How are these earphones receiving power?

[Phloxicon]

I was reading about how radios work and I came across a line I didn't understand on this page:
http://electronics.howstuffworks.com/radio9.htm

Your wire antenna is receiving all sorts of radio signals, but because you are so close to a particular transmitter it doesn't really matter. The nearby signal overwhelms everything else by a factor of millions. Because you are so close to the transmitter, the antenna is also receiving lots of energy -- enough to drive an earphone!

Is it just inductive coupling ( http://electronics.howstuffworks.com/wireless-power1.htm )? If so, how? I thought that needed a closed loop. Is it something else?

 

[Archosaur]

This is an interesting question! I was stuck on this for a while, too.

A speaker of any kind is less of an electrical device and more of an instrument that is played by whatever you plug it into.

The device (lets just say, an ipod) kicks electrons around and very rapidly changes the voltage at the 1/8" headphone jack. If you plug in headphones, the electron-kicking pushes the diaphragm of the speaker back and forth.

If you'll notice, playing the headphones at a higher volume will drain the battery of the ipod faster.

Also, using larger headphones, like studio monitors instead of ear buds, will drain your battery faster. Because the diaphragm is larger, more energy is required to move it back and forth, resulting in a better sound and a shorter battery life.

 

[Archosaur]

Sorry for the double post, but I realize i didn't quite answer your question.

The article you linked essentially tells you to stick an antenna in the ground and place the leads of a pair of headphones around a diode.

The same thing is happening here.

The radio station is powering the headphones (playing them like an instrument).

The radio waves kick electrons up and down the antenna, but the diode makes the headphones only "see" current movement in one direction. So instead of feeling +1 -1 +1 -1... it feels +1 0 +1 0...

The radio waves alone are enough to generate sound in the headphones.

You might now ask, why do you have to plug radios or tvs into the wall at all then??

Radios and tvs use transistors, which are essentially valves that can open or close a circuit.
The radio waves are not strong enough to generate a loud sound, much less the electron stream that (old) tvs use. Instead, the radio waves open and close the valves (transistors) and a more powerful current (the one from the wall) travels through it. So essentially, the signal is duplicated on a more powerful circuit. This is how transistors "amplify" signals.

Sorry if you knew all this. It was hard to tell how much you knew already just based on your post.

 

[mgb_phys]

The Radio transmitter generates a powerful electric field.
In this field there is a voltage difference between two points - in this case the top of the wire and the ground.
A crystal earpiece uses very little power compared to regular headphones because it is only moving a very small slice of piezo crystal (the sound is very quiet and very poor)

The signal from the transmitter spreads out in space as you get further away an so once you are a few miles from there is only enough field strength to move very small amounts of electricity - too small to drive speakers - which needs to be detected and amplified by your radio. This amplification needs power which is why your radio needs batteries.

You can see a similair effect if you hold a fluorescent light tube under a power line with one end grounded, there is enough electric field to light up the tube

 

[Phloxicon]

Brilliant answers, guys. That solved it. Thank you both very much :)