# How does current flow in an AC Circuit.

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.....

## Replies

• narayana murthy
• PraveenKumar Purushothaman
• Ramani Aswath
neo23
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.....
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.
• neo23
Still in doubt...Does that mean that electrons vibrate back and forth their mean position? Do they not travel all the way from source to load and then back? And one more doubt .... if AC reverses direction THEN WHAT DIFFERENCE IS THERE BETWEEN PHASE AND NEUTRAL? AS I HAVE LEARNED THAT PHASE HAS THE POTENTIAL AND NEUTRAL IS 0V. REVERSING DIRECTION OF CURRENT IN THE NEGATIVE HALF CYCLE CLEARLY INDICATES THAT THE NEUTRAL ATTAINS POTENTIAL AND CURRENT STARTS FLOWING BACKWARDS FOR THAT CYCLE?PLEASE CLARIFY THE DOUBTSREGARDS
• Ramani Aswath
neo23
Still in doubt...Does that mean that electrons vibrate back and forth their mean position? Do they not travel all the way from source to load and then back? And one more doubt .... if AC reverses direction THEN WHAT DIFFERENCE IS THERE BETWEEN PHASE AND NEUTRAL? AS I HAVE LEARNED THAT PHASE HAS THE POTENTIAL AND NEUTRAL IS 0V. REVERSING DIRECTION OF CURRENT IN THE NEGATIVE HALF CYCLE CLEARLY INDICATES THAT THE NEUTRAL ATTAINS POTENTIAL AND CURRENT STARTS FLOWING BACKWARDS FOR THAT CYCLE?PLEASE CLARIFY THE DOUBTSREGARDS
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)
• PraveenKumar Purushothaman
Yeah, the flow of current is reversed... ๐
• narayana murthy
@bioramini: sir i think you are saying about bandwidth and jumping of electrons from valence band to conduction band is that the principle
• Ramani Aswath
narayana murthy
@bioramini: sir i think you are saying about bandwidth and jumping of electrons from valence band to conduction band is that the principle
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.
• praba230890
neo23
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.....

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.
• aj_onduty
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! ๐ ,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 ๐ ). 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! ๐
• aj_onduty
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! ๐
• ramgopalverma
Yes you r right gimme . cureent does change its direction alternately
• neo23
praba230890
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.
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.

• Mr.Don
@Aj Superb ๐ but bit complicated. ๐

@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.

File:current Notation.svg

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.

File:neonlamp3.Jpg

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.
• Ramani Aswath
neo23
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.
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.
• aj_onduty
Mr.Don
@Aj Superb ๐ but bit complicated. ๐
๐ 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.
• Mr.Don
You're welcome ajit, please give us some more examples in this thread ๐
• aj_onduty
Mr.Don
You're welcome ajit, please give us some more examples in this thread ๐
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! ๐
• Ramani Aswath
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.
• Ramani Aswath
Excellent. No hurry It is difficult to explain concepts in easy to understand terms.
• aj_onduty
bioramani
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.
Sir,
Here, I was taking about just a single rat, just one unit of electrical energy...
In actuals, there are n number of rats looking for a least resistance path. The competition is very high, and hence, there is a problem of accommodating all the rats in one small path which has least resistance. So some rats try to take the next least resistance path... The story goes on, and more rats come by to find the next best path...
I think this satisfies the condition you mentioned...
Thanks...
• pcd
In this whole discussion where dose a switch stand?
• HANUSH PITTU
if current direction revers in ac supply(supplied for household, single phase) then why don't tester glow when we place it neutral hole of wall socket, as in negative half cycle it should get high potential...
• HANUSH PITTU
if current direction revers in ac (supplied to household purpose) then why don't tester glow when we place it in neutral hole of wall socket, as it should get high potential in negative half cycle
• HANUSH PITTU
• Ramani Aswath
The tester measures the voltage between the zero of the ground and the point. Neutral is always zero.
• HANUSH PITTU
sir, Then what is the voltage that will appeared in phase point during negative half cycle. will it be higher than neutral(above zero) or it will be less than neutral(below zero)
and what will be the direction of current during negative half cycle
• Ramani Aswath
hanush
Then what is the voltage that will appeared in phase point during negative half cycle.
The negative half cycle will show a mirror image of the positive cycle with the voltage negative with respect to neutral.
• HANUSH PITTU
sir, As u said during negative half cycle the voltage is negative with respective to neutral (which is always zero). which means when we place a tester in wall socket hole, During negative half cycle, will current flow from ground to wallsocket(as tester glows) as wallsocket is at less potential compared to ground. please explain ...
• Ramani Aswath
hanush
sir, As u said during negative half cycle the voltage is negative with respective to neutral (which is always zero). which means when we place a tester in wall socket hole, During negative half cycle, will current flow from ground to wallsocket(as tester glows) as wallsocket is at less potential compared to ground. please explain ...
That's right. It will.
• HANUSH PITTU
suppose if we connect any load(say lamp) to the wall socket, During positive half cycle the current will start from phase and go through load and completes it's circuit by going into neutral.
And during negative half cycle will current starts from neutral and go though load and completes it's circuit by entering into phase point?

