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Author Topic: Direction of electrons?  (Read 1250 times)

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Offline bugsbunnyrlzTopic starter

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Direction of electrons?
« on: May 15, 2011, 12:01:56 PM »
I'm sure this is a very stupid question, but still, I'm confused.

If electrons flow from negative to positive in a battery, why would you create a circuit like the one in the attachment?
I saw  someone build this circuit: led connected to negative side of battery, resistor connected to positive side of battery and the positive side of the led.

If electrons flow from negative to positive, then the electrons would flow through the led then the resistor, so what good is the resistor?
Any help would be appreciated.

Offline rbtying

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Re: Direction of electrons?
« Reply #1 on: May 15, 2011, 12:16:49 PM »
The current through all circuit elements in series is equal, it doesn't matter what order the elements are in. 

Offline newInRobotics

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Re: Direction of electrons?
« Reply #2 on: May 15, 2011, 01:30:45 PM »
If electrons flow from negative to positive in a battery, why would you create a circuit like the one in the attachment?
I am pretty sure that electrons flow from positive (more potential) to negative (less potential)  ;D

If electrons flow from negative to positive, then the electrons would flow through the led then the resistor, so what good is the resistor?
If You could imagine pipe with a valve, then in an electric circuit valve acts as resistor, it limits current in whole circuit rather than in the part of the circuit, same as valve, if You start closing valve water flow will be reduced in whole system rather than in a part of it. So it does not really matter if You put resistor before LED or after it  :)

Hope I was clear enough :)

Good luck :)
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Offline Soeren

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Re: Direction of electrons?
« Reply #3 on: May 15, 2011, 01:34:23 PM »
Hi,

Current runs from + to - and was determined way before it was discovered that electrons runs from - to +
But this has nothing to do with the fact that rbtying stated, which is the reason that it doesn't matter which side of the LED the resistor is on.
If you pinch a garden hose to restrict the flow, it will be restricted in both ends (and the middle) no matter where you pinch it -  a circuit loop is similar in that respect.
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Offline MikeK

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Re: Direction of electrons?
« Reply #4 on: May 15, 2011, 02:19:46 PM »
Try to forget that electrons go from the negative side to the positive side.  Conventional flow is + to - and it will save you a lot of headaches if you just go with that. :)

Offline TrickyNekro

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Re: Direction of electrons?
« Reply #5 on: May 16, 2011, 01:17:20 AM »
If electrons flow from negative to positive in a battery, why would you create a circuit like the one in the attachment?
I am pretty sure that electrons flow from positive (more potential) to negative (less potential)  ;D

And I'm pretty sure that they don't... Electrons have negative charge.... The more negative that a point is (with need of a reference to somewhere)
the more electrons there are there...

The more positive (again in respect to somewhere) the more "holes" there are.

That somewhere is usually the ground.

Holes refer to places that electrons can take in materials. And electrons are the only moving part of the system.
So electrons are flowing from negative to positive. And that's the real current flow.

Though, in electronics we have adopted the conventional current flow, and that's in fact the opposite.
So in electronics, the current flows from positive to negative.
But in Physics, the current flows from negative to positive.

It's a matter of the science you are referring to.

But most of the time the conventional current flow is used, so keep that one in mind.

Best Regards, Lefteris
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« Last Edit: May 16, 2011, 01:19:00 AM by TrickyNekro »
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Offline newInRobotics

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Re: Direction of electrons?
« Reply #6 on: May 16, 2011, 08:05:36 AM »
If electrons flow from negative to positive in a battery, why would you create a circuit like the one in the attachment?
I am pretty sure that electrons flow from positive (more potential) to negative (less potential)  ;D

And I'm pretty sure that they don't... Electrons have negative charge.... The more negative that a point is (with need of a reference to somewhere)
the more electrons there are there...

The more positive (again in respect to somewhere) the more "holes" there are.

That somewhere is usually the ground.

Holes refer to places that electrons can take in materials. And electrons are the only moving part of the system.
So electrons are flowing from negative to positive. And that's the real current flow.

Though, in electronics we have adopted the conventional current flow, and that's in fact the opposite.
So in electronics, the current flows from positive to negative.
But in Physics, the current flows from negative to positive.

It's a matter of the science you are referring to.

But most of the time the conventional current flow is used, so keep that one in mind.

Best Regards, Lefteris
Greece
I did not know that :) and now I do... thanks.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

Offline ballbreaker

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Re: Direction of electrons?
« Reply #7 on: May 16, 2011, 08:30:31 AM »
If electrons flow from negative to positive, then the electrons would flow through the led then the resistor

to make it simple the electrons only move when the positive and the negative are connected and all of them will start moving at the same time so the electrons would flow through the led and the resistor the same time because there are electrons everywhere not just on the negative side.
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Offline Billy

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Re: Direction of electrons?
« Reply #8 on: May 16, 2011, 01:14:04 PM »
all of them will start moving at the same time

As a side point not really related to the topic but something I find interesting: The individual electrons move very slowly within the wire. On the order of a cm per hour.  While the electric field will travel quickly through the insulation around the wire, actual electrons are pretty sluggish.

Offline MikeK

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Re: Direction of electrons?
« Reply #9 on: May 16, 2011, 03:34:36 PM »
Yes.  Imagine a line of ping pong balls a mile long.  You push one end and the ball you push doesn't move very far or very fast, but the change happens a mile away instantaneously.

