Is it true that I have a pressure meter that is precise enough and measures fast enough, if I use it to measure the pressure of atmosphere, the measure will vary because gas molecules hit my pressure meter randomly? So can I put a very sensitive piezoelectric material in air and get electricity from it because of the little variation in atmospheric pressure? It looks like it will lower entropy, so it must be impossible. Is it possible to put a very sensitive piezoelectric material in air and get electricity?
Yes, a sensitive microphone will pick up a signal from air molecules hitting the surface. The noise voltage will be small but it will contain useful power. The problem is that your receiver or preamplifier will also generate a similar noise power. If both devices are at the same temperature, they will generate exactly the same amount of noise power density and then you won't be able to extract any net energy from the setup. Only when one of the two devices is at a lower temperature will there be a useful energy flow. This is equivalent to any other conceivable heat engine. The most useful side effect is that one can do a sensitive and precise temperature measurement this way.