tahir4awan:
Voltages are always values compared to some 0-point. In DC sources the same 5V source can give you either -5 V and 0 V or 0V and +5V, that's simple, right? If you consider an AC (giving a signal of A*sin(x*t)) source you can calculate/think of it in different ways:
1) Again one end is considered as 0 V, no matter what the source gives you, as in DC case. Then you can calculate that in the other end is a A*sin(x*t) signal.
2) Or you can choose the 0 V point be in the middle of the source (as in a transformer that has 3 output leads on the secondary winding). In that case both ends of the AC source give the same signal, with a phase difference of 180 degrees. That means that signal on one end is inverted in the other end (that's why there's a "-" sign). Now the signals would be A/2*sin(x*t) and -A/2*sin(x*t). Calculating with actual voltages and RMS-values is a bit different.
3) You can also have a inverted case of the first case, or whatever level to be chosen as 0V. But case 1) gives probably the easiest formulas for calculations and that's why it's often used.
Exactly the same current goes through the AC source from both sides in either case. the current is dependent on the rest of the circuit. Either side can be considered neutral however you please. And you can touch either end (Live or Neutral) if you keep isolated from the other end. You touch the neutral end probably very often (here in Finland every time you touch kitchen sink or whatever). If you were a bird, you would touch the live wire more often, as you would enjoy sitting on a non-isolated power cable.