How to rapidly switch polarity of two plates?
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.