Physics Investigatory Project Class 12

November 30, 2017 | Author: abhishek | Category: Microwave, Chemical Polarity, Vacuum Tube, Electromagnetism, Nature
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physics class 12 investigatory project for cbse board. this project is well edited and almost redy for submission on top...


CONTENTS  Certificate  Acknowledgement  Introduction  Principle  Design  Power Consumption  Heating Efficiency

 Microwave Safe Materials  Effects on Food and Nutrients  Hazards  Bibliography

CERTIFACTE This is to certify that this investigatory physics project on MICROWAVE OVEN is done under my guidance and presence by ABHISHEK AGRAWAL of class XII C within the sripulated time in academic year 2014-15.

Subject Teacher (Dr. Alka Gupta)


It is my duty to record my sincere thanks and deep sense of gratitude to my respected teacher Dr. Alka Gupta for his valued guidance , interest and constant encouragement. I would alsopay gratitude to our lab assistant Mr.Ramesh Gaba for the fulfillment of the project.

Introduction A microwave oven, often colloquially shortened to microwave, is a kitchen appliance that heats food by bombarding it with electromagnetic radiation in the microwave spectrum causing polarized molecules in the food to rotate and build up thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25– 38 mm (1–1.5 inches) of a dense (high water content) food item; food is more evenly heated throughout (except in thick, dense objects) than generally occurs in other cooking techniques. Percy Spencer invented the first microwave oven after World War II from radar technology developed during the war. Named the "Radarange", it was first sold in 1946. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, which was acquired in 1965 by Raytheon. Microwave ovens are popular for reheating previously cooked foods and cooking vegetables. They are also useful for rapid heating of otherwise slowly prepared cooking items, such as hot butter, fats, and chocolate. Unlike conventional ovens, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to produce Maillard reactions. Exceptions occur in rare cases where the oven is used to heat frying-oil and other very oily items (such as bacon), which attain far higher temperatures than that of boiling

water. The boiling-range temperatures produced in high-water-content foods give microwave ovens a limited role in professional cooking, since it usually makes them unsuitable for achievement of culinary effects where the flavors produced by the higher temperatures of frying, browning, or baking are needed. However, additional heat sources can be added to microwave ovens, or into combination microwave ovens, to produce these other heating effects, and microwave heating may cut the overall time needed to prepare such dishes. Some modern microwave ovens may be part of an over-the-range unit with built-in extractor hoods.

Principle 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 radio 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 centimeters (4.80 in)—while large industrial/commercial ovens often use 915 megahertz (MHz)—32.8 centimeters (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 The electric dipole consists of two charges of equal magnitude but opposite sign separated by a distance 2a, as shown in Figure.

The electric dipole moment of this configuration is defined as the vector p directed from –q to +q along the line joining the charges and having magnitude 2aq: p=2 qa

Now suppose that an electric dipole is placed in a uniform electric field E, as shown in Figure 26.20. We identify E as the field external to the dipole, distinguishing it from the field due to the dipole. The field E is established by some other charge distribution, and we place the dipole into this field. Let us imagine that the dipole moment makes an angle θ with the field.

The electric forces acting on the two charges are equal in magnitude but opposite in direction as shown in Figure 26.20 (each has a magnitude F = qE) Thus, the net force on the dipole is zero. However, the two forces produce a net torque on the dipole; as a result, the dipole rotates in the direction that brings the dipole moment vector into greater alignment with the field. The torque due to the force on the positive charge about an axis through O in Figure 26.20 is Fa sin θ, where a sin θ is the moment arm of F about O. This force tends to produce a clockwise rotation. The torque about O on the negative charge also is Fa sin θ ; here again, the force tends to produce a clockwise rotation. Thus, the net torque about O is Because













It is convenient to express the torque in vector form as the cross product of the vectors p and E:

We can determine the potential energy of the system of an electric dipole in an external electric field as a function of the orientation of the dipole with respect to the field. To do this, we recognize that work must be done by an external agent to rotate the dipole through an angle so as to cause the dipole moment vector to become less aligned with the field. The work done is then stored as potential energy in the system of the dipole and the external field. The work dW required to rotate the dipole through an angle dθ is dW=τ dθ (Eq. 10.22). Because τ = pE sinθ and because the work is transformed into potential energy U, we find that, for a rotation from θi to θf , the change in potential energy is

The term that contains cos θi is a constant that depends on the initial orientation of the dipole. It is convenient for us to choose θi = 90°, so that cos θi = cos 90° = 0. Furthermore, let us choose Ui = 0 at θi = 90° as our reference of potential energy. Hence, we can express a general value of U = U f as We can write this expression for the potential energy of a dipole in an electric field as the dot product of the vectors p and E:

In this case, once we rotate the dipole through angle θ, the system tends to return to the original configuration when the object is released. The dipole begins to rotate back toward the configuration in which it was aligned with the field. Molecules are said to be polarized when a separation exists between the average position of the negative charges and the average position of the positive charges in the molecule. In some molecules, such as water, this condition is always present— such molecules are called polar molecules. Molecules that do not possess a permanent polarization are called nonpolar molecules.

