Here, we'll answer "Just how do microwave ovens work?" by looking at the science involved in cooking with microwaves. We have sections on
why is the magnetron an important part of the microwave?what are microwaves?how do microwaves make food hot?why is standing time always recommended?Questions such as
why is there a turntable?how does the door stop microwaves from escaping?what is a stirrer?how is the power controlled?- these questions are answered on the next page "How does a microwave work? - the appliance."Watch this video for a quick 1 minute overview of microwave cooking. More detailed explanations are given below the video.
How do microwave ovens work? - The magnetron
Click on the thumbnail to see a larger picture.Essentially, a microwave oven is a metal box with a door on the front, and a magnetron bolted on the outside of the box. The magnetron's job it is to feed radio waves into the box, which is also called the oven cavity. Without the magnetron, the oven is just a tin box with a glass door on the front.
A magnetron is made up of electric wires wrapped around a tubular metallic shape that includes two large magnets. When a high voltage current is passed through the wires, radio waves are given off. These waves are channeled down a metal pipe called the waveguide, and into the oven cavity.
Because the radio waves are quite small, relative to most radio waves, they are called microwaves. The radio waves can be absorbed by some materials, such as food and vegetables, and their energy is converted into heat. However, they bounce off other materials, like sheets of metal.
Percy Spencer, the man who invented the microwave oven, found that if food was placed close to the source of the radio waves, enough heat could be generated to cook the food. Luckily, he also found out that the thin metal walls of an oven would be enough to stop the radio waves spilling out to the surroundings, and traveling all over the kitchen.
How do microwave ovens work? - What are microwaves?
Microwaves are part of a broad range of radiation known as electromagnetic radiation. These waves travel at the speed of light through space and are present at various intensities throughout our planet. Like waves in the sea, the wavelength is measured as the distance between peaks of each wave, and tells us how big each wave is.
The illustration below is from NASA. At the left hand end of the spectrum, above the small image of a football field, it shows radio waves with a very long wavelength. Moving to the right hand end, the wavelengths get smaller. The spectrum runs through microwaves, infrared, visible light, and then into the high energy waves such as X rays and Gamma Rays.
Electromagnetic spectrum, showing very long waves on the left, and very short waves on the right
Radio waves with very long wavelengths, between 10km and 100km peak to peak, follow the curvature of the Earth, and are used by the military to communicate with submarines traveling underwater. Shortwave radio has wavelengths of between 10m and 100m peak to peak, and is used for long distance communication, ship to shore and so on.
The wavelength of the microwaves used for cooking is about 12.2 cm or approximately 5 inches from peak to peak. There is more information on our page, about microwave oven radiation.
How do microwave ovens work? - How microwaves make food hot
Food contains water molecules. These molecules have a positive charge at one end of the molecule, and a negative charge at the other end. As the microwaves move through the oven cavity, they generate rapidly changing magnetic fields. The water molecules spin over rapidly from end to end, trying to stay in line with the magnetic fields.
This spinning happens 2.45 billion times per second. That is a lot of spinning.
Water molecules rotate as the magnetic fields change direction
How do microwave ovens work? - Why is standing time recommended?
The radio waves only manage to get about 4 or 5 cm, or about 2 inches, into the food before the majority of their energy is absorbed. This is a surprise to most people, but it is true. There is only one way for heat to get to the center if your food is thicker than this. The heat must travel through the outer layers of food to the center. This is exactly the same as all other forms of cooking we use, roasting, baking, broiling and so on.
This is why all manufacturers recommend "standing time" in their recipes. This allows the heat from the outside of the food to penetrate into the center and cook the food there. It is also why it is difficult to defrost thick foods, like whole turkey or chickens. The water molecules are held fast in an icy structure, and prevented from spinning. Only when the molecules change from ice to water can they get "excited" enough to produce heat.
Most regular ovens heat the air surrounding the food. Because it takes time for the heat to travel to the center of the food, the outside of the food is exposed to more heat than the center. This extra exposure of the surface gives foods, particularly meat, the characteristic flavor and browned appearance. The air in the microwave does not get hot, so the outside of the food cooked in a microwave does not brown or take on the roasted or grilled flavors.
To see more explanations about features in modern microwave ovens, go to "how does a microwave work?"
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