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Author Topic: How electricity flows?  (Read 2587 times)

scientist

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How electricity flows?
« on: 16/02/2013 13:59:14 »
It's very basic question but still i have many doubts about it...
1. if electrons are flowing then why chemical properties do not change of that conductor
2 resistance is faced due to collisions to atoms. so what are these collisions? and how an electron collides?
3 if electron drift is only couple of meter per second then how bulb gets on instantly when we push the button

I've many more questions but let them go for moment. But still please help me bcoz these simple questions(not for me) are not allowing me to think of something bigger .

 :(


 

Offline graham.d

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Re: How electricity flows?
« Reply #1 on: 16/02/2013 17:31:06 »
Hello. Here is how I see it...
1. That is a good question. If you are speaking of an element that is a good conductor (like a metal) then it is a material where outer electrons are not tightly bound to the nuclei. This allows the electrons to move freely between the atoms in a random fashion. However when a voltage is applied across such a material the electrons will gradually shuffle along as a result of the electric field. The electrons do not move very fast on average (a few mm per second, though each electron can achieve a very high speed for a short time) but the electric current may be high because of the total numbers of electrons entering and leaving the conductor at its terminals. In general the chemical properties of the material are not affected very much because the average binding force to the nucleus does not change very much as a result of the current. I say not much, but metals are subject to electrochemical effects which are related to this. These are better understood in terms of the voltage between the metal and some other material. A voltage applied between the metal and another material will aid the exchange of electrons with the other material and can thereby enhance a chemical reaction where "Galvanic Corrosion" can occur with an exchange of ions.
2. I think "collisions" are a way of producing a model we can think about when understanding how electrons flow through a conducting material. The electron will have a "mean free path" which is a distance than can be calculated. It is an average distance over which an electron is accelerated by the average field it will see before "colliding" with some material. This model works in that it correctly predicts the behaviour of "Ohmic" conductors. You can consider the electron to be momentarily held and released to start its acceleration again. This is a model though. It does not work to explain the behaviour of superconductors for example. With semiconductors (where the electrons are not so weakly bound) the model is used to calculate the behaviour not only of electrons, but also of "holes"; these can be thought of as positively charged particles that behave in a similar way to electrons but are in fact "the absence of an electron in the atomic outer shell" but which will propagate (bucket brigade fashion) in an opposite direction.
3. Electron drift is much slower that a "couple of meters per second". However there are a lot of electrons so the net movement of charge can be high. Although each electron can take a long time to get from one end of a piece of wire to the other, the electric field propagates at the speed of light (slightly modified by the material's properties). The commencement of the current flow will start as soon as the electric field appears across the light bulb filament (for example) which is therefore very fast an not related to the speed of the electrons.

I hope this helps.
 

scientist

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Re: How electricity flows?
« Reply #2 on: 16/02/2013 19:02:46 »
okay... yes it absolutely helped me... i discussed the first question with my friends. we concluded that the atom which passes an electron immediately receives an electron from atom which is behind it. In other words, an atom passes electron only if another atom(the one behind it) is going to pass its electron to it. So, chemical properties remain unchanged.
 

Offline CliffordK

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Re: How electricity flows?
« Reply #3 on: 16/02/2013 20:55:26 »
It would certainly depend on the conductor, and the amount of power being applied.

Electricity flowing through air can create a plasma.  It also isn't uncommon for chemical reactions to occur including forming ozone.
 

Offline David Cooper

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Re: How electricity flows?
« Reply #4 on: 16/02/2013 23:45:22 »
If you have a rod a metre long and push it along by a millimetre, the far end will also move a millimetre at almost the same instant - you don't have to move the whole rod a metre to make the far end of the rod move.
 

Offline yor_on

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Re: How electricity flows?
« Reply #5 on: 17/02/2013 01:19:32 »
David, approximately true, but the speed of motion in that rod is the speed of sound if I remember right? But scale it down and see what happens, will that still hold true for one atom? Or one electron? Although if you had a infinitely rigid rod the statement would be true, as I get it.
 

scientist

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Re: How electricity flows?
« Reply #6 on: 17/02/2013 05:18:05 »
David, approximately true, but the speed of motion in that rod is the speed of sound if I remember right? But scale it down and see what happens, will that still hold true for one atom? Or one electron? Although if you had a infinitely rigid rod the statement would be true, as I get it.
I can't get what you are talking... can you please make it simpler?
 

Offline Bored chemist

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Re: How electricity flows?
« Reply #7 on: 17/02/2013 15:03:20 »
The "simple" answer is that some of the electrons in a metal are not connected to any particular atom.
when a current flows through the metal some electrons are added at one end of the wire and taken out at the other end and, since very few electrons are involved compared to the overall number, there's no change in the overall structure of the metal.
 

Offline David Cooper

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Re: How electricity flows?
« Reply #8 on: 17/02/2013 19:54:47 »
David, approximately true, but the speed of motion in that rod is the speed of sound if I remember right? But scale it down and see what happens, will that still hold true for one atom? Or one electron? Although if you had a infinitely rigid rod the statement would be true, as I get it.

Yes, the speed with the rod is transmitted at the speed of sound, whereas the atoms in the rod could be moving at one millimetre per hour. With electricity you can make a few electrons move a tiny distance at a very low speed and the electrons a the wire a mile away will move the same distance at the same speed, with the delay being governed by the speed of light (or something very close to that speed - the forces between the electrons keeping them apart are transmitted at the speed of light, but some slight zigzagging may be involved in the transmission chain).
 

Offline yor_on

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Re: How electricity flows?
« Reply #9 on: 18/02/2013 11:07:19 »
The idea (as I get it) is that the 'force' transmitted through that rod ,as you move it, is moving its atoms (compressing/elongating in some atomic undulation) approximately at the speed of sound. A little like marbles hitting marbles, hitting marbles, as the 'force' of your motion propagates through the medium. It's a 'elasticity' inside (as well as in those marbles). But with a 'infinitely rigid rod' you could ignore the 'elasticity', which also should make this new rigid materials name, unobtanium :) as it in principle then should be able to exceed the speed of light. Poking at the moon from earth for example.
 

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Re: How electricity flows?
« Reply #9 on: 18/02/2013 11:07:19 »

 

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