We can understand the permanent polarization of water by inspecting the geometry of the water molecule. In the water molecule, the oxygen atom is bonded to the hydrogen atoms such that an angle of 105° is formed between the two bonds (Fig. 26.21). The center of the negative charge distribution is near the oxygen atom, and the center of the positive charge distribution lies at a point midway along the line

joining the hydrogen atoms (the point labeled X in Fig. 26.21). We can model the water molecule and other polar molecules as dipoles because the average positions of the positive and negative charges act as point charges. As a result, we can apply our discussion of dipoles to the behavior of polar molecules. Dipole Moments of different substances in food Water p = 6.2 × 10-30 Cm Sunflower oil p = 2.007 × 10-30 Cm Olive oil p = 1.957 × 10-30 Cm Palm




1.926 × 10-30


Microwave ovens take advantage of the polar nature of the water molecule. When in operation, microwave ovens generate a rapidly changing electric field that causes the polar molecules to swing back and forth, absorbing energy from the field in the process. Because the jostling molecules collide with each other, the energy they absorb from the field is converted to internal energy, which corresponds to an increase in temperature of the food.

Design A microwave oven consists of:  A high voltage power source, commonly a simple transformer or an electronic power converter, which passes energy to the magnetron  A high voltage capacitor connected to the magnetron, transformer and via a diode to the chassis  A cavity magnetron, which converts high-voltage electric energy to microwave radiation  A magnetron control circuit (usually with a microcontroller) A short waveguide (to couple microwave power from the magnetron into the cooking chamber) A metal cooking chamber A turntable or metal fan Modern microwave ovens use either an analog dial-type timer or a digital control panel for operation. Control panels feature an LED, liquid crystal or vacuum fluorescent display, numeric buttons for entering the cook time, a power level selection feature and other possible functions such as a defrost setting and preprogrammed settings for different food types, such as meat, fish, poultry, vegetables, frozen vegetables, frozen dinners, and popcorn. In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. As such, the choice of power level does not affect the intensity of the microwave radiation; instead, the magnetron is cycled on and off every few seconds. Newer models have inverter power supplies that use pulse width modulation to provide effectively continuous heating at reduced power, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating. The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the industrial, scientific, and medical (ISM) frequency bands set aside for noncommunication purposes. For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in the ITU Region 2 while 2.45 GHz is available worldwide. Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8 GHz and 24.125 GHz, but are not used for microwave cooking because of the very high cost of power generation at these frequencies. The third, centered on 433.92 MHz,is a narrow band that would require expensive equipment to generate sufficient power without

creating interference outside the band, and is only available in some countries. The cooking chamber is similar to a Faraday cage (but there is no continuous metal-tometal contact around the rim of the door), and prevents the waves from coming out of the oven. The oven door usually has a window for easy viewing, with a layer of conductive mesh some distance from the outer panel to maintain the shielding. Because the size of the perforations in the mesh is much less than the microwaves’ wavelength (12.2 cm for the usual 2.45 GHz), most of the microwave radiation cannot pass through the door, while visible light (with its much shorter wavelength) can. The production of electromagnetic waves of a small enough wavelength (microwaves) is done with cavity magnetron. The cavity magnetron is a highpowered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities (cavity resonators). Bunches of electrons passing by the openings to the cavities excite radio wave oscillations in the cavity, much as a guitar's strings excite sound in its sound box. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions. Unlike other microwave tubes, such as the klystron and traveling-wave tube (TWT), the magnetron cannot function as an amplifier, increasing the power of an applied microwave signal, it serves solely as an oscillator, generating a microwave signal from direct current power supplied to the tube.

Power Consumption A microwave oven converts only part of its electrical input into microwave energy. An average consumer microwave oven consumes 1100 W of electricity in producing 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits. Such wasted heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents. For cooking or reheating small amounts of food, the microwave oven may use less energy than a cook stove. Although microwave ovens are touted as the most efficient appliance,[19] the energy savings are largely due to the reduced heat mass of the food’s container.[20] The amount of energy used to heat food is generally small compared to total energy usage in typical residences in the United States.

Heating Efficiency 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 groups 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 have 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 center of the entire mass of food outwards. This idea arises from heating behavior 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 food 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 unbrowned.

