# When electrons that are moving through a wire come to a "crossroads," do they divide or choose?

#### Dolmetscher007

Joined Mar 21, 2019
36
I have been reading an excellent book on "Teach Yourself Electronics," and it really is amazing! It finally answers a lot of those granular details about physics that I've been missing. My brain is so detail oriented, however, that I end up getting really bogged down in very pedantic detail. If I do not 110% understand something, I have the hardest time just "moving on" and hoping to figure it out later. It is a personality trait that I've battled my entire life, but one that severs me quite well in the end; once I know something... I really know it!

Can someone help me understand the following conceptual problem I'm having?

If you are designing a DC power supply or even just a basic lightbulb circuit... let's say you have a transformer that takes in the 120V AC from the wall, and reduces that voltage down to 18V AC (just as an example), and then the 18V AC needs to go through rectification to convert it to DC. When the electrons that are coming through the wire from the secondary windings of the transformer arrive at where the wire is connected to the rectifier diodes, how do the electrons know which way to flow? I've included a schematic that I've drawn up to illustrate what I'm talking about.

IMG URL: https://dolmetscher007.imgbb.com/

I know that electrons can only flow in one direction through a diode, which is why they are used for converting AC into DC, but even if we removed the diodes from the equation, and just replaced them with 1k resistors... do the electrons divide up like a line of ants that fork into two lines of ants? If so, do they fork evenly, or according to something placed further on down the line? Or do they not fork at all, and they always flow in whatever direction poses the least resistance? I know that there is a problem in that electrons flow from negative to positive, but convention has us writing it the other way around. But... for my current question, I don't think it matters which way the electrons are actually flowing when electrons reach a "crossroads" how do they know which way to go? Is the secret to understanding it, that electrons are not being "pushed" but that they are being "pulled", so the "path" they take is dictated by the negative terminal of the... what?... the power source? Ground?

Basically... I am trying to "read" schematics, but I seem to be missing something very fundamental about electron flow.

#### ElectricSpidey

Joined Dec 2, 2017
2,881
The electrons take and follow their paths in accordance with Ohm’s law.

#### nsaspook

Joined Aug 27, 2009
13,552
I have been reading an excellent book on "Teach Yourself Electronics," and it really is amazing! It finally answers a lot of those granular details about physics that I've been missing. My brain is so detail oriented, however, that I end up getting really bogged down in very pedantic detail. If I do not 110% understand something, I have the hardest time just "moving on" and hoping to figure it out later. It is a personality trait that I've battled my entire life, but one that severs me quite well in the end; once I know something... I really know it!

Can someone help me understand the following conceptual problem I'm having?

If you are designing a DC power supply or even just a basic lightbulb circuit... let's say you have a transformer that takes in the 120V AC from the wall, and reduces that voltage down to 18V AC (just as an example), and then the 18V AC needs to go through rectification to convert it to DC. When the electrons that are coming through the wire from the secondary windings of the transformer arrive at where the wire is connected to the rectifier diodes, how do the electrons know which way to flow? I've included a schematic that I've drawn up to illustrate what I'm talking about.

IMG URL: https://dolmetscher007.imgbb.com/

I know that electrons can only flow in one direction through a diode, which is why they are used for converting AC into DC, but even if we removed the diodes from the equation, and just replaced them with 1k resistors... do the electrons divide up like a line of ants that fork into two lines of ants? If so, do they fork evenly, or according to something placed further on down the line? Or do they not fork at all, and they always flow in whatever direction poses the least resistance? I know that there is a problem in that electrons flow from negative to positive, but convention has us writing it the other way around. But... for my current question, I don't think it matters which way the electrons are actually flowing when electrons reach a "crossroads" how do they know which way to go? Is the secret to understanding it, that electrons are not being "pushed" but that they are being "pulled", so the "path" they take is dictated by the negative terminal of the... what?... the power source? Ground?

Basically... I am trying to "read" schematics, but I seem to be missing something very fundamental about electron flow.
Letting it go and moving on is IMO the best thing to do for now.

