For 'big conductor', I mean something like a big steel block. For word 'isolated', I mean that the conductor doesn't be connected to anything else like GND.
It was this situation that made me thinking about the question:
As we all know, voltage will generated between the piezo electric discs' two poles when we touch it. I wonder what happens when it connects one pole to GND and the other pole to a big isolated conductor. Like the picture above.
In my understanding, for an isolated conductor, its voltage is proportional to its charge. So in the figure above, the small charge generated by touching the piezo hardly causes the voltage at the upper pole to rise. Viewed this way, connected to a big isolated conductors can be considered as grounded.
But it seems I was wrong. I tested the voltage U in the figure above with an oscilloscope. Its two poles can not only produce voltage, but also basically the same as without this large isolated conductor. PLEASE TELL ME WHY!!!
Thank you for reading my broken English. If anything is not clear, please help tell me!
yes - effectively you are treating it as a capacitor to ground so Cv = q
However the capacitance is a function of its shape and position relative to other conductors.
And unless its a parallel plate capacitor with large plates and small sepearation, the capacvitance is TINY.
What does "grounded" actually mean?
That something is connected to infinite source of or sink for electrons?
Ground may be an earth ground, or just a common reference point.
If you have an Arduino plugged into a laptop, the Arduino's ground is usually isolated from earth ground. Same thing if you have a battery-operated Arduino. You car's ground is the body, also isolated from earth ground. Sometimes on a schematic, you'll see separate "circuit grounds" and "chassis grounds".
Normally you need a "complete circuit" for current to flow. But a "big conductor" can act as an "antenna" and pick-up capacitive or electromagnetic signals, or noise. If you connect a long open-wire to the Arduino's analog input, you'll pick-up AC power line noise from the power lines all-around. (You might get noise with the digital inputs too, but the noise has to hit a certain voltage level before it will be detected as high or low.)
That won't be useful.
The Earth is a pretty big "isolated conductor", and works very well. Most people pound a metal bar into the ground and make a good connection to it.
Your steel block is NOT a conductor if it is isolated. It has the potential to be a conductor if it is made part of a circuit that carries electrical current.
Thank you sir. My assumption that a large conductor will have a large capacitance is actually based on this formula(the capacitance of an isolated conducting sphere of radius R):
Refer to this website: Show that the capacitance of an isolated charged spherical conductor is directly proportional to its radius. | Homework.Study.com
I just did the math. Even a ball with a diameter of 1 meter, the capacitance is only 1.11e-10 F. It's still a lot smaller than my piezo's(2.6e-08 F)...So its impact is minimal. (Equivalent capacitance 1.11e-10 + 2.6e-08 ≈ 2.6e-08)
I guess when attaching it to a ball R > 100m , the result will be a little different...
I just connected the bottom pole in the picture to the GND of the oscilloscope probe. 
Thank you for your knowledge. What I'm curious about is whether the voltage fluctuation between the circuit ground and the earth ground is large. I always thought a circuit ground with a lot of copper and a lot of conductor would stabilize the potential.
Please understand the difference between static electrical charges and electrical current. Your formulas all deal with static electrical charges and will apply to any material. You can make a large ball of plastic and the formulas will still apply, but no electrical current is flowing.
Work it out for the Earth.
I have seen an "earth" - yes connected to a very good earth in the ground - show a 70V signal at 13MHz.
The CIRCUIT ground is the basis for all your measurements, so the actual potential of the EARTH - or any other planetary body - is irelevant.
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