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How does current flow in an AC Circuit.

Discussion in 'Electrical | Electronics | Communications' started by neo23, Aug 26, 2011.

  1. neo23

    neo23 Apprentice

    I am extremely confused about the direction of ac current flow in the circuit. If current reverses its direction for every positive and negative half , so in that case does that mean that - electrons flow from phase to neutral in positive half cycle and , similarly for negative half cycle electrons flow from neutral to phase.? Please clear my views regarding this.....
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  2. Praveen-Kumar

    Praveen-Kumar Knight

  3. A.V.Ramani

    A.V.Ramani Guru

    You are right. However, the distances moved are rather small.
    In a copper wire the velocity of electrons is about 300 microns/second. In a half cycle at 50 Hz this translates to just 3 microns. So one can say the electrons vibrate about a mean position about 3 microns in 50Hz ac circuits.
  4. neo23

    neo23 Apprentice

  5. A.V.Ramani

    A.V.Ramani Guru

    Probably my reply is rather cofusing.
    1. All metals have what are called free electrons that are randomly moving. Under an applied potential they move in a direction from negative to positive potential. The electron does not have an individual identity. Consider an electron less than 3 microns from the neutrl terminal. During the positive half cycle this will enter the neutral terminal. The number of such electrons entering the neutral terminal is determined by the load current. For every 96,500 amp secs one Avagadro number of electrons will move. One particular electron from the source will not move from the source to load in each half cycle. All that one can say is that if it were possible to tag one particular electron it can be seen to reach the load ultimately.

    2. It is true that the phase wire has the potential and the neutral is at zero potential. (This is only a convention. It has zero potential with respect to earth. That is all.) However, the phase wire goes to negative potentiall in each half cycle. At this time the neutral is positive (though still zero with reference to earth) with respect to the phase wire. This means that there is a potential difference between the two. Whenever there is a potential difference across a load, a current will flow. (Ohm's Law)
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  6. Praveen-Kumar

    Praveen-Kumar Knight

    Yeah, the flow of current is reversed... :)
  7. @bioramini: sir i think you are saying about bandwidth and jumping of electrons from valence band to conduction band is that the principle
  8. A.V.Ramani

    A.V.Ramani Guru

    Not really, Murthygaru. Each conductor has a drift velocity for the conduction electron That is what I referred to. These electrons are already in the conduction band.
  9. praba230890

    praba230890 Apprentice

    Absolutely, that is what happens when A.C flows through a circuit. The electron flow direction changes for every half cycle. If the direction of the electron flow changes for every half cycle then with no doubt the current flowing direction will also be changing(as they always flow in opposite to each other).

    Let me talk about the basic for both the A.C and D.C
    Lets take a lamp to solve this problem. I call the lamp as the energy trapper. Just take it in analogy with the rat trapper. Here we need the cheese to trap the rat. What acts as cheese is the neutral connected to the ground(mostly). That is the current will start flow only when there is a closed path or there is a voltage difference set. After setting the voltage difference or closing a circuit with lamp(energy trapper) at the middle, now the current starts flowing with an idea of reaching the other end. But at the lamp the whole current flow(sometimes part of it, but mostly entire current) will be converted into heat energy. Yes we trapped the electrical energy and converted into heat. We are not letting the current to finish the cycle. But a virtual cycle exists to describe the current flowing phenomenon.
    Now i think you will be catching the point. That is the current flow mostly exists between the lamp and the source.

    It's time to talk about your A.C problem. As in A.C the current flows back and forth, but between the supply and the lamp. Don't worry about the neutral. But remember, you still need a neutral to close the circuit and create a potential difference. Now the lamp gets the electron flow continuously to produce the heat no matter in which direction and traps the current.
  10. aj_onduty

    aj_onduty Certified CEan

    From the start of my learning about the currents, what I felt about the phase and neutral is as follows.
    The alternator (generator) in a power plant has its three phases connected to the switch-yard, from where it is transmitted and distributed. The neutral is connected through the NGR to the ground(The NGR thing I learned slightly afterwards though! ;) ,not during the initial days). This means that the earth potential is the neutral. This saves one extra connecting conductor from the alternator to your home, the neutral connecting conductor. So, when you get a neutral wire from the electricity board, please do understand that actually the substation which is supplying you the power has its own ground connected to the earth, not to the generator, as the potential is equal, or the circuit is completed via ground. Now, when you take the phase wire and then make a connection through any appliance to the neutral, you are actually grounding the phase.
    Please do not confuse the earth wire with the neutral wire.
    The earth wire only conducts the illegal current(as I call it, which is found where it shouldn't be, like body of old electric irons etc), and not the phase. But it can do the same function as the neutral. But for that, you actually need good quality grounding, which no one usually has(In India ;) ). That grounding is provided by your nearest sub station.
    Now, when you connect the phase to the neutral, you are actually shorting it, and at that time, a current flows from the phase wire to the neutral wire, just like a short circuiting current.
    hope you understand that. Please do ask if there are any doubts. My explanation abilities are a bit mediocre.Sorry about that! :D
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  11. aj_onduty

