Question on electricity

[smoothtalker]

I'm not EE trained. I apologies for my ignorance.

I'm trying to understand AC electricity at the moment. But I'm very confused on how alternating current works. I've a hypothetical scenario here. An appliance, the bulb.
For eg. The first instance when we power on and electricity flows.. Live wire (230v) -> bulb (230v 3A) -> neutral wire( 0v 3A)

Am I correct if assuming I measure all 3 points with reference to ground?

I'm confused..

Please refrain from posting unnecessary replies. Reply only if you know the answers to below 4.

Can someone illustrate the full cycle. Using a bulb? Everyone is saying something different.

Here are the details.

1) we have 3 wires. earth. hot. neutral.
2) how does neutral conducts electricity when it's grounded at substation. Doesn't that short circuit?
3) why is neutral always 0v reference to earth? Even when there is a current flow from neutral to live in the half cycle?
4) how does neutral wire power the bulb in the second cycle when it is -230v 0A

[Sigmoid]

I really, really suggest you pick up a basics book. Apparently you are missing the basic of basics, and even though we can try to explain it here, there are great teachers and writers who have done it already.

For example: http://www.amazon.com/Electronics-For-Dummies-ebook/dp/B004IK9XLA/

Now to quickly try and answer your question. Electricity does not "flow" like water. Earth ground has potential too, we just use it as a reference and arbitrarily call it 0V. There is no such thing as "there is no electricity in it".
When the live wire is positive compared to earth return, electrons flow from earth toward the hot terminal of the generator in the power plant (conventional current flow and physical electron flow are opposites, I don't want to dwell on it, read a book and it will be explained in detail).
When the live wire is negative compared to earth return, electrons flow from the hot terminal of the generator through the live wire toward earth return.

[AndreaEl]

Live wire are 230V reference to Neutral wire.
Obvious Neutral wire are 0v reference to neutral wire.
Ground wire is for safety and it not allow that all the conductive material connect to it reach a dangerous voltage which can be dangerous. (for example when in a defective device there are a Live wire that enter in contact with the chassis. In this case current flow through earth wire.)

I know that i explain not good, but my english is not very good...

you can read here, of sure is explain better: http://www.allaboutcircuits.com/vol_2/chpt_1/1.html

[smoothtalker]

I really, really suggest you pick up a basics book. Apparently you are missing the basic of basics, and even though we can try to explain it here, there are great teachers and writers who have done it already.

For example: http://www.amazon.com/Electronics-For-Dummies-ebook/dp/B004IK9XLA/

Now to quickly try and answer your question. Electricity does not "flow" like water. Earth ground has potential too, we just use it as a reference and arbitrarily call it 0V. There is no such thing as "there is no electricity in it".
When the live wire is positive compared to earth return, electrons flow from earth toward the hot terminal of the generator in the power plant (conventional current flow and physical electron flow are opposites, I don't want to dwell on it, read a book and it will be explained in detail).
When the live wire is negative compared to earth return, electrons flow from the hot terminal of the generator through the live wire toward earth return.

I tried reading some articles. They talked about ohms law. Etc.. But none really tells you the mechanics of how it works. I guess those are covered in higher level courses.

I know ohms law. i know AC alternates. But I don't understand how it actually supply the power. I know what are the 3 wires. I know about safety. I know how to use a multimeter.

 But I'm now asking a very specific question. Can anyone explain how electricity power a load during the negative cycle?

[Paul Price]

There are some very good, easy to understand, well-explained tutorials about electricity and the place to find them is Youtube.com

[Nerull]

AC doesn't alternate between a positive voltage and zero, it alternates between a positive and a negative voltage.

When the hot wire swings to -230V, there is a 230V potential from the neutral wire to the hot wire and electrons will flow.

There is no such thing as absolute voltage. There is no such thing as just '0V'. Voltage is always measured relative to something else. Neutral is 0V relative to ground, but at the negative peak of the hot wire, the neutral has quite a lot of positive voltage relative to it.

[tsmith35]

The most basic example may be with a light bulb.

hot = 120VAC RMS
neutral = 0V
ground = 0V

Neutral is typically tied to ground inside the breaker box of your house.