Microwave Safe Materials In most home kitchens, we'll find an assortment of different materials used in our containers ranging from glass to plastic to ceramic to metals. But are all food grade materials similar? Should we care if a container is marked Microwave Safe? Let's take a closer look at some common materials used in food containers and if they are microwave safe.

Glass & Ceramics Glass containers are often marked microwave safe. These containers can be heated in a microwave without a problem. The issue with glass that is not microwave safe is that micro-air bubbles may be present in the glass and as the glass heats in the microwave oven, the bubbles may expand to the point where the glass breaks or shatters. (Obviously, you shouldn't eat food where it's glass container has broken.) Pyrex glassware is an excellent example of microwave safe, heat resistant glass that can also be baked. Even Pyrex glass cannot withstand the intensity of direct heat, such as a range or a broiler, for long, so don't use glassware with such heating methods. Also, all glass is susceptible to thermal cooling shock (rapid cooling, for example, dunking in cold water while hot) and may crack. One popular method of testing if a particular glass is microwave safe is to microwave the container while it is empty for one minute. If the container is hot, then it is not microwave safe. If the container is warm, it should be fine for heating food. If the container remains cool, you can cook in the microwave with that container.

Any glass container with a metallic trim should never be microwaved. The electrical currents induced by the microwave radiation in the metals can cause sparking and pinpoint heating of the glass. Sometimes this can result in marring or even breaking of the glassware. Also, make sure any glassware with a colorful coating, finish, or stain should be marked for use with food or microwave safe before attempting to use in food preparation. The dyes, pigments, or stains may not be food grade. Almost always, decorative plates are not for use with food. Food safe ceramic uses glazes that are made from harmless materials like silica, dolomite, kaolin, feldspar, ball clay, and others. In these glazes, the inevitable leaching that occurs is only a functional and aesthetic issue and has no health impact. Glazes that contain metals such as lithium, lead, or barium may present a health issue. Ceramic containers made with such glazes cannot be sold in the United States without either a permanent marking stating it is "Not for Food Use May Poison Food" or have a hole in the container (presumably rendering it useless for food preparation). If you make your own ceramics, make sure you use a food grade glaze if you plan to use it in your kitchen. Plastics Food grade plastics are made from a specific list of plastics approved by the FDA (which may include dyes and recycled plastic that have not been determined to be harmful to humans). Once a plastic container has been used to store non-food items (like detergent or paint), it can no longer be considered food grade. Plastics containers that are not food grade may leach plasticizers into food on contact. Due to the nature of plastics, they have a high affinity for fats. Plastics that come in contact with an oil-based substance will almost always be irrevocably altered and the plastic may never become truly clean once again. Contact to foods that are high in fat may cause leaching of the original oil-based substance into the food even if the plastic was originally food grade. Microwave safe plastics are food grade plastics (which do not leach plasticizers) that are known to be able to withstand higher than normal temperatures. Plastics that are not microwave safe may leach harmful substances when heated in a microwave oven. (There was an internet e-mail scare/hoax that was passed around

claiming the USDA or FDA and independent researchers showed that dioxin (a plasticizer) leached out of plastic wrap onto food being microwaved. This is untrue since all microwave safe plastics are dioxin free. Saran and Ziploc both maintain that their product lines are completely plasticizer free. The temperatures necessary to create dioxin (around 1500°F) are beyond the normal operating conditions of household microwave ovens.) Lexan is a food grade polycarbonate plastic that has gained a large following in the food service community. It is hard, durable, and resistant to reacting with oils resulting in a virtually stain and odor proof material. It is capable of handling a range of temperatures from below freezing to boiling. Recently, Sierra magazine published a report claiming that polycarbonate plastics leach an endocrine disruptor called Bisphenol-A (BPA). Unfortunately, the studies the article was based on cannot be directly related to use in the food industry since the tests were performed on non-food grade polycarbonate mouse cages (which affected the growth cycle of the mice). No evidence of food grade polycarbonates (such as Lexan) being a health hazard has been uncovered. The S.C. Johnson Company says that the larger Ziploc brand bags are microwave safe. All Ziploc bags are made of microwave safe materials, but bags smaller than 1 quart size may be too thin to withstand the temperature of the food being microwaved.

