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We have the capacity to reverse the polarity of a DC voltage, at least as regards what we apply it to. When we consider a DC motor, like, say, the ones in the little "robot wars" we see on TV, the motors can be run in reverse by reversing the polarity of the applied DC voltage. We normally consider AC as already reversing its polarity. It is, after all, AC, which is alternating current. The current alternates because the voltage alternates. We'll avoid any further discussion of the polarity of AC voltages here and leave it at that.

Answer

The problem, here, is that 'polarity' has several different meanings.

Polarity, in the sense of 'electric charge' cannot really be applied to a voltage (potential difference) although it can be to a potential (although the polarity is relative; it depends on the point of reference).

Polarity, in the sense of 'direction', can be applied to a voltage. For example, is the voltage acting clockwise or counter clockwise around a circuit?

Polarity, in the sense of 'connection', can be applied to a voltage. For example, it's important that the line conductor is connected to the centre terminal of an Edison lamp holder or to one or other of the slots in a receptacle (power socket).

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