If I have two metal plates, with P1 connected to V+, and P2 connected to GND, how can I alternate the polarity of these two plates? I wish to have P1 connected to GND, and P2 connected to V+. Then swap polarity again, and so forth. Is there such a circuit for this type of polarity switching?
The answer will depend on how fast, and what voltage. For occasional switching you can use a dpdt relay wired as a reverser.
I found a tutorial for this here. If that dies just search "DPDT reverse switch" or "DPDT reverse relay".
To go faster (like many times per second), you'll want to use an H bridge. For lowish voltages, you can get a fully integrated module from amazon based on the LM298N motor driver.
For higher voltages, you might have to build your own H bridge using high voltage components.