Why is there a live and a neutral in a wall connector?

Question

I get the ground. Since you can plug the plug in either way there is no way for the appliance to know what's in what wire, right?

Answer 1

Hot and neutral are the supply and return lines. Ground/earth is a safety line and does not normally handle current flow.

What defines a neutral is that some effort has been made to keep it near earth potential. It is not quite at earth potential** and neutral is not ground.

Now, some plugs and receptacles are polarized, either by virtue of differing pins (the wide blade on NEMA 1), or simply a side effect of having a third grounding pin not inline. In that case, the appliance knows which pin is neutral, and can be designed internally with that in mind. The classic example is a screw-in bulb fixture; the easily-touched outer shell of the socket is made neutral, and the harder-to-reach tip is "hot" and also switched.

If its plug is not polarized (Schuko, GU10, GU24), most likely the appliance is required to comply with a higher insulation standard, so the appliance will be nominally safe even if the prongs are reversed.

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** and sometimes more than a little not at earth potential

Answer 2

The appliance doesn't care about the direction, because the current is AC, meaning it is alternating directions 50 to 60 times a second.

The neutral wire takes current back to the circuit breaker which takes current back to the power plant to complete the loop.

See also: https://electronics.stackexchange.com/questions/134869/does-it-matter-which-way-you-plug-stuff-into-the-wall-socket

Answer 3

Because the windings in a split phase transformer are 180 degrees apart with the neutral in the center.

Most appliances use one 180 winding or the other (120V RMS) and some use both windings (240v RMS).

In the case of the latter with pure resistive loads like some heaters, a neutral isn't necessary because both currents on each winding (leg/pole) are balanced.

Some appliances use both windings and require a neutral (240/120) like a dryer or oven because of non pure resistive loads like electronics may create minor imbalances​.

At the end of the day all the imbalanced currents from all the appliances like computers, fluorescents lights etc, goes on out the service neutral to the transformer where it came from.