Question about Piezoelectricity

[superpaul3000]

I was reading the wiki article on Piezoelectricity and this effect is either really odd or I'm missing something obvious here (I know wikipedia is not the best source but I'm feeling lazy). The article states that a Piezoelectric material will produce a voltage in response to mechanical stress. So looking at the picture there it looks like simply applying pressure to something will produce a voltage that I can run an electric load on. If that is true then we can just put one of these under a mountain and humanity's energy needs are taken care of forever!? Something about that just doesn't make sense. Do they mean that the increase in pressure produces a voltage but after that pressure is done increasing the voltage drops again? So dropping a mountain on one of these would make a lot of energy obviously lol.

 

[Dickfore]

I was reading the wiki article on Piezoelectricity and this effect is either really odd or I'm missing something obvious here (I know wikipedia is not the best source but I'm feeling lazy). The article states that a Piezoelectric material will produce a voltage in response to mechanical stress. So looking at the picture there it looks like simply applying pressure to something will produce a voltage that I can run an electric load on.

It seems you missed the third sentence in the article you were reading.

If the material is not short-circuited, the applied stress induces a voltage across the material.

 

[superpaul3000]

It seems you missed the third sentence in the article you were reading.

I see now. That seems to rule out the ridiculous mountain scenario. So if you can't close the circuit then how is it used to power devices like when it is put in peoples shoes?

 

[Dickfore]

Because the stress is variable in time, constantly creating new charges on the plates of the capacitor.

 

[uart]

I see now. That seems to rule out the ridiculous mountain scenario. So if you can't close the circuit then how is it used to power devices like when it is put in peoples shoes?

Hi Paul. The trick is that these piezoelectric devices can only displace the charge in the circuit by a very small amount, so that rules out the possibility of providing the constant DC current that you envisage.

Note however that you can get useful amounts of high frequency AC current flow even with very small displacement of charge! So that's the catch, you can't use them to deliver constant power under a static load (like your "under a mountain" example), you have to continually alternate (or pulsate) the mechanical stress in order to obtain useful output power.

I hope that helps.

 

[superpaul3000]

Hi Paul. The trick is that these piezoelectric devices can only displace the charge in the circuit by a very small amount, so that rules out the possibility of providing the constant DC current that you envisage.

Note however that you can get useful amounts of high frequency AC current flow even with very small displacement of charge! So that's the catch, you can't use them to deliver constant power under a static load (like your "under a mountain" example), you have to continually alternate (or pulsate) the mechanical stress in order to obtain useful output power.

I hope that helps.

Ok that makes a lot more sense. I think that was my understanding I guess the article just confused me. I'll update it to make that more explicit. This is a cool effect. I wonder if it can convert atomic scale kinetic energy into electricity like it can do with macro scale kinetic energy. Like if there was a wall of nano sized Piezoelectric plates that an air molecule could bounce off and some of that momentum would be converted into electricity. Is that possible? If so I wonder if it could be used to take thermal energy out of the air. That way I can chill the beer in my cooler and run my laptop of that energy at the beach. :)

 

[Dickfore]

I'll update it to make that more explicit.

Please do not update anything since it is obvious you are incompetent to do so.

 

[superpaul3000]

Please do not update anything since it is obvious you are incompetent to do so.

Well that's the great thing about wikipedia. Check out what I wrote and fix it if you see anything wrong. Just because I'm incompetent doesn't mean I'm always wrong. So what do you think, is the cooler idea not possible?

 

[DavidSullivan]

Ok that makes a lot more sense. I think that was my understanding I guess the article just confused me. I'll update it to make that more explicit. This is a cool effect. I wonder if it can convert atomic scale kinetic energy into electricity like it can do with macro scale kinetic energy. Like if there was a wall of nano sized Piezoelectric plates that an air molecule could bounce off and some of that momentum would be converted into electricity. Is that possible? If so I wonder if it could be used to take thermal energy out of the air. That way I can chill the beer in my cooler and run my laptop of that energy at the beach. :)

This concept is already in use at the macro-level in piezoelectric microphones. So, it won't cool your beer but you can rock with it!

 

[DavidSullivan]

I was reading the wiki article on Piezoelectricity and this effect is either really odd or I'm missing something obvious here (I know wikipedia is not the best source but I'm feeling lazy)

My experience - Wikipedia is worthless if you're trying to learn something of any complexity. If you already know it, but have forgotten the formula's,etc., it can be right handy.

 

[superpaul3000]

This concept is already in use at the macro-level in piezoelectric microphones. So, it won't cool your beer but you can rock with it!

That much I know. The difference between the two examples being that sound waves create mechanical stress large enough to produce voltages big enough to have detectable currents given the resistances we are dealing with. The voltage from an air molecule bouncing off the piezoelectric device is minuscule. I think the problem with the piezoelectric cooler is that R>>V and thus current is negligible. What if we had a room temperature superconducting closed circuit, could we charge a superconducting battery (not chemical) on that and cool the air as well?

 

[superpaul3000]

Actually a total closed superconducting circuit is not necessary. I guess it must be superconducting from the capacitor plates surrounding the piezoelectric material, through a transformer that can step it up to some usable voltage, and up to the electrodes. Then you can apply any load.