quetions

[snowplough]

Hi there,
Im new to this site but woould appreciate if any of ou could answer one of many quetions i have about electrics as i am currently doing m y level 2 electrics course.

 

[Guest123]

Hi there.

Unless we know what the question is we cant help.

 

[isitintheregs]

Hi there.

Unless we know what the question is we cant help.

Maybe he's a bit snowed under

 

[ezzzekiel]

darn dont think i can help with this one

 

[Andy Smith]

ther answer is, yes i think its, well 42 lol

 

[snowplough]

Sorry guys ,

Heres the first of many questions,

Regarding A.C CURRENT, if the current is alternating in both directions between live and neutrel, why is a switch put on the live conductor when you would surley think that it could just as well be put on the neutral conductor seen as the current is alternating in both directions at 50 times a second.
Hope you can help me with this one as i no its easy when you no the answer.

Many Thanks for your trouble,

snowplough

 

[Guest123]

Hi.

The AC current waveform doesn't alternate between line and neutral, it alternates between the positive and negative ends of the scale

The neutral on the other hand is totally different as all it does is provide a return path for current to travel down.

 

[Keef]

Cuz ther neutral is connected to the centre tap on the main transformer (earth) and the LINE (not live) is not, thus the neutral carries no voltage to earth under no-load (switch off) conditions, the line does.

You want to switch the more 'dangerous' of the two. Make sense?

Totally unrelated, but how do you create new posts on this forum? I can only reply, hence this unorthodox method!

 

[Guest123]

Hi Keef.

Click on which ever forum you want to start a thread in and at the top click on new thread and away you go.

 

[snowplough]

Ta for that answer guys, i must have miss herd my tutor , i thought he said that the A.c current alternates between line and neutral, your saying it only alternates through the three phases on the star which makes sense to me now.

Second question for the night, when the say that for a given amount of power say 20kw for instances , its more efficient to increase the voltage which would decrease the current , hence decrease the heat loss so you can have smaller cables.

But where does ohms law come into this, if ou have maller cables dont ou get a drop in voltage (pressure) after so long with thses power cables stretching for miles, and how much voltage drop do ou get, is it enough to make a great difference to say a standard 400kv transmiision cable.

Many Thanks

snowplough

 

[pushrod]

Sorry guys ,

Heres the first of many questions,

Regarding A.C CURRENT, if the current is alternating in both directions between live and neutrel, why is a switch put on the live conductor when you would surley think that it could just as well be put on the neutral conductor seen as the current is alternating in both directions at 50 times a second.
Hope you can help me with this one as i no its easy when you no the answer.

Many Thanks for your trouble,

snowplough

In a simple circuit of consumer unit to light bulb (live) and back to consumer unit (neutral) you are right that the bulb could be controlled by putting the switch in the live or the neutral. However if you put the switch after the load (bulb) everything up the switch would be live. So even though the bulb was off all the connections at it would be live and so dangerous even though you might think it was switched off. So it is convention that all switches are put in live wires.

.

Second question for the night, when the say that for a given amount of power say 20kw for instances , its more efficient to increase the voltage which would decrease the current , hence decrease the heat loss so you can have smaller cables.

But where does ohms law come into this, if ou have maller cables dont ou get a drop in voltage (pressure) after so long with thses power cables stretching for miles, and how much voltage drop do ou get, is it enough to make a great difference to say a standard 400kv transmiision cable.

Many Thanks

snowplough

Good question. You are correct about the efficiency of high voltage transmission and the reduction of energy loss due to the lower current flowing. I would suspect that there is voltage drop along the large runs of the super and national grid but that they can be taken care of at the substation transformers so that the correct voltage, within limits, reaches the consumer.

 

[snowplough]

Hi There,
I alwas assumed electrons flowed from atom to atom in a circuit replacing the electron from the atom infront as that electro moved on creating current flow, as i read things on the net especially with regards to resistance it says that electrons bump in to the atoms infront which is the resistance to current flow, i am confused now i assumed electrons moved from atom to atom to redress the inbalance not tried to get around them, please help i am really confused.

many thanks

snowplough

 

[Darkwood]

Hi There,
I alwas assumed electrons flowed from atom to atom in a circuit replacing the electron from the atom infront as that electro moved on creating current flow, as i read things on the net especially with regards to resistance it says that electrons bump in to the atoms infront which is the resistance to current flow, i am confused now i assumed electrons moved from atom to atom to redress the inbalance not tried to get around them, please help i am really confused.

many thanks

snowplough

I think you are concentrating too hard on this one as you start to enter the world of quantum physics but il try explain it without confusion as i recall the electrons do flow but only a few meters or miles per hour, now it begs the question how can this be as the light bulb lights instantly after you flick the switch, well its the electromagnetic field that is flowing round close to the speed of light. Its kind of hard to explain in simple form but hope i gave you insight.Speed of electricity - Wikipedia, the free encyclopedia
Found this link that explains it in english better than i did, hope it helps

 

[snowplough]

Man thank for the speedy answer, i wonder if an one else can explain it to me ,

Man Thanx

nowplough

 

[shedbuilder]

Second question for the night, when the say that for a given amount of power say 20kw for instances , its more efficient to increase the voltage which would decrease the current , hence decrease the heat loss so you can have smaller cables.

But where does ohms law come into this, if ou have maller cables dont ou get a drop in voltage (pressure) after so long with thses power cables stretching for miles, and how much voltage drop do ou get, is it enough to make a great difference to say a standard 400kv transmiision cable.

Many Thanks

snowplough

This is one of those rare occasions when nature gives a win-win situation for very little cost.

First consider having a "grid" operating at 10V, and with resistance in the supply cable of 1ohm (numbers just plucked out of the air for easy maths). Let's say you need to draw approximately 10W of power through it (for now ignore the voltage drop of the cable, so assume the load end is 10V). You're drawing a current of 1A. So, the power dissipated in the supply cable (I^2 x R) is 1W, and the voltage drop across the cable (I x R) is 1V, or 10% of the supply voltage.

Next consider a "super grid" operating at 100V, again supplying 10W (therefore 0.1A). Because of the current decrease (10 fold), we can reduce the c.s.a. of the cable by a factor of 10. This increases its resistance by 10 times to 10ohms.
Now I^2 x R is 0.1W, and I x R is still 1V, or (now just 1% of the supply voltage).

So, we've got. Less power loss in the cable, less voltage drop (percentage wise), and cheaper cables because there's less conductor c.s.a. required. The only downside is that we've got a higher voltage to deal with, so insulation needs to be beefed up.

Hope this helps.