Why do you think that electron particle movements in circuits are important at this point in the learning cycle? Yes, electrons are important as a system to move electrical energy but that's a different discussion about fields. Don't think about electron flow or electrons at the particle detail level because the electron model used in most circuit theory texts are so simplified as to be useless for explaining detailed electron behavior. The quickest way to understand 'read' schematics is the see them as energy flow diagrams where the components control electrical energy flows. Don't think about current loops as energy carriers, think about energy flow directions with single pointers from potential energy sources to energy sinks moving across the page as the circuit dictates.

#### WBahn

Joined Mar 31, 2012
30,295
I have been reading an excellent book on "Teach Yourself Electronics," and it really is amazing! It finally answers a lot of those granular details about physics that I've been missing. My brain is so detail oriented, however, that I end up getting really bogged down in very pedantic detail. If I do not 110% understand something, I have the hardest time just "moving on" and hoping to figure it out later. It is a personality trait that I've battled my entire life, but one that severs me quite well in the end; once I know something... I really know it!

Can someone help me understand the following conceptual problem I'm having?

If you are designing a DC power supply or even just a basic lightbulb circuit... let's say you have a transformer that takes in the 120V AC from the wall, and reduces that voltage down to 18V AC (just as an example), and then the 18V AC needs to go through rectification to convert it to DC. When the electrons that are coming through the wire from the secondary windings of the transformer arrive at where the wire is connected to the rectifier diodes, how do the electrons know which way to flow? I've included a schematic that I've drawn up to illustrate what I'm talking about.

IMG URL: https://dolmetscher007.imgbb.com/

I know that electrons can only flow in one direction through a diode, which is why they are used for converting AC into DC, but even if we removed the diodes from the equation, and just replaced them with 1k resistors... do the electrons divide up like a line of ants that fork into two lines of ants? If so, do they fork evenly, or according to something placed further on down the line? Or do they not fork at all, and they always flow in whatever direction poses the least resistance? I know that there is a problem in that electrons flow from negative to positive, but convention has us writing it the other way around. But... for my current question, I don't think it matters which way the electrons are actually flowing when electrons reach a "crossroads" how do they know which way to go? Is the secret to understanding it, that electrons are not being "pushed" but that they are being "pulled", so the "path" they take is dictated by the negative terminal of the... what?... the power source? Ground?

Basically... I am trying to "read" schematics, but I seem to be missing something very fundamental about electron flow.
Like any particle, an electron moves in the direction of the net force acting it. There are many forces, but the dominant one in this case is the electric force which is due to the interaction of a particle's charge with the electric field at that point. Voltage is nothing more than a measure of the net electric field in a region of space in that a voltage difference between two points tells you the average strength and direction of the electric field between those two points.

So now let's consider two paths available for electrons to flow -- say through two resistors for simplicity. As electrons flow through the resistor, the voltage across the resistor changes (Ohm's Law). As the voltages across the resistors change, the electric fields at the junction of the two resistors change. In essence, if "too many" electrons go down one of the resistors, the voltage drop across it will increase such that the electric field will shift so as to favor electrons going to the other resistor. This is a dynamic balancing act that happens so fast that, for most purposes, we can't perceive it and treat it as simply happening instantaneously. The end result is that the current is shared by all possible paths such at the voltage at the junction is the same regardless of which path is considered (this is assuming that the electric fields involved are conservative, which is not always the case).

#### bogosort

Joined Sep 24, 2011
696
Though sometimes it may be helpful to think of electric current as the flow of tiny billiard balls, it's not a physically accurate model. Through increasingly sophisticated experiments, physicists of the 20th century realized that an electron cannot be a traditional particle, i.e., what we'd think of as a scaled down version of a cannonball. In the most accurate model to date, the electron is treated as an excitation in a quantum field (like the photon to the electromagnetic field). So, if you want to try to reason cogently about circuits at the electron level -- and there's really no practical reason to do so -- you'll need to stop thinking about electrons as tiny balls.

I gather, however, that you're more interested in the (macro) electrical behavior than the (fundamental) physics behind it. If so, I'd recommend you zoom out several orders of magnitude and treat electric current as an aggregate statistical phenomenon, much like you'd ignore the behavior of each individual water molecule when considering the dynamics of water. But if you're really interested at what's happening at the electron level, start learning about quantum mechanics (though it won't help your electronics intuition).

