Microwave Oven is truly remarkable feat of engineering. These days most of us own a microwave oven from reheating leftovers to making the meal, they’re pretty common piece of kitchen kit but how microwave oven works and how do they actually cook things?
Microwave ovens heat up food using microwaves, which are one type of electromagnetic radiation like X-rays, radio waves and the visible light that we can see. The heating power of microwaves was first discovered accidentally during the Second World War. One day an American engineer called Percy Spencer was working on a unit for producing high intensity microwaves for using in radar, when he noticed that the chocolate bar in his pocket began to melt. He then placed popcorn kernels near the micro wave producing unit known as a magnetron tube and was rewarded with a flurry of Pop’s. Another experiment involving an egg was a bit messier but this was the case to the idea of what micro wave could do in the kitchen.
So how a magnetron like the one Spencer was using produce does microwaves. The first magnetron was developed in the UK in 1940. It’s made up of a copper tube with a couple, a rod running down the centre and the vacuum between the two. A high voltage is applied across the two conductors which causes electrons to move through the vacuum from the rod to the tube but they don’t move in a straight line. The tube is also held in a magnetic field and this applies a force to the electrons which makes them spirally outwards. As the electron spin out they pass small openings in the sides of the tube. The electrons in the cavities behave like air in a bottle when you make a note by blowing across the top. In the bottle the air breaks in and out of the bottle opening by creating a sound. The electron spiraling pass the cavities of the magnetron or a bit like you blowing across the top of the bottle. They make the electrons in the cavities vibrate in and out producing microwaves. This is a similar way to how a mobile phone aerial works.
When you have a rapidly changing voltage in the wire which forms the aerial that wire produces electromagnetic radiation that allows you to communicate. The micro waves produced are TAPPED OFF from one of the cavities in a radar device that fed to the antenna of transmission and in a microwave oven. Then they are directed into the oven interior to cook the food. After the discovery that microwaves which could be used to heat up food, Raytheon the company spends had been working for tried to market microwaves for home use. Unfortunately these early microwave ovens were far too large and expensive for practical use in the home and it wasn’t until the mid-nineteen sixties that the domestic sized oven was finally made.
But why should blasting something with microwave radiation cause it to heat up. The answer lies in the particular wavelength of the microwaves that are used for this purpose. Waves of this linked in frequency are very well absorbed by water, fats and sugars, basically the main constituent of food. When these molecules absorb the micro waves they get all excited which makes them jiggle about and heat up. This happens to all the molecules that once in the out a couple of centimeters of the food, which is why heating something up in a microwave, are so much quicker than using a conventional oven.
Heat from the oven only heats out a couple of millimeters before being conveyed through the food by the process of thermal conduction.
But the question often asked is “are microwaves dangerous”?
Other types of electromagnetic radiation like gamma rays and x-rays can be harmful to our health so what about microwaves. Well thankfully the microwaves used in a microwave oven are type of non ionizing radiation which means they pose no risk of damaging tissues in causing cancer like Gamma or X-rays would. But just as they heat up food if you got too close you could get burnt, which is why all the microwaves ovens have the metal mesh in the door to prevent the microwaves from escaping and a safety switch to shut off the microwave supply when the door is opened.
Now a microwave contains three main components:
- A vacuum tube called the magnetron. It generates the energy that heats food.
- A waveguide hidden in the wall of a microwave oven to direct that energy to the food.
- A chamber to hold the food which safely contains the microwave radiation.
Magnetron is the heart of a microwave oven. It consists of a vacuum chamber that contains a filament that is heated up by putting power to wires. This filament is a point where it will emit electrons, and around the filament there is an anode which is connected to the fins which take away the heat that doesn’t get used into making power. The magnetron connects into the oven cavity. We call the oven a cavity. So that is how the magnetron becomes the heart of a microwave oven is. Magnetron is the power generator with very high frequency.
So what is the basic principle of microwave oven and how does it make power?
The filament is spirally rounded with tungsten, connecting between the two end points and the electrons that are emitted by the filament, when you put voltage between that filament and the anode, would ordinarily just zing across and nothing would happen. But there is a magnet, when the electrons start to move in one direction the magnet turns them, and depending on the voltage it turns faster or slower. So now the electron starts whirling around the filament due to the magnet, and they are going over to connect with the copper anode. The anode is not a solid piece of metal it’s got some veins, these are designed to be resonant at the frequency that we want.
Resonance is like when you have a person on a swing and you want them to go higher and faster you push them at the right time so that it makes them go faster. The electrons go by each of these veins at a time such that they can kick another electron inside the copper at the right time such that they can build up a voltage. And that’s what happens in a magnetron, electrons whirl around at the right speed to generate wave energy in an anode which is designed as a bunch of resonators. So, this is how the microwaves are generated.
Now how do we get that power to come out to the oven so we can use it?
There is a cap on the end, an insulator, a small rod from the cap connects to one of the veins to connects the power out and by mounting this probe in the sidewall of the oven cavity and stick this inside the oven, you radiate energy into this oven. Now the oven itself is also a cavity, waves bouncing back and forth across all the walls, and if there is a piece of material in the cavity like a food item, the microwaves impinge on the food, and in the process the food absorbs the energy and gets hot. Because the waves comes in from so many different directions it heats all around and penetrates into the food and makes it cook relatively uniformly inside as well as outside unlike other forms of cooking which require heat to come in from the side and conduct in, instead of coming in as an electromagnetic wave and the primary mechanism is that you are heating the water content or fat content rather than heating the dry materials, like the fiber materials.
This is how the principle of heating the food in microwave ovens work.