Dr. Universe: Why do microwaves hum? – T.J., Middle School, Ohio

Dear T.J.,

Long before humans invented microwave ovens, they had to rely on stoves or an open flame to heat up their food.

These days, we can warm up food in a matter of minutes, or even seconds. A lot of parts had to be invented to make that all happen. These parts help electricity flow through the microwave in lots of different ways.

One part called a transformer helps move the electricity from one area to another. As it does so, it may vibrate or shake a little, and that tiny movement can cause a humming noise.

Electricity is actually a big part of what powers microwaves, just as it does refrigerators, lightbulbs, and cellphones. The electrical current travels through a wire, sort of like water through a hose.

Inside of a microwave, we also find tiny parts called diodes which work like gates, allowing electricity to move in one direction but not the other. We might also find some capacitors in the microwave which work to store electrical energy.

You may also be interested to know that microwaves are actually a form of electromagnetic energy.

I found out all about it from my friend Sumeyye Inanoglu, a graduate student researcher at Washington State University. She is very curious about how we can preserve food and use microwave technology to make better ready-to-eat meals.

These kinds of energy move in waves, not too unlike the shape of the wave you see crashing onto the beach.

The light you see with your eyes is also a kind of energy. There are a lot of different kinds of electromagnetic energy, so many that we call them part of the electromagnetic spectrum.

The spectrum is sort of like a rainbow of energy, with the different lengths of waves forming the colors of the rainbow. We find very long waves, like radio waves, at one end of the spectrum, and very short waves, like X-rays, at the other end. Microwaves are in the middle.

Sumeyye told me that many years ago there was a scientist named Percy Spencer who was studying radar and was experimenting with microwaves—the wave, not the appliance.

One day Spencer had chocolate in his pocket and those microwaves ended up melting all the chocolate. That melted chocolate eventually led to the invention of the actual appliance.

The waves are produced inside the appliance from a part called a magnetron. The magnetron is mainly made up of two big magnets. You may also hear some sounds or vibrations coming from a fan that cools the magnetron.

Lots of tiny parts can add up to do really useful stuff. But they’ll also make some noise as they vibrate. Those vibrations are also why moving parts eventually wear out and have to be replaced.

You might just say you are hearing the sounds of engineering. It’s something to think about the next time you are waiting for the microwave timer to beep.

Sincerely,
Dr. Universe