Current Flow Basics: Bulb, Wire, Insulators

[sgstudent]

In a simple circuit where i have a bulb connected to a wire that's connected back to the bulb, how will the electrons flow? I'm thinking it would look like this http://postimg.org/image/6jd2dg2md/full/ where as one electron comes out of the negative end, another electron goes in the positive end. Is this the correct way to think of it?

What about flowing through insulators? Would it be the same thing just that the current is a lot smaller because of the large resistance?

Thanks for the help

 

[ravikannaujiya]

Electron always flows from lower potential to the higher potential. A source, a battery, has two terminal. one has lower potential and the another has higher potential thus giving the total potential difference written on the battery.
potential difference V=V2-V1
where V2 is heigher potential
and V2 is lower potential
This causes the electron to flow from the terminal of lower potential to the terminal of higher potential. The flow of electron turns out to be flow of current but in the opposite direction as electron has negative charge. Instead, if we have a system in which proton is free to move, then we would find the direction of the flow of current would be same as the direction of the flow of proton.
Here one thing must be remembered that the flow of proton is from hiegher potential to the lower potential.
From all above you can find that current always flow from the higher potential to the lower potental whether the cause of current is flow of electron or proton.
It is like you have a flow of water in a pipe and the direction of the flow of water current would always be in the direction of higher presser to lower pressure.

 

[sgstudent]

Electron always flows from lower potential to the higher potential. A source, a battery, has two terminal. one has lower potential and the another has higher potential thus giving the total potential difference written on the battery.
potential difference V=V2-V1
where V2 is heigher potential
and V2 is lower potential
This causes the electron to flow from the terminal of lower potential to the terminal of higher potential. The flow of electron turns out to be flow of current but in the opposite direction as electron as negative charge. Instead, if we have a system in which proton is free to move, then we would find the direction of the flow of current would be same as the direction of the flow of proton.
Here one thing must be remembered that the flow of proton is from hiegher potential to the lower potential.
From all above you can find that current always flow from the higher potential to the lower potental whether the cause of current is flow of electron or proton.
It is like you have a flow of water in a pipe and the direction of the flow of water current would always be in the direction of higher presser to lower pressure.

Hi thanks for the reply but I'm not asking about why current flow but how does it flow. Could you check the first post and see if I'm right?

Thanks

 

[ravikannaujiya]

hmm... it is very clever way of asking... :) anyway...then the answer is very simple... current is nothing more than the flow of charges. if one chrge is flowing in any medium, the current is flowing as well. If you can imagine the flow of a charge, then you have your answer how the current is flowing. More the speed of the charges more the current ... for your visulisation you may play this simulation.. it is a java running program..
http://phet.colorado.edu/en/simulation/battery-resistor-circuit
I think it may work.. :)

 

[BobG]

Yes, the electrons go from the negative terminal to the positive terminal.

However, when you're measuring current, you're interested in the flow of energy and the energy flows from the positive terminal to the negative terminal.

It works kind of like a bunch of students sitting in a row of seats. Kid #1 leaves seat #1, the energy has moved to seat #1. Kid #2 moves from seat #2 to seat #1, the energy moves from seat #1 to seat #2. And so on. And at the end of this, each kid (each electron) has moved one slot forward, while the empty seat (the energy) has moved all the way from seat #1 to the last seat.

As a result, electricity (the energy) moves very fast, even though the actual electrons are moving very slow. (The actual speed of electricity depends on the components in the circuit, etc, but it's much, much slower than the speed of light, just to dispel another myth - which makes sense, since the speed of the energy still depends on the speed of the electrons to a certain extent, even though you're tracking the holes left by the electrons rather than the motion of the electrons themselves.)

 

[CraigH]

I think sgstudent is asking how the electrons move inside a conductor, not asking about conventional current flow vs electron flow, or energy flow.

Conductors, like everything else, are made of atoms. atoms have a positively charged nucleus surrounded by negatively charged electrons. In a conductor the outer shell of electrons surrounding the nucleus can pass from one atom to another. These electrons are called valence electrons, the more valence electrons a certain material has, the better it is at conducting electricity. Remember that, a conductor is already full of electrons, the battery isn't spitting electrons out into it and then sucking them back up. The battery "pumps" the electrons in the conductor around the circuit.

