Question: How much charge is contained in the crystal?

Answer:  A lot.  Every electron constitutes a negative point charge.

Real answer:  Not much.  Most electrons don't want to move.  Energy must be put into the system to get them to leave their nice cozy molecules.  In the case of piezoelectricity, energy is added through stress forces.

Question: How long will the crystal continue to generate a charge when deformed?

Answer: Charge is never generated, it is only moved around.  The key idea is that charge comes in two flavors:  positive and negative.  These two types of charges attract each other with a force many magnitudes stronger than gravity, so that the universe is generally pretty neutrally charged.  However, if we can coax some negative charge to accumulate, then we create a potential force between that accumulated negative charge and a less negatively charged region.  This is called voltage.  Voltage is a potential force that can do work via the electric force, just like water that has been pumped into a water tower can do work via gravity.  So let's rephrase the question.  How long will the charge remain on the surface of the crystal?  Indefinitely, as long as no work is done.

@shadysaint Excellent. Alright, how long will the charge remain on the surface of the crystal if we keep stressing it? In other words, is it like a battery that is eventually exhausted?

So, it is not like a battery and will never exhaust? "As long as you keep inserting energy by adding (and releasing) stress, there will be a charge."

Sorry if I can't read between the lines.

Last edited by turvy (2012 June 14, 9:25 pm)

Clear analogy, so just like I can't run out of water (as long as I keep filling the tank) I can't run out of crystal, right?.

My understanding is that a crystal with pizoelectric potential is kind of like a battery with - / + charges fixed in their positions and when pressure is applied to the material, causing it to change its form, the charges will flow / be exchanged inside the crystal creating voltage. When the pressure is released and the crystal assumes its original form the charges will return to their places.

Last edited by turvy (2012 June 14, 11:59 pm)

You can to some degree think of a piezoelectric crystal as a capacitor (and I think this is more accurate than thinking of it as a battery). Once you apply pressure the crystal deforms in a way that causes the molecules to gain an electric dipole moment, which effectively leads to a charge separation. Just like a charged capacitor, this charge could theoretically remain there indefinitely if there was no way for the charge to move around, i.e. nothing the capacitors could discharge on (assuming the stress is also applied for an indefinite amount of time). As somebody already noted though, the charge will slowly decrease in any real environment as they will slowly discharge on their environment. Just like a capacitor, a charged piezoelectric crystal will lose its charge almost immediately when used to drive a circuit, which is probably a good thing as they would hardly be usable in microphones if this was not the case. Additionally, if you keep applying the same amount of stress to the crystal no further charge separation will occur, which means that once discharged no further electricity can be generated if the stress is kept constant. However, if you were to apply more stress to the crystal, the electric dipole moments grow stronger and the crystal will get charged again. This is not a process that can be continued indefinitely as the crystal will eventually break though (assuming you could apply enough stress to break the crystal in the first place, which would probably require a fair deal of stress).

Edit: If you were to release the pressure and let the crystal return to its original state, it would charge again as the charge separation would disappear, effectively giving you a charge separation in the other direction charging the crystal once again (but with the poles reversed). The energy to do this comes from stored mechanical energy in the crystal's stressed state. Once returned to its original state you can apply stress to it once again to harvest more electrical energy.

Disclaimer: Not 100% on the part about charge being generated with opposite polarity when you release the stress as the source seemed a bit sketchy (and I could only find one that stated it explicitly), but the physics seemed legit to me as a physics major on my first year for whatever that is worth.

Last edited by Isbilenper (2012 June 15, 5:27 am)