Author Topic: Question on electricity  (Read 11406 times)

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Offline Kremmen

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Re: Question on electricity
« Reply #25 on: October 09, 2013, 06:30:52 pm »
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".


Nothing sings like a kilovolt.
Dr W. Bishop
 

Offline tehmeme

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Re: Question on electricity
« Reply #26 on: October 10, 2013, 12:34:10 am »
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)  :-+



 

Offline Kremmen

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Re: Question on electricity
« Reply #27 on: October 10, 2013, 04:17:11 am »
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.
Nothing sings like a kilovolt.
Dr W. Bishop
 

Offline smoothtalker

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Re: Question on electricity
« Reply #28 on: October 10, 2013, 04:58:44 am »
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.
« Last Edit: October 10, 2013, 05:11:04 am by smoothtalker »
 

Offline Kremmen

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Re: Question on electricity
« Reply #29 on: October 10, 2013, 05:24:40 am »
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.
Nothing sings like a kilovolt.
Dr W. Bishop
 

Offline tehmeme

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Re: Question on electricity
« Reply #30 on: October 10, 2013, 05:27:25 am »
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.
 

Offline tehmeme

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Re: Question on electricity
« Reply #31 on: October 10, 2013, 05:39:15 am »
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 :palm:
« Last Edit: October 10, 2013, 06:27:42 am by tehmeme »
 

Offline IanB

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Re: Question on electricity
« Reply #32 on: October 10, 2013, 05:46:03 am »
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.

« Last Edit: October 10, 2013, 06:02:43 am by IanB »
I'm not an EE--what am I doing here?
 

Offline tehmeme

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Re: Question on electricity
« Reply #33 on: October 10, 2013, 05:57:21 am »
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.
« Last Edit: October 10, 2013, 06:30:57 am by tehmeme »
 

Offline Nerull

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Re: Question on electricity
« Reply #34 on: October 10, 2013, 08:47:40 am »
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.
« Last Edit: October 10, 2013, 08:51:31 am by Nerull »
 

Offline smoothtalker

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Re: Question on electricity
« Reply #35 on: October 10, 2013, 03:30:31 pm »
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
 

Offline smoothtalker

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Re: Question on electricity
« Reply #36 on: October 10, 2013, 03:32:24 pm »
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. 
 

Offline tsmith35

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Re: Question on electricity
« Reply #37 on: October 10, 2013, 04:21:32 pm »
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.
« Last Edit: October 10, 2013, 04:23:52 pm by tsmith35 »
 

Offline IanB

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Re: Question on electricity
« Reply #38 on: October 10, 2013, 06:33:22 pm »
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.
I'm not an EE--what am I doing here?
 


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