Think of all the times you've seen the polarity reversed, either in TV, literature, whatever. All that does is literally plug it in backwards. Red is positive; it's not that hard to remember. You can reverse polarity, so to speak, in real life. Standard mains power is alternating current: meaning it flips polarity roughly 120 times a second. So that doesn't do much. You can invert polarity of DC current devices; simple electronics that use spinning motors will just run backwards; more complex circuitry will burn out. You can reverse the polarity of more complicated things, such as a magnetic field, though usually such complex devices are polarized (if they are) for a specific reason, and reversing it (if it is even possible) will likely be a bad idea. Not to mention that simply plugging things in backwards will burn them out, as noted above. If you get a situation where you're reversing the polarity of, say, a neutron beam, then just give up. (Protip: neutrons, as you might deduce from the name, have no electric charge.) I suppose, if you want to be technical, you could inverse a magnetic polarity, but that would have no effect (because, if it did, then you could achieve the same effect by physically rotating the beam 180 degrees). Of course, if you're on a starship, then reversing the polarity of anything through the main deflector will always work, because the deflector can do anything.