For half of the cycle, current flows from hot to neutral and heats the filament. For the other half of the cycle, current flows from neutral to hot and heats the filament.

A heating element could be substituted for the filament with the same results.

To get an idea of how current can flow from the neutral to the hot, picture yourself standing out in the water at a beach while holding a bucket. The water is perfectly calm and level. Your bucket (the hot wire) is full of water. Raise it above the water and pour it out. Water flows from hot (bucket) to neutral (ocean). Now dunk the bucket into the water and let it fill up. Water flows from neutral (ocean) to hot (bucket). Keep doing that and you'll have a constant flow into and out of the bucket.

See? It's all relative. Which source is "higher"? Depends on the position of the bucket.

[smoothtalker]

The most basic example may be with a light bulb.

hot = 120VAC RMS
neutral = 0V
ground = 0V

Neutral is typically tied to ground inside the breaker box of your house.

For half of the cycle, current flows from hot to neutral and heats the filament. For the other half of the cycle, current flows from neutral to hot and heats the filament.

A heating element could be substituted for the filament with the same results.

To get an idea of how current can flow from the neutral to the hot, picture yourself standing out in the water at a beach while holding a bucket. The water is perfectly calm and level. Your bucket (the hot wire) is full of water. Raise it above the water and pour it out. Water flows from hot (bucket) to neutral (ocean). Now dunk the bucket into the water and let it fill up. Water flows from neutral (ocean) to hot (bucket). Keep doing that and you'll have a constant flow into and out of the bucket.

See? It's all relative. Which source is "higher"? Depends on the position of the bucket.

Great! Finally someone answered my question. So next, if for the next half where current flows from neutral to hot.. Why does the dmm shows 0v when we measure neutral to earth?

Secondly, isn't neutral grounded at the substation? How does it conduct electricity?

[Nerull]

Because it is always 0V relative to earth, and neutral is generally grounded at the breaker panel in household wiring.

Think of the ocean example again. Did the ocean jump up when you dipped the bucket? No, the bucket just went below the surface.

When AC is at it's negative peak, current flows from earth to hot, because the potential on the hot wire is -230V.

[Zero999]

The neutral is connected to earth (normally either at the substation or where it enters the property) to limit the voltage between mains and the earth to 120V or 230V depending on where you live. A complete circuit is required in order for current to flow so if the neutral is only connected to earth and nothing else no current will flow. When a load is connected between the phase and either neutral or earth, a current will flow. If the current flow to earth is above the limit set by the earth leakage detector (RCD or GFCI depending on where you live) the circuit will be disconnected to protect you from being electrocuted.

[BurtyB]

Why does the dmm shows 0v when we measure neutral to earth?

I strongly suggest you disconnect the meter from the mains until you understand how things work and the dangers (like death) before hooking it up again!

[tehmeme]

From looking at this and your other posts, you seem to have patchy knowledge of the basics.

These are some general useful links some of my pupils find helpful, they are meant for revision purposes, but they might be helpful to you.

start off with this (1 of the core physics modules):
http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/generation_transmission_electricity/

then go onto these (1 of the advanced physics modules):
http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/controlling_current/
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/

then this will help you further (1 of the core Design & Technology modules)
http://www.bbc.co.uk/schools/gcsebitesize/design/electronics/

Please read some of the resources people have posted. They are useful.

Edit: if you know the basics, you will be able to easily make sense of what people have posted above.
here's a good link too https://en.wikipedia.org/wiki/Ground_and_neutral

[Nerull]

You keep thinking of 0V as if it is an absolute value rather than a relative value. Earth is not void of charge, it is simply a common reference point. Any object can have a positive or negative potential relative to earth - or any other object. Current will flow any time a conductor is between two points of differing potential, with the direction determined by the polarity of the points.

AC is a current that constantly changes polarity. The voltage on the hot wire looks like this:



At the +V peak, current flows from hot to neutral, which is always at 0V. In the center, both are at 0V and no current flows. As the voltage on the hot wire approaches it's -V peak, current begins to flow again, but this time from neutral to hot. This cycle repeats 50/60 times per second.

