principle of Microwave oven?

Answer 1

The microwave Oven works on the principle of electrical heating more precisely saying it's dielectric heating.A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating.

Answer 2

The microwave Oven works on the principle of electrical heating more precisely saying it's dielectric heating.A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz, through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Working: Microwave ovens use various combinations of electrical circuits and mechanical devices to produce and control an output of microwave energy for heating and cooking. Generally speaking the systems of a microwave oven can be divided into two fundamental sections, the control section and the high-voltage section . The control section consists of a timer (electronic or electromechanical), a system to control or govern the power output, and various interlock and protection devices. The components in the high-voltage section serve to step up the house voltage to high voltage. The high voltage is then converted microwave energy. Basically, here is how it works: electricity from the wall outlet travels through the power cord and enters the microwave oven through a series of fuse and safety protection circuits. These circuits include various fuses and thermal protectors that are designed to deactivate the oven in the event of an electrical short or if an overheating condition occurs If all systems are normal, the electricity passes through to the interlock and timer circuits. When then oven door is closed, an electrical path is also established through a series of safety interlock switches . Setting the oven timer and starting a cook operation extends this voltage path to the control circuits . Generally, the control system includes either an electromechanical relay or an electronic switch called a triac. Sensing that all systems are "go," the control circuit generates a signal that causes the relay or triac to activate, thereby producing a voltage path to the high-voltage transformer . By adjusting the on-off ratio of this activation signal, the control system can govern the application of voltage to the high-voltage transformer, thereby controlling the on-off ratio of the magnetron tube and therefore the output power of the microwave oven. Some models use a fast-acting power-control relay in the high-voltage circuit to control the output power. In the high-voltage section , the high-voltage transformer along with a special diode and capacitor arrangement serve to increase the typical household voltage, of about 115 volts, to the shockingly high amount of approximately 3000 volts! While this powerful voltage would be quite 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 carousel. In any case, the effect is to evenly disperse the microwave energy throughout all areas of the cooking compartment. Some waves go directly toward the food, others bounce off the metal walls and flooring; and, thanks to special metal screen, microwaves also reflect off the door. So, the microwave energy reaches all surfaces of the food from every direction. All microwave energy remains inside the cooking cavity. 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

Answer 3

This is due to resonance.

Every object and material has its own particular 'natural frequency'. When this object/material encountes a wave having a frequency similar to its natural frequency, it starts vibrating heavily.

Ever heard about champagne glasses being shattered by high pitched voices? That comes about since the singer emits a sound wave with a frequency similar to the natural frequency of the glass. The latter vibrates heavily and shatters.

Now, the microwave uses the same principle. Water molecules have a natural frequency of 2450MHz. The magnetron inside the microwave emits electromagnetic waves at this frequency. The water molecules inside the food resonate. Due to their heavy vibrations, their temperature rises and they heat up the surrounding food.

In fact, a completely dehydrated piece of food will never heat up inside a microwave oven!

Answer 4

Short wavelength (but longer than infrared) radiation causes some molecules to vibrate faster ... which is the same as saying they get hotter.

The radiation is much like radar. There's a big "tube"-like element in the microwave that puts out the radiation. There's also a "stirrer" in there to keep the pattern of the radiation changing ... helps cook more evenly.

Answer 5

A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency higher than ordinary waves but lower than infrared light. Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are reserved for this use, so they don't interfere with other vital radio services. Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in)—while large industrial/commercial ovens often use 915 megahertz (MHz)—32.8 centimetres (12.9 in). Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e., as both potential energy and kinetic energy of atoms), is heat. Sometimes, microwave heating is explained as a resonance of water molecules, but this is incorrect; such resonances occur only at above 1 terahertz (THz).
Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted. Dielectric heating of liquid water is also temperature-dependent: At 0 °C, dielectric loss is greatest at a field frequency of about 10 GHz, and for higher water temperatures at higher field frequencies.
Compared to liquid water, microwave heating is less efficient on fats and sugars (which have a smaller molecular dipole moment). Sugars and triglycerides (fats and oils) absorb microwaves due to the dipole moments of their hydroxyl or ester groups. However, due to the lower specific heat capacity of fats and oils and their higher vaporization temperature, they often attain much higher temperatures inside microwave ovens. This can induce temperatures in oil or very fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional broiling (UK: grilling) or deep fat frying. Foods high in water content and with little oil rarely exceed the boiling temperature of water.
Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also has dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of synthetic lava. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.
A common misconception is that microwave ovens cook food "from the inside out", meaning from the centre of the entire mass of food outwards. This idea arises from heating behaviour seen if an absorbent layer of water lies beneath a less absorbent drier layer at the surface of a food; in this case, the deposition of heat energy inside a food can exceed that on its surface. This can also occur if the inner layer has a lower heat capacity than the outer layer causing it to reach a higher temperature, or even if the inner layer is more thermally conductive than the outer layer making it feel hotter despite having a lower temperature. In most cases, however, with uniformly structured or reasonably homogenous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast to broiling/grilling (infrared) or convection heating—methods which deposit heat thinly at the food surface. Penetration depth of microwaves is dependent on composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating further.
The previous paragraph notwithstanding, the interior of small food items can reach a higher temperature than the surface because the interior is thermally insulated from the air. It is possible to burn the inside of a cookie while the exterior remains em-browned.

Answer 6

go to http://home.howstuffworks.com/microwave.htm