Course title : Analytical Instrumentation Presentation title: Microwave oven Submitted to: Mr. Dessie Fentaw Prepared by ID.No Yeshiwas Fetene …………..……………………….…….PGConR/009/10 1

 Introduction  Microwaves  Microwave oven  History of invention Microwave oven  Working principle of microwave oven  Advantage and disadvantage  Conclusion  Reference 2

 To cook something we have to make in contact with some heat sources.  But today, Instead of heating food products, we just push the button and cook easily by using a microwave oven.  From its name we just know that it uses microwaves.  Originally, microwaves were used for communication. In 1950, the use of microwave energy to heat materials was discovered. Now microwave ovens have became common for heating food products in the home.  The most prominent characteristic of microwave heating is volumetric heating, which is quite different from conventional heating where the heat must diffuse in from the surface of the material.  Volumetric heating means that materials can absorb microwave energy directly and internally and convert it to heat.  It is this characteristic that leads to advantages using microwaves to process materials. 3

 Microwaves are a form of electromagnetic wave, like light waves or radio waves, and occupy a part of the electromagnetic spectrum.  Microwaves are used extensively in communications such as to relay long- distance telephone signals, television programs and computer information across the earth or to a satellite in space.  They are good for transmitting information because it can penetrate light rain and snow, clouds, and smoke.  Also used in radars and in detecting speeding cars.  Microwave has become most familiar as the energy source for cooking food. 4

5 Satellite communication Communication Tower Imaging Remote Mobiles MICROWAVE OVEN

 Microwaves are defined as a part of electromagnetic waves which have frequency range between 300 MHz and 300 GHz corresponding to wavelength from 1mm to 1m. 6

Microwaves can pass through Glass, Paper, Plastic & Organic materials. Microwaves are reflected by metal surfaces. Microwaves penetrate and are absorbed by some substances like fats, sugar, water molecules. 7

 Inventor Percy Lebaron Spencer  Born in Howland, Maine on July 9,  Thought by poverty.  Invented Microwave oven accidentally when he noticed peanut chocolate bar in his pocket melted while he was working in active radar set.

 A microwave oven, or a microwave, is a kitchen appliance that cooks or heats food by dielectric heating.  This is accomplished by using microwave radiation to heat water and other polarized molecules within the food.  This excitation is fairly uniform, leading to food being adequately heated throughout (except in thick objects), a feature not seen in any other heating technique. 9

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 A microwave oven works as follows: 1)Electrical energy, in the form of low-voltage alternating current is fed to the transformer where it is transformed to the required level.  Next the voltage from transformer is converted into direct current with rectifying circuit.  Other extra 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.  When the oven door is closed, an electrical path is also established through a series of safety interlock switches 11

2) A magnetron uses direct current from 1 and generates microwaves with a frequency of 2450 mega cycles per second or 2.45 GHz (gigahertz).  Magnetron is the heart of a microwave oven is the magnetron. 12

 Inside the magnetron, electrons are emitted from a central terminal called a cathode.  A positively charged anode surrounding the cathode attracts the electrons.  Instead of traveling in a straight line, permanent magnets force the electrons to take a circular path.  As they pass by resonating cavities, they generate a continuous pulsating magnetic field, or electromagnetic radiation. 13

3) The microwaves are directed by an antenna at the top of the magnetron into a waveguide.  The waveguide channels microwaves to a device called a stirrer which disperses them inside the oven cavity.  Mode stirrer is a metallic fan blade used to perturb the field distribution inside the oven with a motorized platter. The food is also rotated with a motorized plate 4) The microwaves then reflect off the metal walls of the oven's interior and are absorbed by molecules in the food.  The microwaves that penetrate the food have an electric field that oscillates 2.45 billion times a second, a frequency that is well absorbed by polar liquid molecules such as water, sugars, fats and other food molecules. 14

5) Once microwave energy is absorbed, polar molecules such as water molecules inside the food will rotate according to the alternating electromagnetic field.  The water molecule is a “dipole” with one positively charged end and one negatively charged end. Similar to the action of magnet, these “dipoles” will orient themselves when they are subject to electromagnetic field.  The rotation of water molecules would generate heat for cooking.  Hence the composition of a food will affect how it will be heated up inside the microwave oven.  Food with higher moisture content will be heated up faster because of the dipolar interaction. 15

 Structure of water molecule  When there is no external electric field.  When placed in external electric field, dipole will turn towards the electric field. 16

 When microwave is irradiated to the permanent dipole. In this case, the permanent dipole changes a little behind the electric field.  During the time delay, water is absorbing energy from micro wave and generates heat. 17

advantages Cooking time is short Uniform cooking No physical change of foods Melting process is easy Sterilization effect exists There is no flame, then treatment is easy. disadvantages Constraint with metal container Heat force control is difficult Water evaporation in closed container is dangerous because it could be burst. Microwave ovens can be harmful if you stand directly in front of them, while they are on 18

 The successful applications of microwave heating technology for processing of various foods can ease life by shortening cooking time.  The microwave heating technology for pasteurization and sterilization contributed to effectively destroy pathogenic microorganisms and significantly reduce processing time without serious damage in overall quality of liquid food as compared to traditional methods.  The use of microwave heating for food processing applications such as blanching, cooking, and baking has a great effect on the preservation of nutritional quality of food.  Microwave technology is now being seriously considered as a viable energy source in process heating.  Safety should be given priority while working with microwave ovens. 19

1) B. Kwon Lee, Y. Cho, “THE PRINCIPAL OF MICROWAVE OVEN AND MICROWAVE HEATING ”, Yonsei University, OCT ) Y. Omosun, “Microwave Application for food Processing”, Michael Okpara University of Agriculture, Umudike, July ) The Microwave Heating Principle ; February 21, 2017, Available from: URL: heating 4) Microwave ovens ; Available from: URL: 5) Pros and Cons of Cooking In The Microwave, Available from: URL: disadvantages/Pros and Cons of Cooking In The Microwave disadvantages/ 20

THANK YOU !!! 21