The important part is that during the negative part of the cycle, there is a 230V potential between the hot wire and the neutral because the hot wire is -230V below neutral. Neutral is always earth referenced, so if you stick a multimeter between earth and neural it will always read somewhere around 0V, but if you stopped time - and the cycle - and stuck the multimeter in DC mode between neutral and hot in that instant, with the positive probe in neutral, you will read +230V.

[AlfBaz]

Please refrain from posting unnecessary replies. Reply only if you know the answers to below 4.

YES SIR!

[Nerull]

Please refrain from spamming the same post over and over again because you can't be bothered to read the replies which have given you your answers.

[tehmeme]

Please refrain from posting unnecessary replies. Reply only if you know the answers to below 4.

YES SIR!

looks like someone has homework to hand in in the morning. 

[AG6QR]

There is no such thing as absolute voltage. There is no such thing as just '0V'.

At the risk of adding to the confusion, there actually IS such a thing as absolute voltage.  This was one of the earliest electrical discoveries, when electricity was more of a scientific curiosity than a useful way of powering things.

An electroscope measures absolute voltage. 

http://www.school-for-champions.com/experiments/static_electricity_electroscope.htm
http://en.wikipedia.org/wiki/Electroscope

The foil plates of the electroscope repel each other when they're positively charged, and they also repel each other when negatively charged.  They don't repel each other when they're at zero volts.

To be fair, the typical amateur-built electroscope is a very crude instrument, and only shows easily measurable repulsion at relatively high (positive or negative) voltages.

The idea that voltages are always relative, never absolute, is a very useful practical model for the level of voltages we typically encounter in most electronic circuits, where current flows.  And it's absolutely true that an ordinary voltmeter always has two terminals, and measures a potential difference between those two terminals.   But that "no absolute voltage" model doesn't explain how an electroscope's foils can repel each other at positive and negative voltage, with no repulsion at zero volts.


---ok, feel free to ignore the above if you must---


Trying to bring it back to smoothtalker's misconception, the "hot" wire swings between about +170V and -170V with respect to neutral.  When the hot wire is higher voltage than neutral, electricity flows one way.  When it's lower voltage than neutral, electricity flows the other way.  Power is dissipated regardless of which direction the electricity flows, so the filament heats up both ways.

[Nerull]

Those are your answers! What do you think a pretty picture is going to explain that those posts won't? Will you suddenly get it through your head that current can flow from ground to negative potentials if only its animated?

[tsmith35]

[AG6QR]

1) we have 3 wires. earth. hot. neutral.

Ignore earth.  It carries no current unless something is very badly broken.  Only two wires (hot and neutral) participate in the transmission of power.

2) how does neutral conducts electricity when it's grounded at substation

Because it is also connected to one terminal of the generator.

3) why is neutral always 0v reference to earth? Even when there is a current flow from neutral to live in the half cycle

Neutral at the light bulb isn't exactly at 0v relative to earth (V=IR applies, if neutral is conducting current, and the neutral wire has a finite resistance, it will develop a small voltage).

But it's close to earth potential because it is connected to earth potential, and there is no voltage source between it and earth.

4) how does neutral wire power the bulb in the second cycle when it is -230v 0A

Because it is connected to one terminal of the generator.   The two terminals of the light bulb are always connected to the two terminals of the generator.  One of those connections happens to be connected to an earthed ground rod, but this detail isn't essential to the transmission of power.

[Phaedrus]

Since you're acting immature and not reading the answers, I shall add to your confusion.

120V AC mains voltage actually alternates between +170V and -170V, not +120V and -120V. This is because the voltage measurement for AC is in V_RMS (root mean square), which for an ideal sine wave simplifies to V_pk/sqrt2

[SAI_Peregrinus]

If it helps, you can think of Earth (the planet) as an infinite source and sink of electrons. You can shove as many electrons into a grounded rod or pull as many out as you need, so long as you have a voltage. So instead of running your two wires of AC from a generator to an outlet you can stick one of the wires into the earth at the generator, and again at the outlet. That way you use the planet as a wire, and only have to buy 1/2 as much copper.