These electrons do move very slowly, this speed is called drift velocity. The energy however moves much quicker. There are lots of analogy's to explain this, a classic one is marbles in a tube: imagine marbles lined up next to each other in a tube, push the first one and the last one seems to instantly move. The marbles may be moving slow but the energy has traveled fast. (not instantly though. It may appear that the last marble moved instantly but the energy actually traveled at the speed of sound for this system).
I'm not sure how valid this analogy actually is though. I don't think the energy through a circuit travels at the speed of sound. Here's how I think of it:
When an electron moves, the electric field it creates moves, causing there to be a net force on the otherwise equilibrium system of electrons. This causes these electrons to move and therefore their electric fields move. The electric field redistribution can travel very quickly around a circuit.
The electric field at some point in the circuit, multiplied by the charge at that point in the circuit, multiplied by the distance moved by that charge due to the electric field, equals the work done by the electric field on that charge. You can see if no charges are moving, no work is being done. You can also see that the electric field redistribution causing the charges to be moved could cause work to be done far away from the original electric field redistribution very quickly. This is why energy flows around a circuit faster than the electrons do.

But to simply answer your question "how does current flow?": charges (electrons) inside conductors are pumped around the circuit because of a potential difference. the electrons can and do pass through the battery(in the form of anions).

 

[ravikannaujiya]

Thanks for describing this beautiful way of demonstration of the flow of electron and the energy as well... :)

 

[jaydnul]

As a result, electricity (the energy) moves very fast, even though the actual electrons are moving very slow.

This is so important. The drift speed of the electrons is very slow. Like Bobg said, it depends on the size of the wire and other factors. The energy is transferred at the speed of light. Its like a pulley system, the instant you turn one end, so does the other end. (well, at the speed of light, not instantaneously)

 

[technician]

its also like opening a tap... the water comes out immediately (speed of light?)
because the water is already in the pipes, it has not been delivered instantaneously from some reservoir.

 

[BobG]

I guess adding a statement to dispel the myth that electricity travels at the speed of light was in vain.

Or at least consider that the speed of light depends on the medium it's traveling through and that different frequencies of light travel at different speeds through the same medium (hence light being refracted into a rainbow). The speed of electricity will always be less than the speed of light through a vacuum (what we normally use for the speed of light).

 

[Crazymechanic]

Well yes but the speed of electricity is still almoust c ofcouse depending on the circumstances , I think the fact why we can't use marbles and water in pipes is that electricity is actually carried by the em force which is mediated by photons which travel at c and so when you apply a charge at one end of a wire the other one is at the same potential almoust instantly becauce the first excited electron made a electric field which made a magnetic field and both of them together make the em field which makes photons so a photon now goes to excite another electron before the actual drift velocity has ever got near that electron so now you have electrons in a wire that move very very slow and yet electricity at the other end which is there almoust instantly.

I mean we don't wait minutes after we turn lights on for them to actually start to "shine"
unless the bulb is not burnt out and the being that turned the lights on is a blonde ...

 

[DrZoidberg]

In a simple circuit where i have a bulb connected to a wire that's connected back to the bulb, how will the electrons flow? I'm thinking it would look like this http://postimg.org/image/6jd2dg2md/full/ where as one electron comes out of the negative end, another electron goes in the positive end. Is this the correct way to think of it?

The battery creates an electric field inside the wire. That field pushes/pulls all the electrons simultanously. So the battery is really transferring energy to all electrons at the same time. The electrons don't have to push each other.

... for your visulisation you may play this simulation.. it is a java running program..
http://phet.colorado.edu/en/simulation/battery-resistor-circuit
I think it may work.. :)

That simulation has an error in it. It suggests that the drift velocity of electrons is higher at the positive terminal and lower at the negative one. As long as the wire has the same diameter, is made from the same material and the current is the same, the drift velocity of the electrons will also be the same.

Yes, the electrons go from the negative terminal to the positive terminal.

However, when you're measuring current, you're interested in the flow of energy and the energy flows from the positive terminal to the negative terminal.

You can not assign a direction to the flow of energy here. All you can say is that energy moves from the battery to the resistor but not which way around it flows through the wire.

