What does positive and negative really mean?

[D.J.]

Hi,

I have some stupid question for anyone willing to answer.

Why do we have a positive and negative wire comming out of power supplies if electricity is the movment of elctrons ( which have negative charge )?

I know in house wiring thier is an hot wire and nuetral. Elctrons move out the hot wire, into the devise and back to the nuetral, Is this correct?

Is it the same way with power supplies? Elctrons move out the negative wire and back through the positive wire.

These questions have pondered me for a while, I gues I really never learned how electricity works.

Regards

D.J.

 

[mstechca]

I think it has to do with an atom

if I remember, an atom has a number of protons, neutrons, and electrons. and somehow, these atoms form positive and negative charges.

I think Earth has a factor as well.

 

[D.J.]

Hi,

Thanks for the quick reply.

Ya, an atoms polarity is determind by the number of elctrons an protons it contains. Electrons have a negative charge and protons have a positive charge.

Do protons flow through a circuit and are called positive or is negative retuning called positive?

How does electricity flow and what is positive and negative?

Thanks again

D.J.

 

[Styx]

Protons are fixed by the material (since wire is solid and not liquid - except mercury...)
There is a psudo proton-flow in semi-conductors called HOLES, but hole movement is just electron movement leaving a hole in its wake, giving the impression of a hole moving


The flow of current around a cct is the flow of electrons and it is in th opposite direction to the flow of electrons (just to really confuse you)

Electricity flows because something somewhere has the potential to source electrons (ie a battery), when something is connected to it, this potential is realsied and we have current flow.

 

[D.J.]

Hi styx,

Let me see if I understand this correctly.

Since elctricity is the flow of elctrons then the positive wire is just the the return path to the atom. Is that right?

Thanks

D.J.

 

[EdwardM]

Hi D.J.

it's a question that has exercised many engineers for a long time...

Back in the days when electronics was new, about 200 years ago, people like Volta and others realised that 'something was flowing' when they connected up their embryo cells and batteries to frogs legs and other kinds of circuit. At the time they didn't know about electronic movement of charge and arbitrarily decided that the flow (current) most sensibly should start at the positive side of a cell and terminate at the negative end, causing much confusion ever since. :roll:

Ed

 

[Nigel Goodwin]

Hi,

I have some stupid question for anyone willing to answer.

Why do we have a positive and negative wire comming out of power supplies if electricity is the movment of elctrons ( which have negative charge )?

I know in house wiring thier is an hot wire and nuetral. Elctrons move out the hot wire, into the devise and back to the nuetral, Is this correct?

Is it the same way with power supplies? Elctrons move out the negative wire and back through the positive wire.

These questions have pondered me for a while, I gues I really never learned how electricity works.

You DON'T have positive and negative in house wiring, not for a great many decades, what you DO have is live and neutral, where neutral is simply the common of the three phase supply (at least in the UK), and Live is one of the three phases.

With DC, conventional current flow is from positive to negative, but electron flow is from negative to positive. With AC (like the mains) the direction continually changes, so it doesn't flow out of one pin and back in the other, it does both on both connections - changing direction 100 (or 120) times a second.

 

[D.J.]

Hi Ed,

Thanks for the help.

I thoguht it was Ben Franklin who said that elctricity was the flow of electrons and becuase electrons have a negative charge, elctricity flows from negative to positive. Correct me if I am wrong.

So the elctron is all that moves. It comes throw the negative wire, through the circuit and out the positve wire?

Another question. If this is true how come you cannot touch the negative and positive wire together on a power supply.

Thanks

D.J.

 

[D.J.]

Hi Nigel,

I know that their is not positive or negative in house wiring, I just wanted to know why, and I think you answered the question.

I live in America were they call it the Hot Wire and Neutral Wire.

This brings up another question can anyone give me a simple explanation of Phases of elctricity? I have looked it up before and just got more confused.

