Why do so many electric things hum?

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Ever notice a light, a stereo, or other electrical appliance monotonously humming at you? Of course you have. We’ll tell you why it happens. Hmmmmmmmmmmmmmmm.

There are all kinds of reasons for your electric devices to hum. Perhaps, for example, they have just gained the degree of sentience and autonomy they need to kill you, and are feeling peppy about it. One of the main reasons, especially for old technology and those ancient fluorescent lights that we see in scary basements in old horror movies, is magnetostriction in the transformers.

Transformers are the things that allow the uniform voltage delivered to all the plugs in a house to be changed into the various different kinds of voltage that are necessary to run various different types of household appliances. They consist to two coils of wire placed very close to each other, and generally both wrapped around a central piece of metal. The in going electricity pours into the first coil – and then back out. And then in again, because we’re dealing with alternating current. This is current which takes advantage of the fact that many appliances don’t care where electrons are moving as long as they’re moving at all. Instead of pouring electrons into the house, alternating current jerks them back and forward to maintain a continuous electric flow. Running alternating current in one coil of wire induces current – gets the electrons to jerk back and forth – in the nearby piece of wire. The number of coils in each set of wires determines the conversion of the voltage. If there are more coils of wire in the part of the transformer that's being inducted than there are in the one that's hooked up to the main power supply, the voltage increases. If there are fewer coils, the voltage decreases.


And that’s all well and good, but what about the metal connecting these two wires? A moving charge creates a magnetic field, and the metal is affected by that. When the charge moves one way, the metal between the coils is under one kind of magnetic field and is stretched out. When alternating current changes the direction, the magnetic field is changed, and the piece of metal contracts. Since alternating current changes many times a second, the resulting vibration causes a hum.

Then again, your house really could be planning to kill you. Good luck!

Via Hyperphysics and Why Can’t Elephants Jump.