Effects on food and nutrients Several studies have shown that microwaves negatively impact food’s nutritional value. Some excellent scientific data has been gathered regarding the detrimental effects of microwaves on the nutrients in your food:  A study published in the November 2003 issue of The Journal of the Science of Food and Agriculture5 found that broccoli "zapped" in the microwave with a little water lost up to 97 percent of its beneficial antioxidants. By comparison, steamed broccoli lost 11 percent or fewer of its antioxidants. There were also reductions in phenolic compounds and glucosinolates, but mineral levels remained intact.  A 1999 Scandinavian study of the cooking of asparagus spears found that microwaving caused a reduction in vitamin C.  In a study of garlic, as little as 60 seconds of microwave heating was enough to inactivate its allinase, garlic's principle active ingredient against cancer.  A Japanese study by Watanabe showed that just 6 minutes of microwave heating turned 30-40 percent of the B12 in milk into an inert (dead) form.  A recent Australian study9 showed that microwaves cause a higher degree of "protein unfolding" than conventional heating.  Microwaving can destroy the essential disease-fighting agents in breast milk that offer protection for your baby. In 1992, Quan found that microwaved breast milk lost lysozyme activity, antibodies, and fostered the growth of more potentially pathogenic bacteria  Another study about breast milk/infant formula by Lee in 198911 found vitamin content becomes depleted by microwaving, and certain amino acids are converted into other substances that are biologically inactive. Some altered amino acids are poisons to the nervous system and kidneys. Other studies show that, if properly used, microwave cooking does not affect the nutrient content of foods to a larger extent than conventional heating, and that there is a tendency towards greater retention of many micronutrients with microwaving, probably due to the reduced preparation time.  Nutrients are leached from food during any form of cooking, especially when the food is cooked for a long period of time with high amounts of water. So, for example, boiling carrots might strip nutrients much more

drastically than microwaving them, because the carrots’ nutrients might get washed away with the boiling water.  Availability of some amino acids in food materials might be improved, since no surface browning occurs. And microwave heating might lead to destruction of trypsin inhibitors in beans.  Microwave cooking forms fewer nitrosamines and AGEs on meats than conventional cooking methods.  Spinach retains nearly all its folate when cooked in a microwave; in comparison, it loses about 77% when boiled, leaching out nutrients.

 Bacon cooked by microwave has significantly lower levels of carcinogenic nitrosamines than conventionally cooked bacon.  Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.  Microwave blanching is 3-4 times more effective than boiled water blanching in the retaining of the water soluble vitamins folic acid, thiamin and riboflavin, with the exception of ascorbic acid, of which 28.8% is lost (vs. 16% with boiled water blanching).

Hazards Homogeneous liquids can superheat[38][39] when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start explosively when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven. Closed containers, such as eggs, can explode when heated in a microwave oven due to the increased pressure from steam. Insulating plastic foams of all types generally contain closed air pockets, and are generally not recommended for use in a microwave, as the air pockets explode and the foam (which can be toxic if consumed) may melt. "Hot spots" in microwaved food can be hot enough to cause burns—or build up to a "steam explosion." This has resulted in admonitions to new mothers about NOT using the microwave to heat up baby bottles, since babies have been burned by super-heated formula that went undetected. Another problem with microwave ovens is that carcinogenic toxins can leach out of your plastic and paper containers/covers, and into your food. Any object containing pointed metal can create an electric arc (sparks) when microwaved. This includes cutlery, crumpled aluminium foil (though some foil used in microwaves are safe, see below), twist-ties containing metal wire, the metal wire carry-handles in paper Chinese take-out food containers, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape. The electric arc has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter (3×106 V/m). The air forms a conductive plasma, which is visible as a spark. When dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are unhealthy in large quantities. The effect of microwaving thin metal films can be seen clearly on a Compact Disc or DVD (particularly the factory pressed type). The microwaves induce electric currents in the metal film, which heats up, melting the plastic in the disc and leaving a visible pattern of concentric and radial scars. Similarly, porcelain with thin metal films can also be destroyed or damaged by microwaving.

Direct microwave exposure is not generally possible, as microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which the oven is constructed. Furthermore, ovens are equipped with redundant safety interlocks, which remove power from the magnetron if the door is opened. The radiation produced by a microwave oven is nonionizing. It therefore does not have the cancer risks associated with ionizing radiation such as X-rays and highenergy particles. Longterm rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from 2.45 GHz microwave radiation even with chronic exposure levels, i.e., large fraction of one’s life span, far larger than humans are likely to encounter from any leaking ovens.[48][49] However, with the oven door open, the radiation may cause damage by heating. One of the worst contaminants is BPA, or bisphenol A, an estrogen-like compound used widely in plastic products. In fact, dishes made specifically for the microwave often contain BPA, but many other plastic products contain it as well. Some magnetrons have ceramic insulators with beryllium oxide (beryllia) added. The beryllium in such oxides is a serious chemical hazard if crushed and ingested (for example, by inhaling dust). In addition, beryllia is listed as a confirmed human carcinogen by the IARC; therefore, broken ceramic insulators or magnetrons should not be handled. This is obviously a danger only if the microwave oven becomes physically damaged, such as if the insulator cracks, or when the magnetron is opened and handled directly, and as such should not be a concern during normal usage.

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