Is this Live/Neutral transition true and instant?

[Kevin J]

On the right hand side the voltage changes polarity everytime, now, that I 'earth' one of the wire, its now called Neutral, which is 0V(for convenience), and voltage alters in the live wire between +240/-240. My question is this transition true and instant?
*
and if you could help, I don't really understand how does 'earthing' the wire suddenly makes it 0V, isn't the wire still connected to the source which offers 240VAC?

 

[sophiecentaur]

Both circuits are equivalent as far as the device you plug in. There is 240V AC across the terminals in both cases.
The left hand diagram does not define any potentials with respect to Earth (for instance, if you touch part of the circuit you could expect anything. The right hand circuit has its Neutral wire connected to 'an' Earth. The potential of the neutral will therefore be at or near 0V with respect to someone touching that side of the circuit. There are many different domestic mains supply standards around the world and do not pay too much attention to someone who insists that things are the way they describe in detail. The only thing all systems have in common is that the Neutral wire should be near 0V with respect to Earth and safe to touch. (But don't try until you have actually tested it!)

Incidentally, you seem to be assuming that the volts w=swing between + and - 240V. In fact they swing between about peaks of + and - 330V. The 240V figure is the equivalent voltage of a DC generator, supplying the same Power to the Load. Look up RMS mains voltage on Google and find many discussions and explanations.
Edit,: In the US you can often find a Neutral wire with two 120`V wires, in anti phase so that there is 240~V between them

 

[CWatters]

You asked how fast the voltages would change when you make the Earth connection. Perhaps make sure you understand how it would work for DC first...

Consider a 9V battery. The two terminals we usually label 0V and 9V. However all voltages are relative so when we say one node is 9V we mean it is at 9V with respect to the other terminal.

However which node we define as 0V is arbitrary. We could easily choose to define the +ve terminal as 0V, in which case the -ve terminal would be at -9V.

We could also define 0V as the voltage on a tent peg hammered into the ground nearby.

So imagine there is now a wire between that peg and one of the battery terminals. The voltages would be either..

0V and 9V
or
-9V and 0V

...depending on which terminal was connected to the tent peg.So your question now amounts to asking how fast the voltages could change between these two cases (assuming the wire can be moved infinitely fast).

The answer is the voltage changes fast but not infinitely fast. The batteries have some capacitance with respect to the world around them and this has to be charged/discharged. The capacitance is small but not zero so it take some small time for the voltages to change when you move the wire from one terminal to the other.

Edit: Here in the UK we have three live wires that are 120 degrees out of phase with each other. Most houses are small and only use one live phase. A few use all three phases. You can't connect any of these to earth. The electric co also provides a Neutral that is usually at a similar voltage to Earth but not always connected to it. Sometimes they do connect them together, sometimes you have to provide your own safety Earth (typically a copper rod hammered into the ground). Whole books can be written on this topic.

 

[Kevin J]

But why does the neutral becomes 0V, why should it 'choose' the 0v, isn't it still connected to a 240V?

 

[CWatters]

All voltages are relative and as Sophiecentaur said...your circuit on the left doesn't define what the 240V AC is measured with respect to.

We are assuming that it's floating (like a 9V battery) so either terminal can be connected to earth/0V.