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How does current flow in a wire?

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  • 13-10-2018 7:45pm
    #1
    Pussyhands
    Registered Users Posts: 8,239 ✭✭✭


    So I'm trying to understand how electricity works and all that.

    I thought I cracked it and that the power flows through the live, through the appliance and then it returns through the neutral and flows into the ground.

    Then I saw another video where it shows the AC reversing direction, so it flow from neutral through the appliance through the live.

    Question:

    How does it actually flow?

    If it flows as above, how come when the AC reverses that it doesn't flow into the ground and not into the appliance?

    Is the neutral connected to the same ground that the ground wire feeds back to?


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    #2
    Michael Collins
    Moderators, Science, Health & Environment Moderators Posts: 1,846 Mod ✭✭✭✭Michael Collins


    Good post!

    There are actually two separate questions here. Your title mentioned "how does current flow in a wire", and in your post you talk about "power". These are separate quantities.

    You're correct that, in an AC system, the current changes direction on a regular basis. How often this happens depends on the frequency, which in this part of the world is 50 Hz. This means that a full cycle happens 50 times every second, which means each cycle lasts 0.020 seconds.

    Let's imagine we have a standard filament-based light bulb, with the live wire connected to one side, and the neutral connected to the other side. For half of the cycle (0.010 seconds), the current is flowing from the live, through the bulb, to the neutral. For the other half (another 0.010 seconds), the current is flowing from the neutral, through the bulb, to the live.

    The power, on the other hand, only flows *to* the bulb. Since the bulb is lighting on both halves of the cycle, this has to be the case - the power must be going to the bulb in order for it to light up.

    You can't say that the power is flowing on the live wire, or the neutral wire (in fact, the power is often considered to be in the air between the live and neutral wire). Over a full cycle, the power delivered to the bulb will go from zero, up to some maximum, and down to zero again, but it will always be going towards the bulb.

    I know I haven't fully addressed your other questions here, regarding ground wires etc, but I think what I've written above should help you understand what's happening better.

    Feel free to ask for more details if you wish.


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    #3
    2PiDesign
    Registered Users Posts: 24 ✭2PiDesign


    I was told in the past that the neutral wire is connected to ground at the substation.

    As long as the live wire doesn't connect to ground, there is no circuit made to ground, so you don't need to worry about it.

    That's my incomplete understanding of it anyways.

    Interesting thread!


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    #4
    Pussyhands
    Registered Users Posts: 8,239 ✭✭✭Pussyhands


    Michael Collins wrote: »
    Good post!

    There are actually two separate questions here. Your title mentioned "how does current flow in a wire", and in your post you talk about "power". These are separate quantities.

    You're correct that, in an AC system, the current changes direction on a regular basis. How often this happens depends on the frequency, which in this part of the world is 50 Hz. This means that a full cycle happens 50 times every second, which means each cycle lasts 0.020 seconds.

    Let's imagine we have a standard filament-based light bulb, with the live wire connected to one side, and the neutral connected to the other side. For half of the cycle (0.010 seconds), the current is flowing from the live, through the bulb, to the neutral. For the other half (another 0.010 seconds), the current is flowing from the neutral, through the bulb, to the live.

    The power, on the other hand, only flows *to* the bulb. Since the bulb is lighting on both halves of the cycle, this has to be the case - the power must be going to the bulb in order for it to light up.

    You can't say that the power is flowing on the live wire, or the neutral wire (in fact, the power is often considered to be in the air between the live and neutral wire). Over a full cycle, the power delivered to the bulb will go from zero, up to some maximum, and down to zero again, but it will always be going towards the bulb.

    I know I haven't fully addressed your other questions here, regarding ground wires etc, but I think what I've written above should help you understand what's happening better.

    Feel free to ask for more details if you wish.

    This actually confused me even more :(

    I understand how there is always current flowing through the appliance for it to work. I just don't understand the route the current takes.

    So let's take a kettle and plug it in. You have 3 wires in the plug, live - neutral - earth.

    Current flows from the electricity line into the house to the socket, it goes through the live wire and through the kettle. Now, where does it go once the electrons are no longer useful (i.e there has to be "useless electricity once it's "used" right or else you could power things infinitely?)

    So that's my first basic question for something simple like DC.

    For AC I know it switches direction...but where does it go?

    From watching youtube videos it looks like say a transformer on a pole outside, both the neutral and live are connected to it...So...does the current flow from the live through the kettle then down through the neutral, the current "bounces" back from the transformer back through the appliance and then to the ground?


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    #5
    Tails142
    Registered Users Posts: 2,103 ✭✭✭Tails142


    My understanding is what you're buying is essentially a maintained potential, i.e 240v RMS between live and neutral, to maintain this potential at 240v RMS as you use power, the ESB expend energy, I.e. watts, which you pay for in kilowatt hours.

    Apparent power and power factor is another interesting topic and I believe with smart meters, which allow for measurment of power factor, this will soon be charged for.


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    #6
    Pussyhands
    Registered Users Posts: 8,239 ✭✭✭Pussyhands


    Tails142 wrote: »
    My understanding is what you're buying is essentially a maintained potential, i.e 240v RMS between live and neutral, to maintain this potential at 240v RMS as you use power, the ESB expend energy, I.e. watts, which you pay for in kilowatt hours.

    Apparent power and power factor is another interesting topic and I believe with smart meters, which allow for measurment of power factor, this will soon be charged for.

    You're charged for it anyways I believe, unless I misunderstand what you're saying and you're saying they'll only charge you for the active power and not the reactive.


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    #7
    Michael Collins
    Moderators, Science, Health & Environment Moderators Posts: 1,846 Mod ✭✭✭✭Michael Collins


    Pussyhands wrote: »
    You're charged for it anyways I believe, unless I misunderstand what you're saying and you're saying they'll only charge you for the active power and not the reactive.

    Electric Ireland charges business customers for reactive power (var) if it goes above a certain threshold (power factor less than 0.95), but I haven't heard of consumers ever having to pay for anything other than the active power (watts).


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