of a complex number (r*,
= i) (dielectric
permittivity) defined as:
Assume that the e.m.wave goes through water. The
applied field potential (E, volts) of electromagnetic
r* = r´ - iLf
radiation is given by;
theory of dielectric heating
E = Emax.cos(
where Emax is the amplitude of the potential,  is the
angular frequency in radians.second-1 and t is the time
(seconds). If the polarization lags behind the field by the
phase (, radians)
tan() = loss current/charging current = Lf/r´
The terms (r*, r´, Lf ) are all affected by the frequency
of radiation; the relative permittivity (r´, dielectric
constant) at low frequencies (S, static regiond) and at
high ( visible) frequencies the (, optical permittivity) are
the limiting values. The relative permittivity changes with
the wavelength (and hence frequency):
then the polarization (P, coulombs) varies as
P = Pmax.cos(t - )
where  is the relaxation time (a measure of the time
required for water to rotate (
where r is the
molecular radius, k is the Boltzman constant and  is the
where Pmax is the maximum value of the polarization.
viscosity), also considered as the delay for the particles
to respond to the field change, or for reversion after
for heating efficienctly
disorientation. The maximum loss occurs when  = 1/ c.
For water at 25°C, is about 8 ps and r is half the
The electromagnetic penetration is infinite in a perfectly
(diffraction-determined) inter-oxygen distance (1.4 Å).
transparent substance and zero in reflective material (e.g.
metals). At the microwave oven frequency (2.45 GHz),
most energy is absorbed by water. The attenuation () is
given by:
This equation may be approximated where the
Figure 4.
attenuation is (approximately) directly proportional to the
Figure 4. Dielectric permittivity and dielectric loss of
loss factor and inversely proportional to the wavelength
water between 0°C and 100°C, the arrows showing the
times the square root of the relative dielectric constant:
effect of increasing temperature (data is indicative only
but based on [64, 135]; exact data is plotted below) or
increasing water activity. As the temperature increases,
the strength and extent of the hydrogen bonding both
decrease. This (1) lowers both the static and optical
dielectric permittivities, (2) lessens the difficulty for the
movement dipole and so allows the water molecule to
oscillate at higher frequencies, and (3) reduces the drag
For a plane wave, incident microwaves decrease to 1/e
(0.36788; i.e. 63% absorbed) in a penetration distance
Dp given approximately by::
to the rotation of the water molecules, so reducing the
:
friction and hence the dielectric loss. Most of the
dielectric loss is within the microwave range of
electromagnetic radiation (~1 - ~300 GHz). The
frequency for maximum dielectric loss lies higher than
the 2.45 GHz (0.0817 cm-1) produced by most
microwave ovens. This is so that the radiation is not
Thus, using water at 25°C, r´ = 78. Lf =12, tan = 0.15
and Dp = 1.4 cm. At 2.45 GHz
The amount of power (P, in watts m-3) that is absorbed is
given by:
P = 2f0LfE2
totally adsorbed by the first layer of water it encounters
where 0 = 8.854x10-12 F m-1, f is the frequency (Hz, =
and may penetrate further into the foodstuff, heating it
/2) and E is the potential gradient (V m -1).
more evenly; unabsorbed radiation passing through is
mostly reflected back, due to the design of the
microwave oven, and absorbed on later passes.
So most of emw oven uses 2.45Ghz frequency wave
Increse of haat per mininute
C : specific heat ρ :density