am I correct in my view.
• Ramani Aswath
hanush
am I correct in my view
Quite correct.
• Ramani Aswath
Actually there is not much movement of electrons in an AC circuit. They just oscillate about a mean position.
Here is an example calculation of what happens in an AC circuit:
Drift Velocity Numerical Example
• HANUSH PITTU
thank you sir....
• HANUSH PITTU
sir, during negative half cycle
how that negative voltage (less than zero)... appear at wall socket..
I am bit confused about that negative voltage...
which means how the voltage is created less than ZERO
• Ramani Aswath
hanush
which means how the voltage is created less than ZERO
Positive and negative are just conventions. Do you know that when a current flows from the positive electrode to the negative the electrons actually move from the negative to the positive?
If you take a battery and connect its positive terminal to the positive of a voltmeter and the negative to the meter's negative you read a positive value.
If the terminals are reversed at the meter you read a negative voltage.
An AC generator consists of a coil rotating in a magnetic field. Consider the centre of rotation to be the origin of a normal X-Y plane. When the coil is perpendicular to the magnetic field the potential is zero. As it starts rotating the potential increases to a maximum at coil parallel to the field. It decreases to zero when the coil is again perpendicular. Beyond this, the voltage decreases to a minimum and ends up with zero at the end of one rotation. The generator runs at 3000 RPM to produce the common 50 Hz voltage in India.
See these video tutorials:
• Kaushik Banerjee
I am confused about which current flows in circuit when the circuit is excited with AC source? Is the current has r.m.s value or the peak value of current? And also is same current flows in all types of circuits that is in RL, RC, LC & RLC circuits? And which current flows in circuit when circuit is excited with DC source? Please help me!
• Stephen B Johnson
AC current "flows" like it is alternating. Small displacements back and forth...repeating over and over with an amost deterministic period. It will usually lag the voltage, but that depends on the circuit.
• S.M. Thomas
WOW - this is very informative info Mr. AV Ramani. The words that we use to describe AC are all wrong and very misleading.

1. AC does not "flow". It does not travel down a long power cable from point A to point B.

2. As you state, positive and negative are just naming conventions. Logic says that you really can't have a "negative" voltage. But that is what we are all taught.

3. The "sign wave" diagram used to describe AC is also very misleading. It would seem to indicate that AC travels or "flows" like a sound wave - which it does not.
• S.M. Thomas
S.M. Thomas
1. AC does not "flow". It does not travel down a long power cable from point A to point B.
Meaning the electrons do not "flow" or move down the cable.
• Jay Kosta
Regarding whether in domestic AC household wiring the 'neutral' (usually white) wire, electrons flow/move/vibrate FROM it to the 'hot' (usually black) wire - WHEN the circuit is complete.
.
YES they do, but only when they are being 'pulled' or 'pushed' to do so by the voltage difference in the hot wire's complete circuit.

