Microwaves and Electromagnetic Radiation - Essay Example

Introduction Electromagnetic spectrum is a continuum of electromagnetic waves arranged according to frequency and wavelength. It comprises of; visible light, microwaves, radio waves, ultra-violet rays, X-rays and gamma rays. This set of waves is produced as a result of energy changes in electrically charged particles. Electromagnetic spectrum comprises of both electric and magnetic fields and the propagation of these waves are at right angles with both electric and magnetic fields (Gupta & Eugene, 2007). Additionally, these waves travel at speeds of light and there has been an illusion that light itself is an electromagnetic wave. These waves are classified based on the wavelengths. A wave with the longest wavelength has the shortest frequency. Microwaves have frequencies ranging from 300 megahertz to 300 Gigahertz and wavelengths ranging from 1 millimetre to 1 metre (Gupta & Eugene, 2007). The term micro-wave was first cited in the electromagnetic waves context in 1931 in a publication called the International Telephone and Telegraph (Sobol & Tomiyasu, 2002). Micro-wave was used to describe a two way radio communication system linking Dover in the U.K. and Calais in France (Sobol & Tomiyasu, 2002). Microwaves have been assorted into three groups based on frequency. These three bands are the 300MHZ to 3GHZ band (Ultra high frequency (UHF)), the 3GHZ to 30GHZ band (Super High Frequency (SHF)) and the 30GHZ to 300GHZ band (Extremely High Frequency (EHF)) (Gupta & Eugene, 2007). Progressive research has led to the discovery of numerous applications of these waves. Currently, Micro-waves have many applications including; food cooking by use of micro-wave ovens, Radar detection technology, Television broadcasting, medical treatment (diathermy) and in the mobile and telecommunications industry. All these applications use the properties of micro-waves; reflection, absorption and transmission. The fact that microwaves are totally reflected by metals and absorbed by non-metals (WHO, 2005) has aided in the design of microwave ovens. This essay will focus on how a microwave oven works and the application of microwaves in food cooking in micro-wave ovens. Microwave ovens are a common appliance in modern day kitchens supplementing modern stoves or cookers. They were originally used in commercial setups before being designed for domestic uses. A common feature of these ovens which is an advantage over other cooking stoves is the cooking speed. There has been an uncertainty in the discovery of use of microwaves in cooking. A common narrative is that of Percy Spencer an engineer who moved past a microwave source in 1945 and realised that a chocolate bar he was having started melting (Marshall Cavendish Corporation, 2003). In 1947 after Percy’s discovery, huge sized microwave ovens (refrigerator size) were designed for use in hospitals and military camps in food cooking (Marshall Cavendish Corporation, 2003). Innovation has led to reduction in sizes and commonness in modern day homes. Domestic microwave ovens are used for de-icing frozen foods, heating up cold foods and primary cooking of food (Ishii, 1995). Microwave ovens are also used in industries for food processing such as rubber vulcanization and cooking of bacon (Ishii, 1995). Parts of Microwave Oven A simple diagram of a conventional microwave oven. Source: Vollmer (2004) A microwave oven consists of various parts such as the microwave generator, waveguide tube, fan, turntable and the power supply as shown in the figure above. Microwave generator is a crucial part of a microwave oven. A common type of microwave source is the magnetron. Magnetron plays a key role of supplying microwave and it can be termed as the ‘heart’ of this appliance. Generally, it is a cathode ray tube similar to that used by television sets. A magnetron is therefore a vacuum tube with an anode and a cathode (Marshall Cavendish Corporation, 2003). In between the anode and the cathode is the accelerating potential (Marshall Cavendish Corporation, 2003). Microwaves are produced at the cathode and controlled by magnetic and electric fields. Magnetrons are designed with varying ratings depending on the size of the oven. Some magnetrons have ratings of around ten million watts. Domestic microwave ovens have magnetrons with ratings of around 750 watts (Marshall Cavendish Corporation, 2003). Domestic microwave ovens are designed to operate at frequencies of about 2450 MHZ which corresponds to a wavelength of about 12.2cm (Vollmer, 2004). The microwaves generated by the magnetron are directed to the cooking chamber which is made of metal to act as a faraday cage (Marshall Cavendish Corporation, 2003; Vollmer, 2004). The metallic inner surface reflects the microwaves which resonate within the cooking chamber forming standing waves. Besides this, The fact that standing waves have nodes and antinodes is the reason behind food getting heated in some regions of the chamber while it remains cold in some (Vollmer, 2004). Conventional microwave ovens have a rotating turntable to counter this effect of nodes and antinodes of the standing wave. The turntable deflects the microwaves leading to even distribution of energy within the cooking chamber. Notably, the front side of the oven is made of glass which could be a possible cause of microwave energy loss and danger to users. Nonetheless, this problem has been counteracted by covering it with metal grids (Vollmer, 2004). These metal grids have holes of smaller sizes compared with the microwave wavelength. Therefore, they help in deflecting the microwaves back to the cooking chamber thus acting like the inner metal lining (Vollmer, 2004). Other safety measures which have been put to ensure that microwave ovens are safe are an automated door which switches off the magnetron upon its opening. The gaps in the door region may also be potential points where microwave energy may escape. The problem has been counteracted by fitting a quarter-wave choke prevent escape of microwaves (Marshall Cavendish Corporation, 2003). How a Microwave Oven works. Microwave ovens use the principle of dielectric heating (Marshall Cavendish Corporation, 2003). Dielectrics are generally non-conductors of electric current but become polarized electrically. Water filled materials such as food readily absorb the micro-wave energy and converts it to heat (Marshall Cavendish Corporation, 2003; WHO, 2005). Heat generation is as a result of absorption of energy which causes the food molecules to vibrate rapidly. The energy absorbed from micro-waves causes the food to vibrate at frequencies of about two million hertz (Mittal, 2015). It is generally the collisions and friction between the moving molecules of food which causes the immense build-up of heat within the oven. Food molecules are generally non-conductors but contain ions which can respond to presence of an electric field (Marshall Cavendish Corporation, 2003). These molecules are therefore affected by magnetic and electric fields. In a normal scenario, these ions display random pattern as shown in fig1. When these ions are put in an electromagnetic field, they get aligned facing one direction as shown in fig 2. Adding an electrostatic field to this arrangement will result to realignment charges in food ions realigning the positive and negative polarities facing one direction as shown in fig 3. When the electrostatic field is reversed, the alignment of food ions reverses as shown in figure 4. A continuous flip in the realignment of food ions due to change in electrostatic field will result to friction between them thus causing a heating effect (Marshall Cavendish Corporation, 2003). Micro-waves therefore induce food to generate heat which results cooking of food. Unlike in other cooking appliances such as stoves where food is heated beginning from the outside surface, microwave cooking generates heat from within the food and heating occurs internally (Marshall Cavendish Corporation, 2003). It is for this reason that some foods which change their colour upon cooking using conventional methods may not display the same characteristics when microwave oven is used. It should be noted that when cooking food using a microwave oven, safety measures should be observed. For instance, metallic cookware should not be used inside the cooking chamber. Besides, reflecting away the microwaves thus preventing them from penetrating foods, they can also lead to magnetron damage by reflecting energy on the magnetron (Mittal, 2015). Fig 1: Random pattern of food ions before being put in an electromagnetic field. Source: Marshall Cavendish Corporation, 2003 Fig 2: Aligned pattern of food ions placed in an electromagnetic field. Source: Marshall Cavendish Corporation, 2003 Fig 3: Pattern formed by food ions when placed in an electrostatic field. Source: Marshall Cavendish Corporation, 2003 Fig 4: Pattern of food ions in a reversed electrostatic field Source: Marshall Cavendish Corporation, 2003 The rate at which food is cooked by a microwave oven is largely dependent on the molecular structure and the thickness of food (Mittal, 2015; Marshall Cavendish Corporation, 2003). As earlier mentioned, micro-waves are absorbed by foodstuffs. However, this occurs only to a limited depth. They can penetrate foods only to a maximum depth of five centimetres (Marshall Cavendish Corporation, 2003; Mittal, 2015). This has an implication that thin sized foods cook faster in microwave ovens. Foodstuffs of thickness of more than 5 cm also get cooked but through a different mechanism which is common in the formal cooking. Heat from the heated-up layers penetrated by the micro-waves is conducted away to other layers in regions where this radiation cannot penetrate (Marshall Cavendish Corporation, 2003). It is a common instruction to the user to leave some foods for some time after removing them from the oven to allow heat to be conducted to regions where micro-waves may not penetrate. This is to allow food to further cook from the heat that had already been absorbed by layers penetrable by these waves. Microwave oven has numerous advantages over other conventional methods. One major advantage of this method of cooking is the speed. Microwave cooking is faster than any other method of cooking. Additionally, they are more effective when reheating some foods such as rice because the heating does not have an effect on the original texture of the food (Marshall Cavendish Corporation, 2003). Nonetheless, microwave ovens are not without some setbacks. One of the setbacks is its unsuitability to be used in cooking some foods such as boiled eggs. Since the appliance relies on heat generated from within the molecular structure of the food, the contents of the egg which are liquid in nature will generate more heat which will cause the shell to break (Marshall Cavendish Corporation, 2003). In addition, this method of cooking is not suitable for those foods which rely on extensive cooking periods for them to have flavour. Also, the fact that there is minimum colour change when these ovens are used will likely give a false impression to the user who might overcook the food. Other setbacks are the health issues raised on foods cooked using microwave ovens. As earlier mentioned, microwaves will only penetrate thin sized foods. It may lead to uneven cooking if it is in the case of thicker foods (WHO, 2005). There have also been questions on safety of these ovens especially if they are faulty leading to leakage of microwaves. Microwaves are known to have serious effects when they get into contact with human tissues. References Gupta, M. & Eugene, W. (2007). Introduction to Microwaves In M. Gupta & W. Eugene (eds), Microwaves and Metals. New York: John Wiley & Sons (Asia) Pte Ltd, pp.1- 23. Ishii, T.K. (1995). Handbook of Microwave Technology Applications (Vol. 2), London: Academic Press. Marshall Cavendish Corporation. (2003). How it Works: Science and Technology (Vol 11). New York: Marshall Cavendish Mittal, A. (2015). Simply Cooking: Theory and Principles. New York: Anand Mittal Publishers. Sobol, H. & Tomiyasu, K. (2002). Milestones of Microwaves, IEEE Transactions on Microwave Theory and Techniques, 50(3): 594–611. Vollmer, M. (2004). Physics of the microwave oven. Physics Education, 39(1): 74-81. World Health Organization (WHO). (2005). Electromagnetic fields & public health: Microwave Ovens. International EMF Project Information sheet. [Online]. Available from: http://www.who.int/pehemf/publications/facts/microwaveovens_infosheet.pdf?ua=1 [Accessed on 5th September 2015] Read More

