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AC and Appliances

  1. Feb 2, 2008 #1
    Could someone answer my question about AC. When current is coming in an appliance in a house then where is it coming from and where is it going after passing through the appliance? And when it changes its direction then what is its source and what is its end point? Since we do not have battery terminals here so what is going on here in terms of source and end point of the current?? Hope I could phrase my question properly?
     
  2. jcsd
  3. Feb 3, 2008 #2
    It is really very similar to a battery if you were to reverse the connections at the battery 120 timer per second.
     
  4. Feb 5, 2008 #3
    Where is the current coming from. Generators - at the moment burners - burn something - heat water and make steam to turn a turbine around. A magnet moving next to a wire induces current.The energy from the steam is turned into electricity by this simple movement.We transform mechanical energy into electrical energy. The current is AC because a magnet has two poles. The current comes to your house(in England) normally as a single phase supply - ie positive wire and neutral. The current flows from the positive to the neutral and then from the neutral to the positive 50 times a second in England and 60 times a second in America. The end point of all current is the source which is the generator.
     
  5. Feb 11, 2008 #4
    OK, What I understand is that the generator pushes the current in to the positive wire and then it goes to the appliance and then neutral wire, am I right? Where does it go from the neutral wire? Where does the neutral wire has an end and what happens to the current at the end of the neutral wire? An what is pushing the current from the neutral wire to the positive?
     
  6. Feb 11, 2008 #5

    mgb_phys

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    That's where the current=water anaology breaks down.
    Electrons don't move in an electrical wire - fields do.

    So the neutral wire stays at zero V and the live wire varies from -340V to 340V above and below this (in a European 240V system). It is the difference in voltage that causes the fields which do the work.

    The US has a slightly different 2nd system for where the neutral has a 110V varying signal so that it is 220V away from the live and you can get twice the voltage.
     
  7. Feb 11, 2008 #6
    Back up a minute. I'm not going to comment about the systems in Europe, but you are way off concerning the US electrical system. The neutal is the reference. It has no voltage on it because the system voltage is always referenced to the neutral.
    -
    In the US, the so-called neutral wire in residential systems is tied to the center tap on the secondary of the transformer. Each hot wire is tied to remaining 2 leads on the secondary of the transformer. There is ALWAYS 120 volts RMS between the neutral and either hot wire. The neutral is also tied to the earth through a ground rod. This is what makes the 2 'hot' wires 'hot'. It isn't about which direction the current flows or anything like that.
    -
    Now to help answer the original posters question, if you understand how a tranformer works, you should understand the system based on what I've just posted. The current flows out of one wire from the transformer and into another. The next half cycle the same thing happens in reverse.
     
  8. Feb 11, 2008 #7

    mgb_phys

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    Sorry - a terminology thing. I thought that there was a 0v ground and one of the hot wires was still called neutral even though it was live.
    Instead they are called neutral and 2 lives?
     
  9. Feb 11, 2008 #8

    russ_watters

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    Anyway, for that second question, again, consider a battery connected to a light. It has two wires, one sorta like the "hot" wire and one sorta like the "neutral". So with the AC power to your house, both wires are connected back to a generator at the power company.
     
  10. Feb 11, 2008 #9
    Yes.
     
  11. Feb 12, 2008 #10

    stewartcs

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    Give this a read...

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html

    CS
     
  12. Feb 12, 2008 #11
  13. Feb 12, 2008 #12
    I didn't read the link in much depth because I don't have time until later today. A 240 volt appliance does not have a NEUTRAL wire. It has a ground wire, but no neutral. Do a search on this here on PF and you will have many explanations.
     
  14. Feb 12, 2008 #13
    This question seems to bring us to all sorts of issues on generation of electricity. For any electricity to flow there must be a ring from the source and back to the source. In the middle is a load which the source 'sees' as resistance. The return wire does not have to be at 0 Volts but there are good reasons for making it so which I will return to later on. As long as there is a voltage difference at the source between the two ends of the wire then current will flow. The amount of current is given by the equation V=IR.
    In England, the return wire is the neutral. This return wire is kept at 0 Volts by essentially sticking it in the ground - earth has no potential. We fix the neutral to earth - ie zero potential at the transformer which serves a series of houses
    The advantage of doing this - please do not confirm this by personal experiment- is that touching the return wire will not give you an electric shock as there is no voltage difference between you standing on the earth and the wire. Also, all neutrals in a house for example are at the same potential. There is current flowing in this return wire though.
    The question you are hinting at is where is the force in electromagnetism - how does a moving magnetic field create electricity, what is its force and how does it do work? This is a difficult question to answer but we can describe adequately what is happening with Maxwell's equations. The reply from mgb_phys is helpful. The changing electric and magnetic field moving at more or less the speed of light moves electrons - we see this as voltage and current - and it is this movement of electrons which causes light and heat in certain materials. Again, comparisons with mechanical systems such as water or air are of no use here.
     
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