Piezoelectricity Question....

[Roen Hayden]

I have been wondering about something about Piezoelectricity.  If you squeeze the crystal does it release electricity as you squeeze or does the crystal have to go back in place before more electricity can be released? Basically if there is continuous pressure on the crystals will the crystals emit electricity? I don?t know if this is best way to explain it without being in person.

[f_dyne]

The crystal converts the pressure energy into charge separation.
For example you could have, when you sqeeze, a plus-minus separation on two faces.
Then, when you release, the crystal does the exact reverse (it 'sucks' inside the charges produced before, or if the charges have been shorted, it creates new 'inverted' minus-plus charges).

F_dyne

[Laserrod]

I agree.
It is a static action; increase or decrease presure and the voltage follows, ignoring losses and inconsistancies.

Attach a mirrors to several modulated piezos and make a laser show!

[Shanti]

The crystal converts the pressure energy into charge separation.

That's true!

(it 'sucks' inside the charges produced before, or if the charges have been shorted, it creates new 'inverted' minus-plus charges).

Well, that's not really correct.

The principle of the piezo is simple. If you press the chrystal, the different charged atoms in the crystal lattice get forced into a non-symmetric configuration, and therefore now the chrystal becomes an electret! It does neither absorb nor output any electrons, for it is an insulator!!
But if it becomes an electret it will generate an electric field! And if you now place a neutral conductor nearby (two plates) they will try to exchange electrons so long until the electric field of the two plates completey cancels the electric field of the "electret" (Piezo) (So you will see a current from one plate to the other). If the two fields cancel each other out, so that the total E-Field is 0 as seen by the two plates, no current will flow anymore. If you now release the force on the piezo, it will loose it's electret properties and become a conventional insulator. But the two plates which still have their E-Field, due to the charge imbalance made before, will now again try to get a 0-Field, and so current in the other direction will flow until the plates are again neutral. (BTW: The second part is nothing else than a conventional discharge of a capacitor with the chrystal as insulator)

[atomicX]

just because something is an insulator doesn't mean electric can't flow through.  A good insulator is an excellent resistor. 

If you press it, you'll get electricity.  If you hold it, it won't give you anything.  However, if you are clever and make a rod that stick to it really tight, then extend the rod to the sky and attach a plate to the rod, the air will cause the rod to go back and forth creating cyclic stress.  Now you have alternating current.  These kind of current are high voltage low ampere current.  Most of the time people ignore it because they don't know how to harness it.