    aj_onduty Certified CEan

    Usually, the AC loops.
    What I mean by saying that is that the polarity of the AC changes. If you ask me,"you said that there is no neutral coming directly from the generator, but why is there two wires of each phase coming?"
    Well, that is the loop. The generator inside has two poles(or many poles of which, if total is 4, 2 are connected with each other, thus electrically, there are only two poles.(this is just an example for understanding purposes)). The two wires coming out are connected to each of the two groups of poles.The change in direction of the current, or I must say, the vibration of current takes place only inside the loop. If you want to use or tap the potential of the same, you have to ground the loop. I can explain this whole thing in many examples, but I think this explanation is enough, because people actually complain about my looong posts.Still, analyse one thing, why do the switch-yard items in a power plant get damaged only when the circuit is shorted with ground or any other low potential source? Isn't there the same amount of power flowing through the conductors when it is not shorted? Please remove the theoritical things like voltage increases and current decreases and such boring things. Please use the resource unique to you, your head! ;)
  12. ramgopalverma

    ramgopalverma Certified CEan

    Yes you r right gimme . cureent does change its direction alternately
  13. neo23

    neo23 Apprentice

    So in DC circuits there is no current flowing from the lamp to the negative terminal , to comlete the path. As all the electrons are flowing from +ve terminal to lamp ; and are dissipated as heat and light.
    Well what i think ; is that the current flow remains same throughout the closed series circuit and what changes is the voltage. voltage gets dropped of the electrons from the lamp to the negative terminal.

    Its like a 1 metre cube water falling from a height on the ground... and then after crashing its potential energy is lost and starts moving slowly.

    Please review and correct me.
  14. Mr.Don

    Mr.Don Star

    @Aj Superb :thumbsup: but bit complicated. :p

    @neo23 Exactly not, AC or DC the flow of charge will be the same in the wire and the flow of charge will be same in any condition. Let we take a simple bulb as example.

    However, the completion of circuit is required for the glow of bulb whether it is DC or AC.

    Now, for DC it is not as complicated as it was on AC. For AC circuit, as you said there will be current flowing from positive half cycle to negative and from negative to positive. This can be clearly observed in neon light decorative band which we obviously see for house decorating or in wedding theaters. There during positive flow of charge, some bulbs will glow and some won't. Like that the glow in AC circuit can also be time(depending on frequency) dependent.

    As we take this neon lamp as example, the corresponding rods will glow according to respective half cycles and if the time period is less then both rods will glow. For example, take a LED light in your home, if present. There will be an OFF, AC and DC switch. If DC the lights will glow and if AC, same the lights will glow much brighter. All this process will go through main wire and the neutral wire will also be present closing the circuit between the load and the source.

    Another example, take an AA battery. connect an led bulb where it was required for you to connect the bulb between the positive and the negative poles of the battery. Hence, the circuit can be closed between the source and the load.

    Point to Remember : Current is about flow of charge where there will be charge in the negative wire also but it is negative energy(or waste energy where it is dissipated as heat in most cases) as it was already utilized by the bulb which based on electron, hole concept. Please Correct me if I am wrong.
  15. A.V.Ramani

    A.V.Ramani Guru

    Please do not be confused. In a closed circuit with a potential difference across the circuit, current flows. The amount of current can be calculated by the Ohm's Law. It is by convention that current flows from the positive to the negative terminal. This is unfortunate, because what really happens is that electrons flow from the negative to the positive terminal. Ifyou put a current meter between the lamp and the negative terminal you will see that the same amperage flows. Even the best conductor has a finite reistance unless it is a super conductor.
    All that happens is that, according to Ohm's law, the total potential is divided into individual potential drops across each portion of the circuit. This is equal to I x R where I is the current in the circuit and R is the individual resistance. If you use a sensitive voltmeter you can measure the voltage drop even between the ends of the individual pieces of connecting copper wires. Because the lamp has the highest resistance, the maximum voltage drop takes place across this. The power of the lamp is given by I x R where R is the bulb resistance. It is this power consumed by the lamp that appears as heat and light.
    Just think. If no current flows from the lamp to the negative terminal why have that connection at all? Electrons cannot be dissipated. So does it mean that they are getting accumulated in thre element of the bulb? Like water in a tank?
    In a given circuit the number of electrons moving at any time is the same everywhere.
    There is an ecxception. If you have a capacitor in circuit, it acts like the water tank I mentioned earlier. Till the tank is full (that is, till the capacitor is fully charged) there is accumulation of electrons in the capacitor. Once fully cahrged, a capacitor behaves like a resistance of high value.
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  16. aj_onduty

    aj_onduty Certified CEan

    :) I told you, my explanation skills are a bit mediocre, you can actually let me know if you want simpler examples, which I can give you.
  17. Mr.Don

    Mr.Don Star

    You're welcome ajit, please give us some more examples in this thread :confused:
  18. aj_onduty

    aj_onduty Certified CEan

    I read above about the rat-trap analogy given by praba.
    Lets take that analogy, but in a different perspective.
    Lets change electricity (the energy) with rats. The rats have one path, which is from the pole of the generator to the NGR (Neutral grounding resistor) at the generator's end. If you have to catch the rat, you got to lure it into the trap. You need cheese which will show the rat the way. Here, we provide the cheese by showing the rat a path of lower resistance. We connect a conductor to the path through which the rat was going, and we connect it to ground via an appliance in your home. The rat falls for it, it follows the shortest path to the neutral, and when it does, it gives out everything it has got to the appliance.

    Hope this is clear enough, if no, I can give you a diagrammatic representation. But I feel this is simple enough.

    P.S. The diagram can be a bit more complicated than this example! :p
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  19. A.V.Ramani

    A.V.Ramani Guru

    Not quite. If there are n number of resistors of various values in parallel with the power supply each resistor will carry a current inversly proportional to the resistance. The entire current will not flow through the least resistance alone.

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