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How a Microwave Oven Works Published 12/10/2011

Curious about how your microwave cooks food without any visible cooking surfaces? Microwaves use high frequency waves to cook your food quickly - allow Matt to explain.

Video Transcript

Hi, I'm Matt from eSpares.

In this video I'm going to show you how a microwave oven works.

So what are microwaves? They're just a form of energy that move in a wave-like way, much like visible light. Let's have a look at the electromagnetic spectrum, here's visible light, right in the middle and as we move this way along the spectrum, the wavelength gets longer and our microwaves are here, between infrared and radio waves.

Now the microwaves that are produced by our ordinary microwave oven are of just the right wavelength to be absorbed by water, and foods like potatoes are full of water. Now imagine for a second that this isn't a potato but is actually a water molecule, it's polar, which means it's positively charged on one side and negatively charged on the other. As the microwave passes by, the water molecule vibrates to stay aligned with the oscillation from the wave, and it's this vibration that causes heat.

Now contrary to popular belief, microwaves don't heat foods from the inside out. The microwaves still penetrate the outer part of the food first, which is why you'll see on cooking instructions it often tells you to leave the food to stand for a little while, and that allows the heat to penetrate evenly throughout the food.

Microwaves don't heat up plastics, glass, or ceramics, that's because their molecules are not polar. If the bowl does get hot, it's through conduction of heat from the food itself, and I'm sure you know not to put metal objects inside the microwave. That's because many metals reflect microwaves, and you get that arcing or sparking and that's actually dangerous and can cause damage to the microwave oven.

So, let's open ours up.

Now I'm doing something we recommend you don't do. You shouldn't ever take off the outer casing off your microwave, like I'm doing, it should only ever be done by a qualified appliance engineer. We've got high voltages inside and it can be potentially quite dangerous. But I'm doing that so I can show you the flow of the electrical current through the microwave - obviously I'm unplugged from the mains.

So it comes in here into this little board here, where we've got a fuse and a filter and that just cleans any noise from the electrical signal. It comes across here to this which is a thermal cut-out switch, just means if everything is getting too hot inside it will cut the power before things start to catch fire. It then comes down to a little micro-switch, which if I open and close the door, hopefully you can see that, it's cutting the current to stop the microwaves when the door is open. That also comes up to a bulb here, so we can see what we are doing inside and down to our main control board here.

Now the control board is obviously where we set the power level and duration that we want to cook for. From there it comes down to this big lump at the bottom here which is a transformer. That's just jumping the voltage up from ordinary 240 volt mains electricity, up to several thousand volts - it depends on what make and model of microwave you've got. But we need that really high voltage to power this part which is called the magnetron, and the magnetron is what actually produces the microwaves.

They are filtered through this part here, channelled through this, which is called the wave guide and out into the cavity of the microwave. Now there's a cover on this called the wave guide cover - it can be either a plastic one like this, or you might find that you have this slightly metallic card, and it's just to stop any food getting inside the wave guide and into the magnetron, because you don't want that to happen. But it's one of the few parts which is quite easily replaceable on a microwave.

Now there are just a couple of motors I want to show you. There's a motor here which is driving this fan. That's just to draw ambient air across the magnetron, stopping it getting too hot. That comes past the bulb and across the cavity and then vents out this side of the microwave. Now the other motor is right in the middle underneath here and that's just what turns the plate around whilst the microwave is on and the food is cooking.

And the plate is very often broken, it's the most commonly replaced spare part on a microwave. So hopefully you now know how a microwave works. Wave guide covers and microwave plates, and other spare parts for microwaves are available for all makes and models and you can find them on the eSpares website.

Thanks for watching.

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