As a result, electricity (the energy) moves very fast, even though the actual electrons are moving very slow. (The actual speed of electricity depends on the components in the circuit, etc, but it's much, much slower than the speed of light, just to dispel another myth - which makes sense, since the speed of the energy still depends on the speed of the electrons to a certain extent, even though you're tracking the holes left by the electrons rather than the motion of the electrons themselves.)

Wait - are you suggesting here that electrical energy is transferred much slower than the speed of light? The speed at which the energy is being transferred depends on the wave propagation speed
http://en.wikipedia.org/wiki/Wave_propagation_speed
That is, it depends on the speed with which a change in voltage moves through a wire. And that can be more than 99% of c.

 

[technician]

"sense, since the speed of the energy still depends on the speed of the electrons to a certain extent, even though you're tracking the holes left by the electrons rather than the motion of the electrons themselves.)"

What do you mean by 'the holes left by electrons'...I do not think this is part of the explanation of conduction in metals.
I know that 'holes' are essential to explaining conduction in semiconductors and I know that +ve ions are part of the explanation of conduction in liquids, but 'holes' in metals?...please enlighten me.

 

[CraigH]

hmm... it is very clever way of asking... :) anyway...then the answer is very simple... current is nothing more than the flow of charges. if one chrge is flowing in any medium, the current is flowing as well. If you can imagine the flow of a charge, then you have your answer how the current is flowing. More the speed of the charges more the current ... for your visulisation you may play this simulation.. it is a java running program..
http://phet.colorado.edu/en/simulation/battery-resistor-circuit
I think it may work.. :)

That simulation has an error in it. It suggests that the drift velocity of electrons is higher at the positive terminal and lower at the negative one. As long as the wire has the same diameter, is made from the same material and the current is the same, the drift velocity of the electrons will also be the same.
.

DrZoidberg this is something I have wondered about for a while now. If the drift velocity doesn't change, how can there be a potential difference across a resistor?

If (in series) the current is the same on both sides of the resistor, and the drift velocity is the same, then how can there be a different electric field at each side of the resistor?

There is definitely a different electric field at each side of a resistor, because how else will there be a different electric potential at each side of the resistor. And if the charge is exactly the same on both sides what causes this change in electric field.

Also, surely the electrons inside the resistor are being slowed down due to heating. (there kinectic energy's being transferred to the resistor (ohmic heating), so the drift velocity will slow down.

I think this may be something to do with waves. The wave front of the displacement wave being reflected at the impedance mis-match, and causing slowing on both sides of the resistor. Am I correct?

 

[DrZoidberg]

The field inside the resistor is different from the one in the wire, but the field inside the wire is the same on both sides of the resistor. Potential difference is not a measure of the strength of the electric field. It is the integral of the electric field over a distance. Inside the resisitor the field is stronger because the battery is not the only field source. There is also static charge that accumulated on the surface of the wire that produces a field of it's own. At the ends of the resistor you have a "pile up" of charge. The field created by that charge strengthens the field inside the resistor but weakens it in the wire. The integral of the e. field over the entire circuit stays the same of course and is equal to the P.D. of the battery.
Inside the resistor the drift velocity may be different but after the electrons leave the resistor their velocity goes back to their previous value. The e. field is constantly transferring energy to the electrons and the electrons are constantly tranferring energy to the atoms around them. However their kinetic energy at any point in time is virtually zero. Because they move very slowly and are very light. It's like pulling a heavy object with a rope. The rope never has much kinetic energy.
Waves do not occur in a static situation i.e. when the current is constant. So no waves are being reflected.

 

[BobG]

"sense, since the speed of the energy still depends on the speed of the electrons to a certain extent, even though you're tracking the holes left by the electrons rather than the motion of the electrons themselves.)"

What do you mean by 'the holes left by electrons'...I do not think this is part of the explanation of conduction in metals.
I know that 'holes' are essential to explaining conduction in semiconductors and I know that +ve ions are part of the explanation of conduction in liquids, but 'holes' in metals?...please enlighten me.

Figuratively speaking. When the conductor loses an electron, it leaves a 'hole' that needs to be filled by stealing an electron from its neighbor. Probably the more accurate description is it becomes positively charged until it steals an electron from its neighbor, leaving its neighbor positively charged, etc.

