Science Buddies: "Ask an Expert"

Hello everyone. I hope you are all doing well and staying safe. My question is can piezoelectricity work and be harvested under water? I'm tinkering with piezoelectricity in my free time but still have plenty to learn. But from what I currently understand normally you would have a piezoelectric material like quartz crystal sandwiched between two metal pieces. Those metal plates would then have copper wires connecting them to either an electric device or storage battery. When the crystal is pressed on it generates a positive and negative charge on opposite sides of the crystal which are then passed through the metal plates and through the wires to be utilized in some manner. What' I'm curious of is it possible to have piezoelectric materials submerged in water; specifically salt water since salt transfers electricity, apply pressure to those piezoelectric materials while submerged in water but not have any metal or copper wires physically connected to any of the piezoelectric materials. Would the submerged piezoelectric materials still generate an electric charge when pressed against? And if so would it be possible to have a copper cord or something in the water a few feet away absorb that electricity through the water even though it's not directly touching the piezoelectric materials? I've searched and could find no direct answer or experiment in this admittingly curious situation. If any of you have information to aid me in understanding this I would be grateful for your advice.

Hoping you are all well,

James333

Hi James333,

This is an interesting question. You're right that salt water can be used to conduct electricity.

To answer your question, we should briefly review voltage and current. Voltage is a charge difference between two points, and current is electrons flowing between those charge differences. In piezoelectricity, squeezing a quartz crystal creates a voltage difference. However, we need to "close the loop" with wires to create an electric current.

Another important concept to keep in mind with electronics is thinking about the loops where current flows. When using piezoelectricity in air, your wires from the positive and negative terminals are isolated (because air is an insulator). In salt water, your "wires" (ie salt water) are all connected (and it doesn't matter if the copper wire is nearby) so you short your circuit. This is equivalent to connecting your positive/negative terminals on your piezoelectric sensor, so we can't harness the energy from the piezoelectricity.

Squeezing the piezoelectric crystal would still create a voltage difference across the crystal structure. This would probably attract local salt ions on the positive and negative regions, instead of creating a current through your nearby copper cords, so you would not be harnessing that power.

I hope that helps. Please let us know if you have any more questions.

Best,
Charles and Renee