Thanks

D.J.[/quote]

 

[upand_at_them]

Two wires are needed because the electrons have to flow. That is, into and out of something. It is the flow of electrons that does work.

Mike

 

[David Bridgen]

can anyone give me a simple explanation of Phases of elctricity? I have looked it up before and just got more confused.

A single phase a.c. generator will produce one complete cycle of voltage for each revolution of its rotor. In other words, during each and every revolution the voltage increases to a maximum, decreases to zero, increases again to maximum but with the opposite polarity, decreases to zero, increases to maximum with the original polarity, etc etc.

The rotor moves through 360° in each revolution and we can identify points on the waveform it produce as it interacts with the fixed field by saying that each cycle takes, or lasts, 360°. We refer to these points as the phase or phase angle.

The value of voltage, expressed as a fraction of its peak, at any point in the cycle is equal to the sine of the phase angle.

For instance at one twelfth of a revolution, or 30°, the voltage will be 0.5 (sin30) of the peak. At one quarter of a revolution, or 90°, the voltage will be at its peak value or 1. If you check, by taking the sine of greater angles you will see that after 90° the amplitude decreases to zero at 180 and then increases in magnitude (size) but with a negative sign indicating that it is of the opposite polarity. The magnitude increases in a similar manner, rising to a maximum at 270 then decreasing to zero again at 360/0°.

In order to understand it, you might like to think of a 3 phase generator as being 3 single phase generators on the same shaft but one of them is physically oriented on the shaft 120° further round than the first one. And the 3rd one is mounted 120° from the 2nd (240° from the first.)
One end of each generator's winding is connected together, this being the common, or neutral, connection.
The three windings interact with the same field winding.

Now, when the output of the first winding, which we will call phase A, is at zero, because the other windings are symetrically displaced around the rotor shaft they will produce zero volts at symetrically different points of the shaft's rotation. For phase B to be at zero the shaft will have to rotate 120° from the point where phase A produced zero, and for phase C to produce zero the shaft will have to rotate a further 120°, or a total of 240°. 120° later phase A will have completed one complete cycle and the sequence repeats.

Maybe the picture will help.

 

[eblc1388]

A single phase a.c. generator will produce one complete cycle of voltage for each revolution of its rotor.

Only if it is of a two-pole design which most a.c. generators are.

 

[David Bridgen]

A single phase a.c. generator will produce one complete cycle of voltage for each revolution of its rotor.

Only if it is of a two-pole design which most a.c. generators are.

Very well, read that as a simple single phase a.c. generator which is what I would expect to encounter in any basic a.c. theory book.

 

[williB]

david i dont really want to criticize your graph , but , it shows 120 degrees being over half way to 360..

 

[David Bridgen]

david i dont really want to criticize your graph , but , it shows 120 degrees being over half way to 360..

You are quite right Willie. I shall correct it in a couple of minutes.

 

[D.J.]

Hi everyone,

Thanks guys for all your help!

I think I understand it better now.

Thanks Again,

D.J.

 

[ljcox]

Hi D.J.

it's a question that has exercised many engineers for a long time...

Back in the days when electronics was new, about 200 years ago, people like Volta and others realised that 'something was flowing' when they connected up their embryo cells and batteries to frogs legs and other kinds of circuit. At the time they didn't know about electronic movement of charge and arbitrarily decided that the flow (current) most sensibly should start at the positive side of a cell and terminate at the negative end, causing much confusion ever since. :roll:

Ed

Current is the flow of charge carriers. The most common is the electron, but others such as protons, holes in semiconductors, positive and negative ions in liquids, etc.

So it does not matter which direction you choose for the direction of current flow.

However, I find the conventional direction (ie. + to -) more convenient when analysing circuits. I find it is easier to write the mesh and nodal equations if I use conventional flow.

Len

 

[D.J.]

Hi,

Thanks for the reply.

That makes since too.

thanks agian.

D.J.