Why does electricity make noise?
The short answer
Electricity makes noise because the alternating flow of electrons causes a vibration to occur in electromagnetic devices. If the device has a component that picks up this vibration (like a transformer), it causes the component and the air around it to vibrate which produces a humming or buzzing noise.
The long answer
To understand why electricity makes noise, we first need to get into how electricity works. In order for an electric current to flow, charges (electrons) must be moving.
In direct current (DC), the charges move in one direction only, whereas in alternating current (AC), the charges move in one direction for a very short time and then reverse direction. This cycle of switching directions is known as the frequency, which is measured in Hertz (Hz). In the United States, AC power cycles 60 times per second.
This diagram by Chris Woodford is licensed under CC BY-NC-SA 3.0
Okay, so now we know about electricity. What exactly is vibrating to create that hum?
Electricity’s hum is usually caused by a magnetic element inside an electrical device. For example, when you're near power lines, you might hear an electric hum or buzz coming from an electromagnetic device called a transformer. A transformer has an inductor inside it, which is an electromagnet made up of a piece of iron with a coil of wire wrapped around it.
This is an example of a wire-wrapped inductor. "Inductor filter - Line filter " by Marcnovac is licensed under CC BY-SA 4.0
An inductor vibrates at the frequency of the AC power flowing through it. And these vibrations cause the air around the device to vibrate as well. So electricity makes noise because of electrons moving back and forth resulting in a vibration that becomes more audible as more surrounding parts vibrate.
Bonus brain points
Does DC power ever produce a sound?
Yes, it's possible for a DC-powered device to hum if it has parts that are susceptible to vibrations, such as a DC motor. But the frequencies are much lower, so it's less noticeable than an AC-powered device.
Sources
Hamer, A. (2022, January 24). Why does electricity make a humming noise? LiveScience. Retrieved March 5, 2023, from https://www.livescience.com/electricity-humming-noise
Rader, A. (2015, August 22). Electricity & Magnetism | Alternating Current. Physics4Kids.com. Retrieved March 5, 2023, from http://www.physics4kids.com/files/elec_ac.html