Offline alzabo

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Re: Direction of electrons?
« Reply #10 on: May 20, 2011, 08:28:08 AM »
Just thought I'd stop in and add my 2 cents on current vs. moving electrons...
I've had to answer this type of question a lot to people interested in electronics/physics but that aren't necessarily students of it.  First let's remember what exactly "current" is.  We define current in terms of amps, or amperes. An amp, in turn, is defined as the amount of charge to pass through a given point for a given amount of time -- it's a rate not unlike gallons per minute, or miles per hour.  

In particular, amps are defined as coulombs/sec where a coulomb is the metric unit for charge.  A single proton has a charge of +1.602 x 10^(-19) Coulombs and a single electron has a charge of -1.602 x 10^(-19) Coulombs (-- very small number!  Working in terms of Coulombs makes the math much easier than working with the number of electrons or protons).
As mentioned earlier, it is in fact the electrons that move within a given electrical system (well, sort of -- when discussing transistor technology, sometimes we look specifically at "drift currents" which refer to the movement of holes, or areas of positive charge, vs electrons, areas of negative charge; but, all of that is much beyond the scope of this question).  A moving electron is, in fact, a moving negative charge.  

Just like on a number line, if you move in the negative direction, or left, negative 2 units, you are actually moving in the positive direction, or right, by positive 2 units.  Therefore, a net movement of negative charges from the negative terminal of a battery to the positive terminal (which we say is the negative direction) is just like a net movement of positive charges from the positive terminal to the negative terminal (which is the positive direction).  The important thing to remember is current refers to the net movement of charge, not necessarily the net movement of electrons.

As a side note, you may be asking yourself, "Man this is confusing, why don't we just let electrons be positive and avoid all this double negative stuff?!!"  The reason for this is that, the study of current and moving charges was well established before scientists discovered that it was actually the movement of the negatively charged parts of atoms that generated current.  Also, if you're into physics, let's note that if we did "correct" the signs on the particles, we'd have to use our left hand for all our usual "right-hand-rule" equations -- learning those was such a pain in the butt in the first place, please don't make me go back and use a different hand now!
« Last Edit: May 20, 2011, 08:30:31 AM by alzabo »

Offline TrickyNekro

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Re: Direction of electrons?
« Reply #11 on: May 20, 2011, 09:25:31 AM »
Yes.  Imagine a line of ping pong balls a mile long.  You push one end and the ball you push doesn't move very far or very fast, but the change happens a mile away instantaneously.

Hmm, nop. Not instantaneously. Although, this happens extremely fast and when I say really fast I mean within very few femtoseconds, if not
attoseconds (in ideal wire circumstances).

You see, general relativity and electromagnetism have been proved to be correct....

That's a general info though... Nothing in nature happens instantaneously. Physics is only the closest estimation of what happens.
Final word, you only count time, or anything else, when it matters to be counted.

Best Regards, Lefteris
Greece
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P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

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Offline alzabo

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Re: Direction of electrons?
« Reply #12 on: May 20, 2011, 10:49:40 AM »
Yes.  Imagine a line of ping pong balls a mile long.  You push one end and the ball you push doesn't move very far or very fast, but the change happens a mile away instantaneously.

Hmm, nop. Not instantaneously. Although, this happens extremely fast and when I say really fast I mean within very few femtoseconds, if not
attoseconds (in ideal wire circumstances).

You see, general relativity and electromagnetism have been proved to be correct....

That's a general info though... Nothing in nature happens instantaneously. Physics is only the closest estimation of what happens.
Final word, you only count time, or anything else, when it matters to be counted.

Best Regards, Lefteris
Greece
You're off by several orders of magnitude here.  If we consider a best-case scenario, i.e. a lossless coax. line, a DC wavefront will travel at the speed of light.  That means that for a mile long transmission line there will be an initial delay equal to
(3.0 x 10^8 m/s) / (2.5 m) = 1.2 x 10^8 seconds ~ 12 nanoseconds.
This isn't much, but when we consider much larger distances, like the distance a satelite TV service signal has to travel through air, the delay becomes significant.
Also, when we consider time-varying AC signals, we must consider the effects of wave reflections, interference, and the effects of standing waves whenever the length of the line is significantly greater than 1 percent of the wavelength.  For example, if we consider a 16 MHz microcontroller with ideal conditions such that the internal signal propagates at the speed of light, the effects of standing waves become significant when any part of the signal within the controller travel further than 18.75 cm.
( length > 0.01 * lambda = 0.01 * (c/16 MHz) = 0.1875 m)
Of course, such a microcontroller doesn't exist; so, for  16 MHz we don't need to consider these things.  But, what about a 2 GHz processor?
length > 0.01 * (c/2 GHz) = 1.5 mm.
Processors on the order of 1.5 mm certainly do exist and for this reason standing wave effects are taken into consideration when designing these chips. 

Offline Soeren

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Re: Direction of electrons?
« Reply #13 on: May 20, 2011, 05:30:32 PM »
Hi,

Processors on the order of 1.5 mm certainly do exist and for this reason standing wave effects are taken into consideration when designing these chips. 
PCB layouts for PC's has been using microwave techniques for a number of years for the same reason.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline alzabo

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Re: Direction of electrons?
« Reply #14 on: May 21, 2011, 06:54:50 AM »
Hi,

Processors on the order of 1.5 mm certainly do exist and for this reason standing wave effects are taken into consideration when designing these chips. 
PCB layouts for PC's has been using microwave techniques for a number of years for the same reason.

I didn't really want to get into that, but you're absolutely correct.  If the chip is on the order of 1.5 mm, the leads from the chip will more than likely also be on that order, and elements on a PCB can be on that order as well. 

 


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