In 1947 after Percy’s discovery, huge sized microwave ovens (refrigerator size) were designed for use in hospitals and military camps in food cooking (Marshall Cavendish Corporation, 2003). Innovation has led to reduction in sizes and commonness in modern day homes. Domestic microwave ovens are used for de-icing frozen foods, heating up cold foods and primary cooking of food (Ishii, 1995). Microwave ovens are also used in industries for food processing such as rubber vulcanization and cooking of bacon (Ishii, 1995).

Parts of Microwave Oven A simple diagram of a conventional microwave oven. Source: Vollmer (2004) A microwave oven consists of various parts such as the microwave generator, waveguide tube, fan, turntable and the power supply as shown in the figure above. Microwave generator is a crucial part of a microwave oven. A common type of microwave source is the magnetron. Magnetron plays a key role of supplying microwave and it can be termed as the ‘heart’ of this appliance. Generally, it is a cathode ray tube similar to that used by television sets.

A magnetron is therefore a vacuum tube with an anode and a cathode (Marshall Cavendish Corporation, 2003). In between the anode and the cathode is the accelerating potential (Marshall Cavendish Corporation, 2003). Microwaves are produced at the cathode and controlled by magnetic and electric fields. Magnetrons are designed with varying ratings depending on the size of the oven. Some magnetrons have ratings of around ten million watts. Domestic microwave ovens have magnetrons with ratings of around 750 watts (Marshall Cavendish Corporation, 2003).

Domestic microwave ovens are designed to operate at frequencies of about 2450 MHZ which corresponds to a wavelength of about 12.2cm (Vollmer, 2004). The microwaves generated by the magnetron are directed to the cooking chamber which is made of metal to act as a faraday cage (Marshall Cavendish Corporation, 2003; Vollmer, 2004). The metallic inner surface reflects the microwaves which resonate within the cooking chamber forming standing waves. Besides this, The fact that standing waves have nodes and antinodes is the reason behind food getting heated in some regions of the chamber while it remains cold in some (Vollmer, 2004).

Conventional microwave ovens have a rotating turntable to counter this effect of nodes and antinodes of the standing wave. The turntable deflects the microwaves leading to even distribution of energy within the cooking chamber. Notably, the front side of the oven is made of glass which could be a possible cause of microwave energy loss and danger to users. Nonetheless, this problem has been counteracted by covering it with metal grids (Vollmer, 2004). These metal grids have holes of smaller sizes compared with the microwave wavelength.

Therefore, they help in deflecting the microwaves back to the cooking chamber thus acting like the inner metal lining (Vollmer, 2004). Other safety measures which have been put to ensure that microwave ovens are safe are an automated door which switches off the magnetron upon its opening. The gaps in the door region may also be potential points where microwave energy may escape. The problem has been counteracted by fitting a quarter-wave choke prevent escape of microwaves (Marshall Cavendish Corporation, 2003).

How a Microwave Oven works. Microwave ovens use the principle of dielectric heating (Marshall Cavendish Corporation, 2003). Dielectrics are generally non-conductors of electric current but become polarized electrically. Water filled materials such as food readily absorb the micro-wave energy and converts it to heat (Marshall Cavendish Corporation, 2003; WHO, 2005). Heat generation is as a result of absorption of energy which causes the food molecules to vibrate rapidly. The energy absorbed from micro-waves causes the food to vibrate at frequencies of about two million hertz (Mittal, 2015).

It is generally the collisions and friction between the moving molecules of food which causes the immense build-up of heat within the oven.

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