Can anyone help me develop some insight as to what positive and negative voltage means? I've heard of the analogy to water flowing where amps are viewed as units of water flowing through a certain point per unit time and volts are viewed as how high the water is flowing downwards to the ground. Thus, I'm inferring that the positive voltage is how high it is relative to the ground but where is the negative in this picture? Also, in AC, since the polarities are switching back and forth, wouldn't that mean that the charges are going to ground as well? How would this provide useful work?