electric current and flow of electrons through a conductor

[G.H]

While reading about electric current,i got the information that electrons flow from negative terminal of a battery to positive terminal.it has been mentioned in the book that electrons jump from atom to atom with a very slow speed called drift.we know that metals have a tendency of losing electrons.Then why and how would atoms of metallic wire gain electrons from the negative terminal of battery?And one more question arises that if they drift slowly then why is electricity so fast?

[derek w]

Its a bit like a stationary train as soon as the front end moves the back end moves.Electrons input at one end of wire causes an electron output at other end.A battery creates free electrons by chemical reaction.

[Suxamethonium]

When this was explained to me in high school I was told that the valance electrons in metals are fairly delocalised such that metal could be represented basically as a lump of matter with electrons flowing in, through and around it. So when you connect a circuit, the electron from the battery is not the same electron collected by the battery at the completion of the circuit. To explain this better I'm going to set this out as a 'flow diagram' kind of thing:

 

1. Electron from battery is deposited to the metal (i.e. lump of matter with free electrons).

 

2. The metal now has an excess of electrons.

 

3. At some other location of the metal an (i.e. ANY) electron jumps off to restore the balance.

 

This electron can only do this in a completed circuit. This electron is probably not the same electron that originated from the battery.

 

4. This electron passes it's energy to the next part of the circuit, and so on until an electron completes the redox reaction in the battery.

 

 

From this, it may be more apparent why this process can be so instantaneous regardless of electron speed. Also, "slow" for an electron is still insanely fast in our books.

[swansont]

The electrons move slowly but the interaction is at the speed of light. There are also a lot of electrons.

[zapatos]

Can you please clarify where the electrons come from? If from a battery it sounds like they come from a chemical process, but what if the electricity comes from something like a hydro-electric source?

 

Since the current flows in a circuit, does that mean that an individual electron can/will end up back at the power plant?

 

If I had a problem with my electrical device and the current was flowing to the ground, are the total number of electrons involved somehow 'reduced'?

[Dekan]

Its a bit like a stationary train as soon as the front end moves the back end moves.Electrons input at one end of wire causes an electron output at other end.A battery creates free electrons by chemical reaction.

 

Your "train" comparison raises a point that has always puzzled me. Why are thin wires more resistant to current, than thick wires. Surely a thin wire should transmit the current very quickly.

 

Consider a very thin wire. In it, there's just a single line of electrons, from one end of the wire to the other. All electrons close together.

So when you a "push" an electron at one end, the push gets rapidly transmitted down the line, to the electron at the other end.

 

Like in a "Newton's Cradle" toy - where the swinging balls hit each other in almost instantaneous succession.

 

If this analogy is valid, then thin wires should theoretically be the best conductors, with the least resistance. However this doesn't seem to be the case in the real world. Thick wires offer less resistance than thin ones - why is that?

[derek w]

You have answered your own question a very thin wire with just a single line of electrons,an input of 1 electron at one end produces an output of 1 electron at other end.But a wire that is 1 million times thicker will allow an input of 1 million electrons at 1 end and an output of 1 million electrons at other end.

1 coulomb being a flow 6.241 x 10^18 electrons per second past a point in an electric circuit.

Edited April 11, 2012 by derek w

[G.H]

When this was explained to me in high school I was told that the valance electrons in metals are fairly delocalised such that metal could be represented basically as a lump of matter with electrons flowing in, through and around it.

Thanks for using the term 'delocalised electrons'. it cleared half my doubt after watching it in wikipedia.

From this' date=' it may be more apparent why this process can be so instantaneous regardless of electron speed. Also, "slow" for an electron is still insanely fast in our books.[/quote']

but i have read somewhere that a wave is created along the surface of conductor which is a little less than that of light which transfers electrical energy.but i am not sure WHETHER THAT IS FOR ALTERNATE CURRENT OR DIRECT CURRENT.i think it must be varying with respect to AC and DC.is your explanation for speed of conduction is for DC ONLY?

[swansont]

From this, it may be more apparent why this process can be so instantaneous regardless of electron speed. Also, "slow" for an electron is still insanely fast in our books.

 

The speed of an individual electron may be fast, but the drift velocity, which is what is important for current flow, is much less than 1 m/s

 

Your "train" comparison raises a point that has always puzzled me. Why are thin wires more resistant to current, than thick wires. Surely a thin wire should transmit the current very quickly.

 

Consider a very thin wire. In it, there's just a single line of electrons, from one end of the wire to the other. All electrons close together.

So when you a "push" an electron at one end, the push gets rapidly transmitted down the line, to the electron at the other end.

 

Like in a "Newton's Cradle" toy - where the swinging balls hit each other in almost instantaneous succession.

 

If this analogy is valid, then thin wires should theoretically be the best conductors, with the least resistance. However this doesn't seem to be the case in the real world. Thick wires offer less resistance than thin ones - why is that?

