Consider a piezo material, say quartz. It produces electric charges when deformed. Suppose a static force is applied to it, which results is charges on two surfaces. If these charges are drained away but the force (and hence the deformation) is not removed, then will the piezo produces charges again on the surfaces?
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A piezoelectric material does not "produce charges".
The strain causes charges to shift within its structure, creating an electric field that can be sensed externally. For example, you can put electrodes on either side of the material; the field will cause charges to shift from one electrode to the other through an external circuit.
If you short the two electrodes together, you simply create another field that opposes the original one, and this eliminates the voltage difference between the electrodes. After that, nothing further happens until the strain on the material changes again.
Dave Tweed
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so what is the source of the electric field created after the strain? is it not the accumulation of charges on opposite surfaces? – akm Sep 06 '16 at 04:03
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No. Piezoelectric materials are generally insulators -- charges are not free to move in that way. The strain creates tiny electric dipoles throughout the material, each with a small field associated with it. These fields add up to produce the external field that can be sensed. – Dave Tweed Sep 06 '16 at 10:29
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So what is the source of current when the circuit is closed using a strained piezo material? is it the electrode? – akm Sep 06 '16 at 18:13
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Like said, the field will push charges through an external circuit -- that's current. Yes, those charges come from the electrodes. However, the movement of the charges eventually cancels the field, so the current drops to zero with a time constant that is proportional to the capacitance of the electrodes multiplied by the resistance of the circuit. – Dave Tweed Sep 06 '16 at 18:28
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so what you are saying is this. a static strain creates potential difference across the two surfaces of the piezo. when two electrodes along with an external circuit is attached to the piezo, current flows because of potential difference between the electrodes, which further leads to development of equal and opposite potential difference between the electrodes effectively countering the piezo potential difference. is my interpretation correct? – akm Sep 06 '16 at 18:46
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Yes, that's correct. – Dave Tweed Sep 06 '16 at 18:59
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then this would mean if the electrodes and the circuit is removed the potential difference across the piezo would still exist. is this correct? – akm Sep 06 '16 at 20:00
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Yes. What is your point? – Dave Tweed Sep 06 '16 at 21:17
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Consider this. Put a piezo under static strain. connect it to electrodes and a circuit to do some work (output), say light a bulb. disconnect the electrodes from the piezo, now current will flow in opposite direction and will do the work again. Infinite sequences of connection and disconnection will do the work for infinite time, while only input would be energy spent in connection and disconnection. this input can be powered by the output itself. is this not a perpetual energy machine? – akm Sep 06 '16 at 23:09
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No. Physically removing the electrodes from the piezo requires you to do work against the electric field. Each time you do that, you add energy to the system. It's just like an electret microphone, which incorporates a film that has a permanent E field. The energy in the signal coming from the microphone is supplied by the air pressure variations (acoustic energy) that move the diaphram. – Dave Tweed Sep 06 '16 at 23:17
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If you simply open and close a connection, then the charges do not "flow back" in the manner you describe. – Dave Tweed Sep 06 '16 at 23:24