In AC, the potential of the hot wire cycles between +to- ... so it functions by pulling electrons during the + portion, and pushing electrons during the - portion.

The neutral wire functions by 'allowing' those electrons to be pushed or pulled in/from it as is dictated by the hot wire. The neutral wire DOES NOT do any pushing or pulling of electrons.

Think of the neutral as sort of damp sponge - it can accept more water that is pushed into it, and it will release water that is pulled from it. But if there is no external pushing or pulling, the sponge just sits there being 'neutral'.
• Shemin Jose
Sir i have a doubt plz help
Do AC current must need a closed circuit for flow? If you say yes then how a voltage tester works .Its one end to phase other end to earth. Everybody says stray capacitance sombedy says it travels through earth and enters earth wire then back to neutal?
• Jay Kosta
Yes, a closed circuit is needed for current in AC or DC.
In the USA, the AC neutral wire is connected (bonded) to a physical earth ground at the main service entry panel - and also at many other locations from the electric generating source to the service panel.

So, when one wire of a voltage tester is connected to 'earth ground' it is unclear (to me) 'exactly' what path the current flows - I think it most likely flows along the 'earth ground' on the path of least resistence to where there is a physical 'earth ground / neutral wire bond' and then continues along the neutral wire. I doubt that 'stray capacitance' is involved.

HOT -----------------------------------------------------
NEUTRAL ------------------------ voltage
BOND I tester
EARTH GROUND --------------------------------------------

so the current is along the earth ground from the tester until the location 'I' where there is a physical bond of earth ground with the neutral wire.

Jay Kosta
Endwell NY USA
• Jay Kosta
Yes, a closed circuit is needed for current in AC or DC.
In the USA, the AC neutral wire is connected (bonded) to a physical earth ground at the main service entry panel - and also at many other locations from the electric generating source to the service panel.

So, when one wire of a voltage tester is connected to 'earth ground' it is unclear (to me) 'exactly' what path the current flows - I think it most likely flows along the 'earth ground' on the path of least resistence to where there is a physical 'earth ground / neutral wire bond' and then continues along the neutral wire. I doubt that 'stray capacitance' is involved.

HOT -------------------------------------------
NEUTRAL ------------I------------................................voltage
BOND .................................I........................................................... tester
EARTH GROUND -----I----------------------------

so the current is along the earth ground from the tester until the location 'I' where there is a physical bond of earth ground with the neutral wire.

Jay Kosta
Endwell NY USA
• Ramani Aswath
The neon light in a tester requires very little current to light up. All AC electrical supplies are ground referenced. When the probe touches a source higher than a minimum voltage and the finger of the person is touching the metal cap at the bottom of the tester enough current flows through the lamp and the person to the ground to light it up.
• Jay Kosta
The situation with a neon voltage tester is interesting.
With my 2-probe neon tester, if I attach a short piece of wire (a 0.5m test lead) to a bare probe and the other probe into the phase side of a 120VAC outlet -

1) With the test lead hanging in air - the neon bulb will glow slightly.

2) If I touch the metal contact of the test lead, the bulb glows brighter - but I am wearing rubber soled shoes and standing on a plastic box, so my body is well insulated from ground.

Is the extra length of wire in #1 (hanging in air) acting as a type of broadcast antenna?
In #2, is my body surface acting as a larger antenna?

Jay Kosta
Endwell NY USA
• Ramani Aswath
Even though you are 'insulated' there is still a finite resistance. That will conduct some current. With the wire hanging there is a capacitance between the wire and the person. For AC there is a frequency dependent capacitive inductance. This allows a small current to flow.
• Jay Kosta
Thank you for the information about capacitance and capacitive inductance.
What is your opinion about my earlier post regarding current flow from ground to neutral thru the neutral-ground-bonding at the service panel when a high-draw device (e.g. 100w) replaces the low-draw neon tester?

Jay Kosta
Endwell NY USA

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