 

[DrZoidberg]

@DrZoidberg In a dc circuit the field after the resistor will be less than that of before the resistor , otherwise you would never get reduction in current/voltage.
Actually you can see the field weakening while moving along the resistor, as the em field is the one whi9ch excites new electrons to "move " the electricity so if the field would be the same after the resistor the resistor would be useless.

Take a look at Ohm's law.
http://en.wikipedia.org/wiki/Ohm's_law
The generalized version of Ohm's law is J=σE
This means the current density inside of metal is ALWAYS equal to the field strength times the conductivity.
The other way around that means if the current density and the conductivity are both the same, the field strength MUST be the same. So, what I said was absolutely correct. The field in the wire is the same on both sides of the resistor. The resistor leads to a reduction of the field inside the wire because of the "pile up" of charges. So it is not useless.

Even though the actual electrons are slow they are the ones that create the em field which creates the photons and according to how strong the electron that made the em field was the corresponding photon will be of the exact energy the electron had not higher so when we use a resistor we decrease the energy of the electrons so that every next electron has less energy and can make a lower energy photon which indeed excites the next electron to be of lower energy and so the em field decreases the actual current/voltage in a resistor.

I'm hoping you are talking about virtual photons here. But even then your theory sounds rather weird.

 

[nsaspook]

However, when you're measuring current, you're interested in the flow of energy and the energy flows from the positive terminal to the negative terminal.

'Energy' flows from both terminals to the load. The direction of charge movement (current) can be either way or both ways at the same time (battery).

 

[sgstudent]

I think sgstudent is asking how the electrons move inside a conductor, not asking about conventional current flow vs electron flow, or energy flow.

Conductors, like everything else, are made of atoms. atoms have a positively charged nucleus surrounded by negatively charged electrons. In a conductor the outer shell of electrons surrounding the nucleus can pass from one atom to another. These electrons are called valence electrons, the more valence electrons a certain material has, the better it is at conducting electricity. Remember that, a conductor is already full of electrons, the battery isn't spitting electrons out into it and then sucking them back up. The battery "pumps" the electrons in the conductor around the circuit.

These electrons do move very slowly, this speed is called drift velocity. The energy however moves much quicker. There are lots of analogy's to explain this, a classic one is marbles in a tube: imagine marbles lined up next to each other in a tube, push the first one and the last one seems to instantly move. The marbles may be moving slow but the energy has traveled fast. (not instantly though. It may appear that the last marble moved instantly but the energy actually traveled at the speed of sound for this system).
I'm not sure how valid this analogy actually is though. I don't think the energy through a circuit travels at the speed of sound. Here's how I think of it:
When an electron moves, the electric field it creates moves, causing there to be a net force on the otherwise equilibrium system of electrons. This causes these electrons to move and therefore their electric fields move. The electric field redistribution can travel very quickly around a circuit.
The electric field at some point in the circuit, multiplied by the charge at that point in the circuit, multiplied by the distance moved by that charge due to the electric field, equals the work done by the electric field on that charge. You can see if no charges are moving, no work is being done. You can also see that the electric field redistribution causing the charges to be moved could cause work to be done far away from the original electric field redistribution very quickly. This is why energy flows around a circuit faster than the electrons do.

But to simply answer your question "how does current flow?": charges (electrons) inside conductors are pumped around the circuit because of a potential difference. the electrons can and do pass through the battery(in the form of anions).

Hi sorry for the late reply, I went to bed :)

The marbles analogy is great but I would like to add on on this analogy. If there were 3 marbles touching each other and I push one, all 3 would move the same distance right? So is this the same for the battery and wire circuit whereby the electrons are the marbles and the push is the potential difference? So as one charge exits the negative side of the battery at the same time another charge enters the positive side instantaneously? If so then current is Q/t so how should be visualize this? Would it be like fixing a point and counting the number of marbles that pass that point per unit time?

So increasing resistance would only decrease the current. So can I think of it like this: 2 toy tunnel filled with marbles (like in the analogy) one has a larger diameter than the other. As a for the smaller tunnel less marbles can be lined up, so less charge can flow so the current is smaller?