 

Electrons bump into atoms. Atoms are big, relative to electrons.

 

The picture of electrons on some sort of straight freeway is not a good analogy. Electrons are going through a forest of atoms, and trees are in the way. A thin wire is a forest with walls on the side. It constrains your motion even further.

[derek w]

Question.Would not the drift velocity increase with an increase in voltage or resistance?

For example in the filament of an electric light bulb,would there be a higher drift velocity?

Or is it that the filament reaches maximum flow capacity.

Edited April 11, 2012 by derek w

[Joatmon]

 

A change in voltage, e.g. a pulse, will travel along a conductor at less than the speed of light. This speed is sometimes pre-decided by using capacitors and inductors to make a "delay line".

 

Typical velocity factors

"Velocity factor is an important characteristic of communication media such as Category 5 cables and radio transmission lines. Plenum data cable typically has a VF between 0.42 and 0.72 (42% to 72% of the speed of light) and riser cable around 0.70. A VF of 0.70 corresponds to a speed of approximately 210,000,000 m/s or 4.76 ns to travel one meter.

 

Some typical velocity factors for radio communications cables are provided in the ARRL Handbook^[4]:"

http://en.wikipedia...._of_propagation

Edited April 11, 2012 by Joatmon

[derek w]

Ok.Thanks Joatman for your answer,but pulse velocity and drift velocity are not the same thing.

[G.H]

Thanks to everyone those who replied in the thread.The concept of flow of electrons in a conductor,in the case of DC current is CLEAR TO ME TILL SOME EXTENT.I read it in a book and also read some articles on internet.however,i have not totally understood,how does AC flow inside a conductor.could you please explain or provide some LINKS.

[Joatmon]

Ok.Thanks Joatman for your answer,but pulse velocity and drift velocity are not the same thing.

I know, but I threw this in because #4 might give the impression the effective speed of an electric current is instantaneous and #4 might give the impression that the effective speed is that of light.

[Dekan]

Thanks to everyone those who replied in the thread.The concept of flow of electrons in a conductor,in the case of DC current is CLEAR TO ME TILL SOME EXTENT.I read it in a book and also read some articles on internet.however,i have not totally understood,how does AC flow inside a conductor.could you please explain or provide some LINKS.

 

Yes, I also find AC current flow hard to understand.

 

It's easy to understand a DC current. It has a constant, steady flow of electrons. Streaming direct from one end of the circuit to the other.

 

Whereas in an AC circuit, the electrons are apparently alternating in direction - first streaming one way, then the other.

So the electrons in an AC circuit, have to go up and down the circuit wire many times, before they eventually come out.

 

Shouldn't this make AC current slower than DC current?

[swansont]

Yes, I also find AC current flow hard to understand.

 

It's easy to understand a DC current. It has a constant, steady flow of electrons. Streaming direct from one end of the circuit to the other.

 

Whereas in an AC circuit, the electrons are apparently alternating in direction - first streaming one way, then the other.

So the electrons in an AC circuit, have to go up and down the circuit wire many times, before they eventually come out.

 

Shouldn't this make AC current slower than DC current?

 

If the electrons are drifting 1 mm/sec and the frequency is 50-60 Hz, they have a net travel of only about 20 microns superimposed on their thermal motion..

[Joatmon]

Yes, I also find AC current flow hard to understand.

 

Whereas in an AC circuit, the electrons are apparently alternating in direction - first streaming one way, then the other.

So the electrons in an AC circuit, have to go up and down the circuit wire many times, before they eventually come out.

 

Shouldn't this make AC current slower than DC current?

Your idea of the electrons going to and fro is correct. Ignoring the relatively few electrons at the ends of the conductor which enter and leave the alternator driving the ac current you can say the electrons do not come out. The same electrons just go to and fro. On average the speed of the electrons is zero. The actual speed of the drift of electrons as they go to and fro changes from zero to a maximum and back to zero, first in one direction and then in the other direction. This keeps repeating. You can find quite a lot of information using Google - for example:- http://www.allaboutcircuits.com/vol_2/chpt_1/1.html

[G.H]

Your idea of the electrons going to and fro is correct. Ignoring the relatively few electrons at the ends of the conductor which enter and leave the alternator driving the ac current you can say the electrons do not come out. The same electrons just go to and fro. On average the speed of the electrons is zero. The actual speed of the drift of electrons as they go to and fro changes from zero to a maximum and back to zero, first in one direction and then in the other direction. This keeps repeating. You can find quite a lot of information using Google - for example:- http://www.allaboutcircuits.com/vol_2/chpt_1/1.html

Thanks for your help.The site provided a good information about how the polarity of AC changes with respect to time(or rotation of the magnet).

Edited April 13, 2012 by vedprakash..