I understand how to do calculations involving potential difference but I've never been able to find any information that helps me understand what is physically happening to cause it. My assumption is that for a PD to occur, there must be a build up of charge to cause the difference in potential. But if the charge is physically there to cause the PD, what stops is flowing? If a 220V wall socket can deliver enough current to be fatal, how can a 1,000,000V stun gun only cause a shock? Assuming the internal resistance of the individual experiencing the shock is the same in both situations. Am I right in asking whether the stun gun does have such a high PD because of a build up of charge it posses? How does this result in a low enough current flowing to make sure its not fatal? Is it due to the way the charge is delivered, possibly its duration of discharge? Why is it even necessary to have a device possess such a large PD, what benefit would it have over a 100,000V version? Now that I've typed all this, it sounds really stupid. I can't think of a better way to explain my uncertainty though.