Thanks so much for the help :)

 

[technician]

Figuratively speaking. When the conductor loses an electron, it leaves a 'hole' that needs to be filled by stealing an electron from its neighbor. Probably the more accurate description is it becomes positively charged until it steals an electron from its neighbor, leaving its neighbor positively charged, etc.

Physically speaking, this is completely wrong. This is not the accepted explanation for current flow through a metal.
I suggest you check the textbook explanation.

 

[sgstudent]

The battery creates an electric field inside the wire. That field pushes/pulls all the electrons simultanously. So the battery is really transferring energy to all electrons at the same time. The electrons don't have to push each other.



That simulation has an error in it. It suggests that the drift velocity of electrons is higher at the positive terminal and lower at the negative one. As long as the wire has the same diameter, is made from the same material and the current is the same, the drift velocity of the electrons will also be the same.



You can not assign a direction to the flow of energy here. All you can say is that energy moves from the battery to the resistor but not which way around it flows through the wire.


Wait - are you suggesting here that electrical energy is transferred much slower than the speed of light? The speed at which the energy is being transferred depends on the wave propagation speed
http://en.wikipedia.org/wiki/Wave_propagation_speed
That is, it depends on the speed with which a change in voltage moves through a wire. And that can be more than 99% of c.

Your first explanation is great just to clarify all the electrons would be pushed by the potential difference equally, so its just like http://www.kugelbahn.info/bilder/haupt/aufzug.gif where each marble gets pushed equally? (And sorry for the small gif i couldn't find any gifs about moving marble I also found this link http://www.askaboutireland.ie/learn...lass/science/electricity/what-is-electricity/ but you have to go all the way to the end to show the current flow

But now I'm having some trouble understanding ligtnings from this gif here, http://www.gifbin.com/982185 from here it appears that the electron (lightning) actually goes down slowly unlike in a regular circuit where all the electrons are being pushed at the same rate. Because if the lightning was like a regular circuit, I thought the electrons would flow like this http://postimg.org/image/3ljzyp61h/full/ so all of them are being 'pushed' at the same rate. But now it appears that not all the electrons are being 'pushed'? I hope this paragraph makes sense.

Thanks so much for the help :)

 

[DrZoidberg]

Your first explanation is great just to clarify all the electrons would be pushed by the potential difference equally, so its just like http://www.kugelbahn.info/bilder/haupt/aufzug.gif where each marble gets pushed equally?

Yes, as long as the wire is the same, i.e. same diameter, same resistivity. If for example the wire is thicker in some part of the circuit the electrons will move more slowly there since there are more electrons that can move. That means in the thicker wire the electric field is weaker.

But now I'm having some trouble understanding ligtnings from this gif here, http://www.gifbin.com/982185 from here it appears that the electron (lightning) actually goes down slowly unlike in a regular circuit where all the electrons are being pushed at the same rate.

It's called a streamer discharge.
http://en.wikipedia.org/wiki/Streamer_discharge
The air is being ionized. That ionization process moves down slowly, not the electrons.

 

[sgstudent]

Hi thanks so much

Could you explain what would happen if I have an irregular wire? Because I thought the current would remain the same in the whole wire (provided it is series)?

As for the lightning question, what would the process of that discharge be like actually? I read that the air in between the clouds and the Earth are ionized so air would be split into an electron and a positive ion. But where would that ionization occur and how does the electrons move?

Thanks for the great response :)

 

[DrZoidberg]

The current remains the same of course, that's true. But if the wire is thicker the current distributes over a larger cross sectional area, so the current density is smaller. You have more electrons moving but they move at a slower speed. So the total number of electrons that pass through the wire per second remains the same.
How the ionization works is explained quite well in the wikipedia article about streamers. Is there a part you didn't understand?

 

[sgstudent]

Hi

Why would the force on those electrons be smaller? Because in this case I'm not sure if an electron would leave and enter the battery at the same time.
Edit: I thought about it for a while and it makes mote sense if the wire connected to the posited end was thicker than the speed of electrons have to be smaller it or else more electrons would enter the positive end than electrons leave the negative end per unit time. But I'm not not what causes the smaller force.

Oh I was just read the physicsclassroom explanation on it. I'll check out the Wikipedia explanation for streamers now.

Thanks so much for the help

 

[DrZoidberg]

Look at the generalized Ohm's law.
http://en.wikipedia.org/wiki/Ohm's_law
J=σE
If the wire is thicker, the current density must obviously go down. So according to Ohm's law the electric field strength must decrease. The reason why this works is because of a "pile up" of charge on the surface of the wire. You can say there is static charge on the wire that causes the electric field to have the "right" value everywhere. e.g. at a resistor charges pile up. That charge creates a strong field inside the resistor and since voltage is equal to field strength integrated over distance that means a relatively high voltage drops over the resistor.
The number of electrons per second (i.e. the current) that leave the negative terminal of a battery is usually equal to the number that enters the positive terminal.

 

[Crazymechanic]

Well the current density is indeed proportional to the diameter of he wire or conductor it is going through , but explain then DrZoidberg what causes the loss of current in long transmission lines where the effects are measurable and also in every other conductor component even though usually they are too short to measure such a small loss.
Now if the current stays the same as you say then why do all the transmission line and power station operators try so hard to keep the lines as short as possible and if not short then atleast with high voltage so that the current could be decreased and the loss from transfer minimized?

Maybe I'm getting you wrong here but according to what I read I could conclude that after 2000 miles of wire the current would be the same as at the input? But this clearly is not the case.It is not the case because every wire if not superconducting is a resistor , a small one but it has resistance and that resistance takes away the potential of each electron slowly over distance and so every next one can create a lower em field which indeed has less power to push the electricity forward.
Electricity is like the dominos , when you start the dominos from one end they fall quite slowly until they reach the end of the line and so does electrons flow slowly in a DC circuit and they have no net movement in an AC one but now imagine the dominos being pushed not by the one before the one that fell but by an unseen force which travels much much faster han the dominos themselves so that every next dominos can "fall" before the last one has even touched the next one physically and that is the photon of the EM field.

Whoever said here in this thread that the speed of electricity is slow is wrong the speed of electricity through perfect vacuum would be hat of the speed of light or c. In a copper cable it is something like 95%, according to a fast google search.But I believe the figure should be about right.

 

[sgstudent]

Well the current density is indeed proportional to the diameter of he wire or conductor it is going through , but explain then DrZoidberg what causes the loss of current in long transmission lines where the effects are measurable and also in every other conductor component even though usually they are too short to measure such a small loss.
Now if the current stays the same as you say then why do all the transmission line and power station operators try so hard to keep the lines as short as possible and if not short then atleast with high voltage so that the current could be decreased and the loss from transfer minimized?

Maybe I'm getting you wrong here but according to what I read I could conclude that after 2000 miles of wire the current would be the same as at the input? But this clearly is not the case.It is not the case because every wire if not superconducting is a resistor , a small one but it has resistance and that resistance takes away the potential of each electron slowly over distance and so every next one can create a lower em field which indeed has less power to push the electricity forward.
Electricity is like the dominos , when you start the dominos from one end they fall quite slowly until they reach the end of the line and so does electrons flow slowly in a DC circuit and they have no net movement in an AC one but now imagine the dominos being pushed not by the one before the one that fell but by an unseen force which travels much much faster han the dominos themselves so that every next dominos can "fall" before the last one has even touched the next one physically and that is the photon of the EM field.

Whoever said here in this thread that the speed of electricity is slow is wrong the speed of electricity through perfect vacuum would be hat of the speed of light or c. In a copper cable it is something like 95%, according to a fast google search.But I believe the figure should be about right.

For the dominos analogy, would it be correct

 

[DrZoidberg]

In an hv transmission line you have 3 different kinds of losses.
http://en.wikipedia.org/wiki/Electric_power_transmission#Losses
Resistive losses, corona discharge and reactive losses.

You can model a power line as a combination of inductances, resistors and capacitors.
Here is a graphic
http://www.microwaves101.com/encyclopedia/images/Transmission%20lines/general_Tline.jpg
It's from this site
http://www.microwaves101.com/encyclopedia/t-line_model.cfm
The resistor G' represents the corona discharge, C' is the capacitance of the wire relative to ground and R' and L' are the resistance and inductance of the wire.
C' and G' cause a current drop because they are in parallel, not in series to the line. R' causes a voltage drop.
A resistor in series with other components never causes a drop in current, only a drop in voltage.

However I was talking about a low voltage dc circuit consisting of a battery, a resistor and wires in series. There is no corona discharge there. There also are no capacitive or inductive losses because those can only occur with ac.
Therefore in such a dc circuit the current is always equal everywhere. There is no loss of current.
Maybe looking at Kirchhoff's laws will make this more clear.
http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

 

[ZapperZ]

Well the current density is indeed proportional to the diameter of he wire or conductor it is going through , but explain then DrZoidberg what causes the loss of current in long transmission lines where the effects are measurable and also in every other conductor component even though usually they are too short to measure such a small loss.
Now if the current stays the same as you say then why do all the transmission line and power station operators try so hard to keep the lines as short as possible and if not short then atleast with high voltage so that the current could be decreased and the loss from transfer minimized?

Maybe I'm getting you wrong here but according to what I read I could conclude that after 2000 miles of wire the current would be the same as at the input? But this clearly is not the case.

The current remains the same because there is still an external field that is being applied to create the current. If this external field doesn't exist, then any kind of preferential drift will stop.

Zz.

 

[Crazymechanic]

Ok you got me lost in a forest of 2 and a half trees , now if the current stays the same after like 1000 miles of wire then what decreases? Just don't say nothing and everything stays the same as at the start of the line?

 

[sgstudent]

Well the current density is indeed proportional to the diameter of he wire or conductor it is going through , but explain then DrZoidberg what causes the loss of current in long transmission lines where the effects are measurable and also in every other conductor component even though usually they are too short to measure such a small loss.
Now if the current stays the same as you say then why do all the transmission line and power station operators try so hard to keep the lines as short as possible and if not short then atleast with high voltage so that the current could be decreased and the loss from transfer minimized?

Maybe I'm getting you wrong here but according to what I read I could conclude that after 2000 miles of wire the current would be the same as at the input? But this clearly is not the case.It is not the case because every wire if not superconducting is a resistor , a small one but it has resistance and that resistance takes away the potential of each electron slowly over distance and so every next one can create a lower em field which indeed has less power to push the electricity forward.
Electricity is like the dominos , when you start the dominos from one end they fall quite slowly until they reach the end of the line and so does electrons flow slowly in a DC circuit and they have no net movement in an AC one but now imagine the dominos being pushed not by the one before the one that fell but by an unseen force which travels much much faster han the dominos themselves so that every next dominos can "fall" before the last one has even touched the next one physically and that is the photon of the EM field.

Whoever said here in this thread that the speed of electricity is slow is wrong the speed of electricity through perfect vacuum would be hat of the speed of light or c. In a copper cable it is something like 95%, according to a fast google search.But I believe the figure should be about right.

Hi thanks for the reply

For the domino anology, would we imagine all of the blocks fall together before they even touch each other? But actually why would resistant take away the potential? Because in my syllabus (which is only a secondary level physics haha) just tells us that in a series circuit the potential difference divided by the total resistance would give us the current. Perhaps my syllabus is too simplified?

 

[Astronuc]

Ok you got me lost in a forest of 2 and a half trees , now if the current stays the same after like 1000 miles of wire then what decreases? Just don't say nothing and everything stays the same as at the start of the line?

There is a voltage drop, and for long distance, there may be current leakage from the transmission line. Think about a transmission line in terms of a mechanical analogy in a water-filled pipeline. The water (current) in = water (current) out, but along the pipe is a pressure (voltage) drop.

 

[DrZoidberg]

You mean in a dc circuit without any corona losses?
What decreases is the potential. If you have a battery and two wires each 1000 miles long connected to the two terminals of the battery and a resistor connected at the end, the potential difference i.e. the voltage across the resistor is smaller than the one across the battery because part of the voltage drops across the wires.

 

[Crazymechanic]

And voltage is proportional to current so what do we get ? :)

@sgstudent No the blocks don't fall all at once the speed of Em waves is that of c or almoust c depending on the medium they are traveling but c even though the highest speed physically is still not infinite remember that.
Light and electricity travel very very fast but still they travel and it takes time and goes over a certain distance.

 

[ZapperZ]

And voltage is proportional to current so what do we get ? :)

This is a severe misunderstanding. It is a POTENTIAL DIFFERENCE, not an absolute potential value! That is what is in V=IR. The drop in the potential across a resistor is dictated by the value of the resistor itself. Do a Kirchoff Law around a simple series loop and see how you can get a consistent result even when I is a constant all around the circuit.

Zz.

 

[sgstudent]

And voltage is proportional to current so what do we get ? :)

@sgstudent No the blocks don't fall all at once the speed of Em waves is that of c or almoust c depending on the medium they are traveling but c even though the highest speed physically is still not infinite remember that.
Light and electricity travel very very fast but still they travel and it takes time and goes over a certain distance.

Oh sorry about that :) I meant they start to fall together instead of hsving one domino pushing down the next domino.

As for the formation of a lightning I read the Wikipedia article on it and I don't quite get their explanation. I don't really understand where the ionization would take place and whether the excess negative charges from the cloud actually go down the air. And where would the positive ions go (also because I'm not too sure where those ions start to form as well..)

Thanks so much for the help :) seems like this thread sparked off a lot of discussion (pun intended haha)

 

[DrZoidberg]

And voltage is proportional to current so what do we get ? :)

A constant current but different voltages across the components of the circuit.
For example. Say the battery has 10V and you have 3 resistors in series connected to it.
2 Ohm, 3 Ohm and 5 Ohm. Now the total resistance is 10 Ohm so the current is 10V/10Ohm = 1A
Voltage drop across the 2Ohm resistor is 2Ohm*1A = 2V. The 3Ohm resistor has 3V dropping over it and the 5Ohm resistor 5V. The sum of all the voltages is 10V.
If you want to include the wires into that calculation, well wires are just resistors.

 

[ZapperZ]

Oh sorry about that :) I meant they start to fall together instead of hsving one domino pushing down the next domino.

As for the formation of a lightning I read the Wikipedia article on it and I don't quite get their explanation. I don't really understand where the ionization would take place and whether the excess negative charges from the cloud actually go down the air. And where would the positive ions go (also because I'm not too sure where those ions start to form as well..)

Thanks so much for the help :) seems like this thread sparked off a lot of discussion (pun intended haha)

Unfortunately, and this is my personal opinion, it has distracted from your original question, which is really straightforward and one that I hope has been answered.

I just hope you were able to weed through the responses you received and able to figure out which ones are the correct answers to your question.

Zz.

 

[sgstudent]

Unfortunately, and this is my personal opinion, it has distracted from your original question, which is really straightforward and one that I hope has been answered.

I just hope you were able to weed through the responses you received and able to figure out which ones are the correct answers to your question.

Zz.

Yup that is true. I managed to understand the mechanism of a simple circuit and a little on lightning as a bonus. I learned that generally electrons in a circuit would all move equally around the circuit back to the other end of the battery unless the wire increases in size, then accordingly the electrons speed of movement might decrease or increase to match the current. So it allows the total electrons leaving the battery to be equal to the total number of electrons entering the battery.

Should wait to learn more about capacitors to get a better understanding before I continue asking about the mechanics of a lightning strike?

And lastly, thanks so much everyone for all their replies I gained a lot from this post Haha :)

 

[DaveC49]

The field inside the resistor is different from the one in the wire, but the field inside the wire is the same on both sides of the resistor. Potential difference is not a measure of the strength of the electric field. It is the integral of the electric field over a distance. Inside the resisitor the field is stronger because the battery is not the only field source. There is also static charge that accumulated on the surface of the wire that produces a field of it's own. At the ends of the resistor you have a "pile up" of charge. The field created by that charge strengthens the field inside the resistor but weakens it in the wire. The integral of the e. field over the entire circuit stays the same of course and is equal to the P.D. of the battery.
Inside the resistor the drift velocity may be different but after the electrons leave the resistor their velocity goes back to their previous value. The e. field is constantly transferring energy to the electrons and the electrons are constantly tranferring energy to the atoms around them. However their kinetic energy at any point in time is virtually zero. Because they move very slowly and are very light. It's like pulling a heavy object with a rope. The rope never has much kinetic energy.
Waves do not occur in a static situation i.e. when the current is constant. So no waves are being reflected.