HOW A MICROWAVE WORKS AND HOW IT COOKS
History
The heart of the microwave oven is the magnetron, which is an oscillator that creates electromagnetic waves that cook your food. Invented around 1910, the magnetron was first used by the military for radar systems. RADAR is an acronym for RAdio Detection And Ranging. It is used to identify distant objects by detecting the difference between the transmitted waves and the returning ones. They were mainly used around the time of WWII. In fact, if they had been understood more reliably, the Pearl Harbor attack could have been realized much sooner. There was a radar system in use that detected the attacking Japanese planes coming in, but was theorized to be a flock of birds or maybe a glitch in the signal.
The government stopped using these radar systems after the war as other types of radar systems were developed. A company named Raytheon was building a large order of radar antenna systems for the government, when the order was canceled. They came to realize the heating properties of the magnetrons while testing. Left with a large number of magnetrons that were just sitting on their shelves, they ended up designing an appliance for heating food that they called "Radar ranges". Later on they came to be called microwave ovens.
The government stopped using these radar systems after the war as other types of radar systems were developed. A company named Raytheon was building a large order of radar antenna systems for the government, when the order was canceled. They came to realize the heating properties of the magnetrons while testing. Left with a large number of magnetrons that were just sitting on their shelves, they ended up designing an appliance for heating food that they called "Radar ranges". Later on they came to be called microwave ovens.
HOW IT COOKS
The microwave oven uses microwaves, which are a type of electromagnetic wave that is shorter than radio waves and longer than infra-red waves. 2.45 GHz is the frequency most commonly used (12.2 cm or 4.80 in. wavelength).
An antenna transmits the waves to the cooking chamber, where they bounce around inside and collide with the food and heat it up. Water, fat, and other things absorb energy from the microwaves. These substances are dipoles with a partially positive charge on one end and a partially negative charge on the other end. Dipole molecules in the food rotate and try to align themselves with the alternating electric field in the microwaves. The rotating molecules hit other molecules and put them into motion. In this way, the food gets heated with both kinetic (energy of motion) and potential energy (energy of potential motion). Heating happens from the outside first and then travels inward. Turntables help to rotate the food so that it heats more evenly and does not sit in hot spots and cold spots of the microwave. These hot spots are areas where the waves bounce through most frequently. The cold ones are where they miss most frequently and are distant from the ricochet of excited molecules.
An antenna transmits the waves to the cooking chamber, where they bounce around inside and collide with the food and heat it up. Water, fat, and other things absorb energy from the microwaves. These substances are dipoles with a partially positive charge on one end and a partially negative charge on the other end. Dipole molecules in the food rotate and try to align themselves with the alternating electric field in the microwaves. The rotating molecules hit other molecules and put them into motion. In this way, the food gets heated with both kinetic (energy of motion) and potential energy (energy of potential motion). Heating happens from the outside first and then travels inward. Turntables help to rotate the food so that it heats more evenly and does not sit in hot spots and cold spots of the microwave. These hot spots are areas where the waves bounce through most frequently. The cold ones are where they miss most frequently and are distant from the ricochet of excited molecules.
What doesn'T COOK
Plastic, glass, and ceramic do not absorb these waves, and metal reflects them. The shield on the door has holes that are significantly smaller than the wavelength of the microwaves. This allows you to see inside the microwave, but the microwaves cannot radiate out of the oven. The rest of the walls have insulting sheets of shielding.
The reason that foil lined microwave containers cook food more evenly is because they reflect the waves back to the food. These containers can even crisp and brown crusts. They have open areas that are not shielded. Only a very small amount is reflected, and most of the waves are still allowed to escape.
Thick metal is damaging for a microwave oven. Many metals are conductors, because they have a lot of electrons contained in them that can move freely. This means that they conduct electricity well, and that is why they reflect the microwaves. When the waves hit the metal, the electrons get excited and reflect them away.
The reflected waves can build up to a charge density that can cause an electrical arc inside the microwave oven. The arcs can make holes in the shielding. They can burn out the magnetron, or generate a surge that can damage vulnerable electronics. This can destroy your microwave oven or, even worse, make it unsafe to be used.
The reason that foil lined microwave containers cook food more evenly is because they reflect the waves back to the food. These containers can even crisp and brown crusts. They have open areas that are not shielded. Only a very small amount is reflected, and most of the waves are still allowed to escape.
Thick metal is damaging for a microwave oven. Many metals are conductors, because they have a lot of electrons contained in them that can move freely. This means that they conduct electricity well, and that is why they reflect the microwaves. When the waves hit the metal, the electrons get excited and reflect them away.
The reflected waves can build up to a charge density that can cause an electrical arc inside the microwave oven. The arcs can make holes in the shielding. They can burn out the magnetron, or generate a surge that can damage vulnerable electronics. This can destroy your microwave oven or, even worse, make it unsafe to be used.
magnetron
How it works
A cavity magnetron works by means of a high-powered vacuum tube, which provides a stream of electrons that interact with a magnetic field while moving past openings in the container (resonant cavities). The vacuum tube is a sealed glass tube with all the gases removed, where electric current flows between two electrodes. The electrons from the filament-heated cathode (negative) electrode to the high-voltage anode (positive) electrode. The vacuum tube generates thousands of watts of power, and this makes them difficult to replace. They are very old electronic device design that may someday be replaced with solid state devices, but for now they are still the most important component in this type of oven. The magnetic field is generated by a powerful magnet. They have a fan to cool any components that heat up in operation, but also may be useful in circulating air when using a convection mode for cooking. Convection cooking is a means of cooking that uses heated air. The antenna carries the microwaves to the cooking chamber of the oven.
The electrons passing the resonant cavities and interacting with the magnetic field, cause the waves to oscillate. The frequency of the microwaves (resonant frequency) is produced by the dimensions of these cavities. This is a simplified explanation of an incredibly complex device. The fact that we can cook food with waves akin to radio waves must have seemed very Star Trek to people when they first became available. They are now a part of every day life for most of us.
The electrons passing the resonant cavities and interacting with the magnetic field, cause the waves to oscillate. The frequency of the microwaves (resonant frequency) is produced by the dimensions of these cavities. This is a simplified explanation of an incredibly complex device. The fact that we can cook food with waves akin to radio waves must have seemed very Star Trek to people when they first became available. They are now a part of every day life for most of us.