Single phase circuit theory

Hello,
i just want to confirm
Is it right that in a single phase circuit 220v A.C 50 Hz
we have two wires one phase and one neutral. and in a whole one cycle 20ms one time phase wire becomes neutral then comes back again to the phase and same thing happened with the neutral wire???

for a half cycle of ac suply, in positive and other half cycle in negative. in postive cycle the current flows from phase line to neutral line& for other half cylce the current flows from neutral to phase line..

It's important (for safety reasons) to know that the "Neutral line is ALWAYS the neutral line". The "phase" line is also known as the "hot" line, because relative to earth ground, it obtains lethal voltage levels. In a properly wired system, the neutral line will always be at zero volts (or close enough to zero to be non-lethal).

I'm offering this because of the way your question was languaged. Just in case that was a misunderstanding, rather than awkward English

Dear Sir steve ,

Before i read for three phase system i just want to clear about single phase. what ever said by Naresh K is right for single phase?

In a complete cycle single phase system neutral wire and phase wire interchange and then come back to original position. This all happened in 20ms and we cant observe it ?

No, the neutral wire and the phase wire do not interchange. The _voltage_ between them changes through a cycle as follows: arbitrarily lets start from zero difference -- so it begins with a zero volt difference between the neutral and phase lines, then to a difference going positive until the difference reaches around 310V (220 * 1.41), then the difference begins to fall until it reaches zero again and then the difference goes negative until it reaches 310V in the opposite polarity, and then diminishes again until it reaches zero.

I used the word "difference" over and over because I wanted to emphasize that it is the voltage _difference_ between the neutral and phase (or 'hot') lines that cycles, NOT the _neutrality_.

Across this same cycle, the neutral line will remain very close to earth ground potential (if the system is wired properly). It may go slightly above and below Earth ground because of voltage drops in the wires due to current flow, BUT the neutral line is always the _neutral line_.

And, BTW, in case this isn't clear, there are NO absolute voltages, only relative voltages. Another way of saying it is: voltages are always _across something_ or _from one point to another_. Even when you say, "the voltage is..." you are speaking in terms of a relative value. The complete phrase is, "the voltage is X relative to _ground_ [or _common_ or _neutral_]. It's just easier to omit referring to the reference. But, it's understood by convention.

I belabour this because it's important to get the language right, otherwise your colleges may misunderstand your meaning.

Think of the graph of a sine wave.
The zero line represents the voltage on the neutral wire.
The sine curve represents the changing voltage on the live wire, going between its positive and negative values relative to the neutral wire.
At 50 Hz, the complete cycle starting from zero, going to its positive maximum, returning through zero and going to its negative maximum, then back through zero, takes 20ms.

Attached: sine drawing process from Wikipedia.
(Animated GIF - download this image to view animation if it doesn't animate in your browser.)

My respected sir steve and frank
Can you please clear me one thing when half cycle complete and sine wave moves in negative direction then which wire is on higher potential?
phase or neutral?
i think, although in negative side max difference is 310v as said above sir Steve, it means after half cycle current start to move from from neutral to phase?

You need to learn to think in terms of "in relation to". In relation to the neutral line, the phase (or "hot") line is at a higher potential than the neutral line both during the positive swing and during the negative swing. In relation to the hot line, the neutral line is at a higher potential.

Now you may be screaming "WHAT?!?" at this point, so let me elaborate: In BOTH cases, the neutral line is at zero or near zero potential in relation to earth ground (again, IF the system is wired properly) -- that's what _makes_ it the neutral line.

You can say "the hot line is at a high potential" at some particular time, without mentioning the neutral line, if it is clear that this is in reference to the neutral line. It's ALWAYS the case that a potential is in reference to some node, whether implied or not.

Also, the term "higher potential" refers to the magnitude... so, both during the positive swing and the negative swing the potential difference is greater than zero between the neutral line and the hot line.

The question "which is at higher potential, hot or neutral?" is nonsensical! Higher than what?

Think _potential difference_.