Microwaves

To celebrate the end of my summative week, today we’ll be talking about an appliance I’ve been using to make a lot of late-night stress snacks to help me cope with said summative week:

The Microwave oven!

I’ve always found microwaves to be an everyday magic, you put food in and it comes out hot, but you’ll notice that when you put your hand in to retrieve your food the air inside the microwave isn’t heated, nor is the actual bowl you put the food in itself most of the time. It’s as though the heat supplied by the microwave somehow can selectively heat up only the food, but nothing else that goes inside it!

And like any other form of magic, it also comes with many mysterious rules: why shouldn’t we microwave metals? Or certain types of plastics?

To answer these questions, we’ll start with how microwaves actually work:

Most of the food we eat contains water in some form, and water molecules have what is called a “dipole moment” (a fancy word that basically means there’s a separation of charge in the molecule), where one side of the molecule is positively charged whereas the other is negatively charged, similar to a magnet. This is due to the way that the water molecule is covalently bonded: in H2O, an oxygen atom shares electrons with two Hydrogen atoms and since the Oxygen is more electronegative (attracts electrons more strongly in comparison), both the electrons which have a negative charge will want to move closer to the Oxygen atom, giving the side of the water molecule with oxygen a negative charge and the side with the two oxygen atoms a positive charge.

Picture a standing wave. A Microwave that is generated by the oven is around 12 cm from each peak to the next, or alternatively, from the deepest point in the wave to the next (Which yes, isn’t very micro but compared to radio waves which can be 10-ish kilometers from peak to peak the name is understandable). The top of each wave is positively charged and the bottom of the wave is negatively charged. This way, the water molecules will rotate into position as its negatively charged side is attracted to the positively charged side of the wave and vice versa. However, by nature of the wave itself being electromagnetic (composed of both electric and magnetic waves) it is constantly changing direction, at a rate of around 4.9 billion times each second. This means that the water molecules are also constantly rotating and flipping in an attempt to remain aligned with the microwaves, this constant motion creates heat due to friction as the motion agitates the Hydrogen bonds water molecules form with other water molecules nearby. (Hydrogen bonds are the attraction that the water molecules feel for each other as the negative and positive charges on each molecule can also be attracted to the negative and positive charge on other water molecules).

The reason why the air or the containers you put your food in when microwaving doesn’t get hot is because air, plastic, glass and ceramics are not as polar (ie. don’t have as much of a charge difference) as water molecules in your food, therefore they’re not as affected by the microwaves.

So where do these microwaves come from? They come from the Magnetron, which is a really cool name for this complicated little device. So picture in your head a tube, with a filament at its core, a magnetised ring surrounding it, and then a larger copper ring surrounding both of those with spokes that extend close to the filament. When electricity is supplied to the microwave it applies a voltage across filament and circular copper outside. This voltage creates an overall force acting on the electrons which causes electrons to leave the filament and move towards the outer copper ring. However, the magnets inside the Magnetron surround the stream of electrons so that they are repelled and bend back to the filament. The constant stream of moving electrons rushing past the gap in the spokes will rhythmically block the gaps and clear from the gaps in the spokes, and this rhythmic motion is what creates microwave radiation, or microwaves which are then reflected into the chamber of the microwave where the food is where it bounces around, being reflected by the metal wall (hopefully into the food), which then as stated prior, heats your snack up.

So we come to our last question, why can’t we microwave metal? At first glance, this rule seems hypocritical, the microwave oven IS a giant hunk of metal itself. Well, microwave radiation has a strong tendency to bounce off metal, so if you put a metal spoon or fork inside it will try to rapidly bounce off those and sometimes the waves can concentrate in one area, creating a flash or a spark that can harm the microwave. As for the metal the microwave is composed of, those are actually to keep the waves bouncing inside the microwave so they don’t escape and start to cook other things that contain water (namely, you) 🙂

So that’s a brief overview on how microwaves work, there’s many other interesting things to be said about them, for example microwave ovens were invented after this guy noticed that his chocolate bar was melted after being near a device that emitted microwaves. Or the fact that apparently in the 1950’s some British scientists used microwaves to bring frozen hamsters back to life.

But the dumplings I just microwaved are done, so I’m off to enjoy them and I’ll save those facts for some other time!

Sources: