* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The applied field potential (E, volts) of electromagnetic radiation is
Survey
Document related concepts
Magnetic core wikipedia , lookup
Terahertz metamaterial wikipedia , lookup
Opto-isolator wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Rectiverter wikipedia , lookup
Index of electronics articles wikipedia , lookup
Mathematics of radio engineering wikipedia , lookup
Superconductivity wikipedia , lookup
Wireless power transfer wikipedia , lookup
Waveguide (electromagnetism) wikipedia , lookup
Cavity magnetron wikipedia , lookup
Transcript
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 = 2f0LfE2 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