[tehmeme]

Here's a good 4 minute video explaining why and how the 3 wires are the way they are.


Then as SAI said  "think of Earth (the planet) as an infinite source and sink of electrons."

[EEVblog]

1) we have 3 wires. earth. hot. neutral.

The earth does not matter. Current flows in in the loop from active to neutral.

2) how does neutral conducts electricity when it's grounded at substation. Doesn't that short circuit?

Ignore the earth, it has nothing to do with the current flow.

3) why is neutral always 0v reference to earth? Even when there is a current flow from neutral to live in the half cycle?

Because the (safety) earth is tied the neutral wire at your meter box. They are the same potential.
Once again, ignore the earth until you figure out how current can flow from active to neutral as a loop.

4) how does neutral wire power the bulb in the second cycle when it is -230v 0A

It's a current loop, current flows one way and then the other, hence the name AC.

[AlfBaz]

You seem to be ok with the concept of a Positive potential causing current to flow to 0V, would it not follow that a Negative potential will cause current to flow from 0V

[Kremmen]

I'm not EE trained. I apologies for my ignorance.

I'm trying to understand AC electricity at the moment. But I'm very confused on how alternating current works. I've a hypothetical scenario here. An appliance, the bulb.
For eg. The first instance when we power on and electricity flows.. Live wire (230v) -> bulb (230v 3A) -> neutral wire( 0v 3A)

Am I correct if assuming I measure all 3 points with reference to ground?

I'm confused..

Please refrain from posting unnecessary replies. Reply only if you know the answers to below 4.

Can someone illustrate the full cycle. Using a bulb? Everyone is saying something different.

Here are the details.

1) we have 3 wires. earth. hot. neutral.
2) how does neutral conducts electricity when it's grounded at substation. Doesn't that short circuit?
3) why is neutral always 0v reference to earth? Even when there is a current flow from neutral to live in the half cycle?
4) how does neutral wire power the bulb in the second cycle when it is -230v 0A

1:
Only at the premises. The distribution grid is a different story and your mileage varies based on the applicable standards in each region.

2:
see the "DaveCAD" attached. The energy comes from the transmission lines via transformers. Transformer coil ends as such have no "live" or "neutral". They just have voltage between the coil ends. No ground. No neutral. Just a voltage (AC) between the coil ends. Every half cycle the polarity of the voltage changes, but only in relation to the coil ends themselves, nothing else.

3:
"Neutral" only becomes "neutral" once it is explicitly tied to the ground using a physical conductor. Until that moment, it is just one of the transformer coil ends, floating free in the universe. The act of connecting a coil end to something references the coil potentials of those points. You could connect one coil end to the positive pole of a 400 V battery, and at that moment the coil end would be at +400V in relation to the - pole of that same battery. Measure the other coil end against the - pole of the battery, you would see your coil AC voltage + the constant 400 V of the battery. But nothing else.
So when you ground one of the coil ends, it then becomes the neutral by virtue of being connected to the "universal" neutral voltage, i.e planet earth. We all share that potential by being in galvanic connection to the planet, so that is why it is called "Neutral" or "Ground" - there is no potential difference between points tied to a common point. For all we know, Earth could be charged to a potential of 10^12 V by the particles of the solar wind, but who cares since we all sit in that same potential. To us it is all equal and we can call it 0 if we want to (and we do).

4:
"Wires" don't power anything. Current circulates in a conducting loop (circuit) where there is a potential difference in one or more places. Thus the circuit you are thinking is made of: the step-down transformer supplying your house. That is the generator of the potential difference by virtue of magnetic induction in the transformer secondary, giving rise to the voltage in the secondary coil ends. The coil ends are brought into the house where the light bulb is then connected as the load between the wires. For convenience, there is the switch interrupting one wire when you want to sleep. So now we have a complete circuit: The voltage between the coil ends drives a current through the load via the connecting wires. The direction of the current at each instant is determined by the polarity of the voltage between the transformer coil ends. Since the transformer operates with AC voltages, the polarity and thus the direction of the current will alternate at 50 or 60 times per second, depending on where on the planet you happen to light your bulb.
Now, for several reasons i won't go into, it is usually considered a good idea to reference the above circuit to the ground potential. To get this effect, we simply tie one of the transformer coil ends physically to planet earth using a conductor. It makes no difference which coil end you select, because they are equivalent in this respect. You just pick one and that then becomes your "neutral". But note that it in no way influences the above circuit at all. The circuit is just referenced to earth potential (i.e. made same as) in one point, the node that became the "neutral".

