I thought maybe they got power from the phone but then I remembered old headphones plugged into a phone jack separate from the charging port so how come one type never needs to be charged and one does?
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Oh boy. This is a fun rabbit hole.
Tl;dr: the sound waves from the music are converted to electrical waves and sent to the headphones down the cord to the speakers in the ear buds.
The longer version –
Do you understand how a speaker works? There are magnets in speakers. The magnets are connected to the speaker cone.
When an electrical signal is sent to the speaker, the electrical signal goes to a coil in near the magnet.
Now remember, magnets have north and south poles – Positive and Negative. When the electrical signal goes through a wire it creates an invisible electrical field. This electrical field, depending on the orientation, will attract or repel the magnet.
The magnet is attached to the speaker cone. Instead of the coil of wire moving, the magnet and the speaker cone moves. This causes vibrations in the air that we hear with our ears.
So when you put a sound wave as an electrical signal into the wire it will cause the speaker to move.
Here’s an example video illustrating how it works. https://youtu.be/CN6lmC6bgxE
When you have air pods or wireless ear buds, in order to generate the electrical signal to move the magnet you need some sort of power.
FUN FACT: Did you know that a speaker could also be used as a microphone? It may take a lot of yelling to move the speaker cone, but it can be done. The air waves would move the speaker which moves the magnet in the coil. That magnet moving in the coil of wire induces electrical current inside the coil and creates an electrical signal.
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