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Mysteries of the Microwave

Is a microwave magical? Everyday Einstein explores the science behind microwaves. 

By
Lee Falin, PhD,
Episode #121

In nearly every American home, there sits a microwave oven. We pretty much take it for granted.

But have you ever stopped to consider how it works?

Maybe you know exactly how it works, but if it all seems a bit magical, this week’s episode is for you.

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Why Is a Microwave Called a Microwave?

The reason a microwave is called a microwave is because it uses energy in the form of microwaves to heat food.

Microwaves form part of the EM spectrum, which I talked about in the very first Everyday Einstein episode on radiation. Microwaves are on the low energy side of the spectrum, right between infrared and radio waves.

What makes microwaves special is that they are just the right size to interact with water molecules.

How Does a Microwave Make Microwaves?

It's magic.

OK, maybe not exactly magic, but pretty close. The technology behind microwaves was developed while trying to perfect radar technology around World War II.

Inside every microwave oven is this thing called a magnetron (which sounds a bit like the name of an evil robot). The magnetron consists of a metal rod called the cathode which is surrounded by a metal doughnut-shaped thing called an anode.

The anode has some holes cut into it around the inside edge, which makes it look like a mouse crawled into the doughnut hole and nibbled a bunch of sections away from the middle. These holes are called resonating cavities. Cavities because they are holes and resonating because of something I’ll tell you in a bit.

While there are a bunch of other parts involved in a magnetron, the last part I’m going to mention are the magnets found above and below the anode doughnut thingy. (Doughnut thingy is not an official term).

To make microwaves, energy is pumped into the cathode rod. As it heats up, electrons start flying off. These electrons are caught in the magnetic field created by the magnets and start spinning around and around the inside of the cathode doughnut hole.

As the electrons pass by the resonating cavities, they resonate with the cavities. This resonance causes the cavities to give off their own energy in the form of microwaves. These waves are then directed into the body of the oven, where they cook your food.

How Do Microwaves Heat Food?

Microwaves are small enough to penetrate a few inches through food. As they do so, they bump into water molecules contained within the food. Since water molecules are polar, one of their ends has a positive charge and the other has a negative charge. This polarity causes the water molecules to try and rotate as they seek to align themselves with the microwave radiation.

As they rotate, they bump into other molecules, causing friction, which generates heat, thereby heating your food.

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