### Author Topic: Question on electricity  (Read 11329 times)

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#### smoothtalker

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##### Question on electricity
« on: October 09, 2013, 08:23:23 am »
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..

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
« Last Edit: October 09, 2013, 10:44:11 am by smoothtalker »

#### Sigmoid

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##### Re: Question on electricity
« Reply #1 on: October 09, 2013, 08:36:13 am »
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.
« Last Edit: October 09, 2013, 08:39:59 am by Sigmoid »

#### AndreaEl

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##### Re: Question on electricity
« Reply #2 on: October 09, 2013, 08:48:48 am »
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...

My equipment:

Multimeter: HP 34401A, HP 3478A, HP 3466A, Fluke 115
Oscilloscope: Rigol DS2072 (DS2202)
Function generator: SRS DS335
Power supply: TDK-Lambda ZUP 20-20, 2x Atten TPR3602A, Atten APR1505A, Atten APR12001A, Atten AT1001D

#### smoothtalker

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##### Re: Question on electricity
« Reply #3 on: October 09, 2013, 08:53:39 am »
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?
« Last Edit: October 09, 2013, 08:58:35 am by smoothtalker »

#### Paul Price

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##### Re: Question on electricity
« Reply #4 on: October 09, 2013, 09:09:58 am »
There are some very good, easy to understand, well-explained tutorials about electricity and the place to find them is Youtube.com

#### Nerull

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##### Re: Question on electricity
« Reply #5 on: October 09, 2013, 09:10:17 am »
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

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##### Re: Question on electricity
« Reply #6 on: October 09, 2013, 09:12:52 am »
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

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##### Re: Question on electricity
« Reply #7 on: October 09, 2013, 09:32:42 am »
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

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##### Re: Question on electricity
« Reply #8 on: October 09, 2013, 09:36:01 am »
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.
« Last Edit: October 09, 2013, 09:37:33 am by Nerull »

#### Zero999

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##### Re: Question on electricity
« Reply #9 on: October 09, 2013, 09:51:03 am »
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

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##### Re: Question on electricity
« Reply #10 on: October 09, 2013, 09:52:58 am »
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

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##### Re: Question on electricity
« Reply #11 on: October 09, 2013, 09:56:24 am »
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):

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
« Last Edit: October 09, 2013, 10:05:01 am by tehmeme »

#### Nerull

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##### Re: Question on electricity
« Reply #12 on: October 09, 2013, 10:05:23 am »
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.
« Last Edit: October 09, 2013, 10:13:10 am by Nerull »

#### AlfBaz

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##### Re: Question on electricity
« Reply #13 on: October 09, 2013, 10:12:23 am »

YES SIR!

#### Nerull

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##### Re: Question on electricity
« Reply #14 on: October 09, 2013, 10:14:06 am »
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

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##### Re: Question on electricity
« Reply #15 on: October 09, 2013, 10:15:26 am »

YES SIR!

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

#### AG6QR

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##### Re: Question on electricity
« Reply #16 on: October 09, 2013, 10:19:22 am »
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

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##### Re: Question on electricity
« Reply #17 on: October 09, 2013, 10:20:36 am »
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

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##### Re: Question on electricity
« Reply #18 on: October 09, 2013, 10:21:21 am »

#### AG6QR

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##### Re: Question on electricity
« Reply #19 on: October 09, 2013, 10:28:39 am »
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.

Quote
2) how does neutral conducts electricity when it's grounded at substation
Because it is also connected to one terminal of the generator.

Quote
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.

Quote
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

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##### Re: Question on electricity
« Reply #20 on: October 09, 2013, 10:34:49 am »

120V AC mains voltage actually alternates between +170V and -170V, not +120V and -120V. This is because the voltage measurement for AC is in VRMS (root mean square), which for an ideal sine wave simplifies to Vpk/sqrt2
« Last Edit: October 09, 2013, 10:36:25 am by Phaedrus »
"More quotes have been misattributed to Albert Einstein than to any other famous person."
- Albert Einstein

#### SAI_Peregrinus

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##### Re: Question on electricity
« Reply #21 on: October 09, 2013, 10:52:52 am »
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

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##### Re: Question on electricity
« Reply #22 on: October 09, 2013, 10:59:57 am »
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."
« Last Edit: October 09, 2013, 11:10:41 am by tehmeme »

#### EEVblog

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##### Re: Question on electricity
« Reply #23 on: October 09, 2013, 03:26:32 pm »
1) we have 3 wires. earth. hot. neutral.

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

Quote
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.

Quote
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.

Quote
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

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##### Re: Question on electricity
« Reply #24 on: October 09, 2013, 03:56:56 pm »
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

Smf