[tehmeme]

This is one weird thread

sure is,
Although the op said "hypothetical", I had the impression the op was actually poking probes in the wall socket :/
It also wasn't clear what level of knowledge he/she has. From some of the questions it seems the very basics were missing, such as the concept of DC & AC current flow.

I must say, I've learnt a couple of things today about this forum and forum etiquette.
1- The way most people handled this thread, is admirable, showing a certain maturity and patience and some people really did put some effort in to provide a decent and relevant response (compared to another forum which would have pummelled the op) 

[Kremmen]

It is not always the case that we answer just to the OP asking the question. Especially in the beginner section there is the risk that a totally cracked set of questions, if left unanswered, would lead other beginners to think that those questions or the flawed thinking behind them has merit.

For that reason if nothing else, i sometimes respond the way i did now. To dispel misconceptions or outright falsehoods in the questions themselves.
It is then quickly obvious from the followup whether we have a troller or a genuine n00b at the other end.

[smoothtalker]

may i know the reason for grounding the neutral? And why doesn't the grounded neutral breaks the circuit?   

since both hot and neutral wire alternate between 0V and 240V in 1/60 seconds. the circuit still works even if neutral is not grounded.

[Kremmen]

may i know the reason for grounding the neutral? And why doesn't the grounded neutral breaks the circuit?   

since both hot and neutral wire alternate between 0V and 240V in 1/60 seconds. the circuit still works even if neutral is not grounded.

Sorry, that information is classified.

[tehmeme]

It is not always the case that we answer just to the OP asking the question. Especially in the beginner section there is the risk that a totally cracked set of questions, if left unanswered, would lead other beginners to think that those questions or the flawed thinking behind them has merit.

For that reason if nothing else, i sometimes respond the way i did now. To dispel misconceptions or outright falsehoods in the questions themselves.
It is then quickly obvious from the followup whether we have a troller or a genuine n00b at the other end.

what a great attitude to have.

[tehmeme]

may i know the reason for grounding the neutral? And why doesn't the grounded neutral breaks the circuit?   

since both hot and neutral wire alternate between 0V and 240V in 1/60 seconds. the circuit still works even if neutral is not grounded.

The answer to the first question is at 00:30 in this video:



The answer to the second part (both parts in fact) were given above by a number of responders.

Edit: I keep falling for it every time..hehe

[IanB]

Just for grins, here is a picture.

The neutral wire is not neutral until we ground it. Grounding it is what makes it neutral. We do this (attach one wire to the ground) for reasons of safety. It has nothing to do with the operation of the circuit.

As you can see from the picture a circuit is just that, a closed path for electricity to flow around. If we attach one part of the circuit to ground we don't change anything about the rest of the circuit. It is still there and unbroken.

Breaking the circuit looks completely different. If we break the circuit by making a gap in a wire electricity no longer flows.

[tehmeme]

Just for grins, here is a picture.

The neutral wire is not neutral until we ground it. Grounding it is what makes it neutral.

As you can see from the picture a circuit is just that, a closed path for electricity to flow around. If we attach one part of the circuit to ground we don't change anything about the rest of the circuit. It is still there and unbroken.

Breaking the circuit looks completely different. If we break the circuit by making a gap in a wire electricity no longer flows.

I think smoothtalker is puzzled by the fact that neutral and earth are attached to "Earth" and the concept of potential differences relating to relative voltages vs absolute voltages.

People have responded to these questions above and some of the links provided might be what smoothtalker needs instead of this constant spoon feeding.

