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How can polarity of AC inverse but keep the live hot and neutral cold?

  1. May 7, 2014 #1
    In an AC circuit, we know that the polarity inverses, and what i know is that the flow of current also will therefore inverse.. which means that the live will become negative and the neutral will become positive.. What i can't understand is how the polarity inverses but the live is still the hot wire and the neutral stays the cold one? i mean the polarity of each should have been reversed..
    I need a very good explanation of it, I've asked many people the same question but any clear answer.. someone here may help me understand this?
  2. jcsd
  3. May 7, 2014 #2
    The answer to your question is simple. Cold wire is connected to ground (earth). So there is now potential difference between cold wire and ground. And this is why cold wire always stay cold.

    As a remainder

    To measure the voltage we need two point in the space. One of this point is treat as a reference point. We have a very similarity situation when we try to measure a height of an object. We need a reference point. The most common reference pint is "above mean sea level". But when you measure the height of the table in your house the floor now becomes your reference point.



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  4. May 7, 2014 #3
    It may be best to refer to a diagram - The Neutral is typically connected to Ground - so when we measure the Neutral - relative to the ground - we see no voltage ( under normal / ideal conditions).
    Note the normal load current still flows in the ground, and this current changes direction with the AC Cycle.

    There are rare cases where the neutral is not grounded, in that case it will have a voltage ( relative to ground) and must be treated the same as a "hot" ( Also note - from a safety standpoint- the neutral should always be treated as a "hot" lead - e.g. calling it "cold" is not correct)
  5. May 7, 2014 #4

    jim hardy

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    Jony and Windadct both hit it.
    But there's an easier way to remember. Awareness of it will keep you out of trouble later in your studies.
    It goes back to the definition of voltage, which we tend to forget.

    Voltage is a potential difference.

    Your "Cold" wire is earthed, so it can't develop(or sustain) a potential difference between itself and earth.

    Your "Hot" wire can develop (or have applied to it) potential difference between itself and earth,, and of either polarity.

    So when you speak of voltage, always ask yourself "Between where and where else?"

    That's why voltmeters have two leads.
  6. May 8, 2014 #5


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    That says it all. The fact that one of the conductors is 'chosen' to be held at near Earth potential has nothing to do with what's going on 'between' the two conductors. If you were bouncing up and down on a bungee cord, you wouldn't be aware of whether you jumped off a high bridge or had been let go from a hook on the ground.
    Some electrical supplies use two 'floating' conductors (the outputs from an isolating transformer). The advantage is that, if you happen to get connected to either conductor, then it quickly comes to your (Earth) potential and you do not get a shock. Floating systems are only suitable in certain situations, where just one piece of equipment is being fed.
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