Electricity For Dummies is a good book for this.

[Nerull]

may i know the reason for grounding the neutral? And why doesn't the grounded neutral breaks the circuit?   

since both hot and neutral wire alternate between 0V and 240V in 1/60 seconds. the circuit still works even if neutral is not grounded.

Neutral does not alternate at all, and as many people have tried to tell you, hot alternates between 240V and -240V, not 240V and 0V.

At the +V peak, current flows from the 240V hot to the 0V neutral, as you would expect as there is a 240V potential between them.

At the -V peak, current flows from the 0V neutral to the -240V hot, because there is still a 240V potential between them, just reversed.

[smoothtalker]

may i know the reason for grounding the neutral? And why doesn't the grounded neutral breaks the circuit?   

since both hot and neutral wire alternate between 0V and 240V in 1/60 seconds. the circuit still works even if neutral is not grounded.

Neutral does not alternate at all, and as many people have tried to tell you, hot alternates between 240V and -240V, not 240V and 0V.

At the +V peak, current flows from the 240V hot to the 0V neutral, as you would expect as there is a 240V potential between them.

At the -V peak, current flows from the 0V neutral to the -240V hot, because there is still a 240V potential between them, just reversed.

Is that correct? There is no real negative voltage. - means the direction changes

[smoothtalker]

Just for grins, here is a picture.

The neutral wire is not neutral until we ground it. Grounding it is what makes it neutral. We do this (attach one wire to the ground) for reasons of safety. It has nothing to do with the operation of the circuit.

As you can see from the picture a circuit is just that, a closed path for electricity to flow around. If we attach one part of the circuit to ground we don't change anything about the rest of the circuit. It is still there and unbroken.

Breaking the circuit looks completely different. If we break the circuit by making a gap in a wire electricity no longer flows.

yea i was just confused. i get the impression that grounding of the neutral seems to short the circuit. because AC is +240v is also coming from neutral in the half cycle. 

[tsmith35]

yea i was just confused. i get the impression that grounding of the neutral seems to short the circuit. because AC is +240v is also coming from neutral in the half cycle.

Ah, here's an example that will make more sense: a simple bellows can be used to represent the hot line. Air is "electricity". Squeeze the bellows handles and you have flow from hot to neutral (neutral being the air all around us). Open the bellows handles and you have a vacuum in the bellows that causes flow from neutral (air around us) into the bellows. The air around us doesn't change at all.

The airflow represents current. The air pressure (or vacuum) represents voltage.

As for what "earth" or "ground" is, that's just the air around us. Interestingly, normal air pressure is about 15 psi above zero pressure (such as you might find in space). The fact that we live in such a highly pressurized environment doesn't mean we have any difficulty squeezing air out of the bellows or sucking air back into the bellows -- it's all relative to the pressure of the air around us.

Think about car tires: they usually say something like "maximum pressure: 35 psi". That number, of course, means 35 psi relative to the air around us, not 35 psi relative to space or anywhere else. To indicate that the pressure is relative to our surroundings, it is typically labeled as "psig" or "psi gage". All that means is that an air gauge that you would use for measuring the pressure starts out indicating zero psi when exposed to ambient air pressure.

Same thing goes for electricity: a meter that is touching ground will indicate zero Volts when exposed to the environment around us. The Earth (and therefore the environment around us) may have an electrical potential of 10,000,000 V relative to some other object out in space, but that's of no concern unless you somehow connect the two (Earth and the other object). For us on Earth, it's just 0 V.

[IanB]

Is that correct? There is no real negative voltage. - means the direction changes

Take a standard 1.5 V battery and measure it with your meter on DC V, red lead to (+) and black lead to (-). The meter will show about 1.5 V. Now swap the meter leads, connect red lead to (-) and black lead to (+). This time the meter will show -1.5 V. But did anything change? Is the battery any different?

Connect a flashlight bulb between the terminals of the battery. It will light up. Now turn the battery around and connect the bulb the other way. Does it light up any differently?

Answer: no, there is no negative voltage. +240 V means the current flows one way, -240 V means the current flows the other way.