#### Papabravo

Joined Feb 24, 2006
21,307
Electrons respond to the forces on them, but classical (Newtonian) mechanics cannot account for all of the observed behavior. The laws of probability and quantum mechanics can give insight into possible behaviors but cannot predict how an individual particle (wave) will move. There are devices called tunnel diodes that exhibit this behavior. The same sort of effect is observed with photons in a dual slit experiment, which suggests that photons exhibit both particle and wave behavior.

https://en.wikipedia.org/wiki/Double-slit_experiment

This experiment speaks directly to the phrasing of your question.

#### BobaMosfet

Joined Jul 1, 2009
2,123
Letting it go and moving on is IMO the best thing to do for now.

Why do you think that electron particle movements in circuits are important at this point in the learning cycle? Yes, electrons are important as a system to move electrical energy but that's a different discussion about fields. Don't think about electron flow or electrons at the particle detail level because the electron model used in most circuit theory texts are so simplified as to be useless for explaining detailed electron behavior. The quickest way to understand 'read' schematics is the see them as energy flow diagrams where the components control electrical energy flows. Don't think about current loops as energy carriers, think about energy flow directions with single pointers from potential energy sources to energy sinks moving across the page as the circuit dictates.
Because it is important. If most of the people in electronics concentrated on understanding why electrons move the way they do, they would understand deeper meanings in KVL and KCL (that 99.9% of people in electronics today don't even know a clue about)- and ultimately because if you reduce everything to electronic flow, electronics becomes incredibly simple.

How electronics is currently taught is it's greatest impediment to how it actually works.

#### nsaspook

Joined Aug 27, 2009
13,552
How electronics is currently taught is it's greatest impediment to how it actually works.
That is true and talking about electrons won't improve it. That's really needed is 'field theory' lite where we understand the SYSTEM of electrical energy transportation in circuits using charge as the wave-guide for a closer abstraction to the physics of electrical science. The electron fixation is mainly counter-productive because it hides the limitations of KVL and KCL outside of closed circuits in explaining electromagnetic phenomena. KCL and KVL are special cases of Ampere's circuital law and Faraday's laws of induction. So if you have varying E or H field you can't use KCL and KVL where electric fields could be induced and emf could be produced, in which case Kirchhoff’s loop rule breaks down.

The word electron is used once.

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#### BobTPH

Joined Jun 5, 2013
9,265
I hate the water analogy, but in this case it might help.

You hook up a pump to a pipe and the pump reaches a junction that goes 3 ways.

1. A 1 inch diameter pipe.

2. An eighth inch diameter pipe.

3. A pipe that ends with a valve that allows flow only toward the junction.

You observe a strong flow of water out of the one inch pipe, a trickle out of the eigth inch pipe, and nothing coming put of the pipe with the valve.

Do you obsess over how the water molecules “know” which way to go?

Bob

#### BobaMosfet

Joined Jul 1, 2009
2,123
That is true and talking about electrons won't improve it. That's really needed is 'field theory' lite where we understand the SYSTEM of electrical energy transportation in circuits using charge as the wave-guide for a closer abstraction to the physics of electrical science. The electron fixation is mainly counter-productive because it hides the limitations of KVL and KCL outside of closed circuits in explaining electromagnetic phenomena. KCL and KVL are special cases of Ampere's circuital law and Faraday's laws of induction. So if you have varying E or H field you can't use KCL and KVL where electric fields could be induced and emf could be produced, in which case Kirchhoff’s loop rule breaks down.

The word electron is used once.
The above isn't proving a breakdown in Kirchoff's Laws, but is rather demonstrating a pure lack of understanding of what KCL and KVL actually mean as well as the restatement by Thevenin with his theorem (which should actually be a law because it is essentially corroborated by Kirchoff). Consider this your matrix moment- either you will bend around the spoon or it will bend around you.

Just remember, there is only the electron, and that which impedes it's movement. Grasp this grashopper, and you will finally understand Kirchoff.

#### crutschow

Joined Mar 14, 2008
34,841
Electrons don't "know" anything.
Don't complicate things.
They go where Ohm's law and the voltage drop dictates.
If you know the voltage drop (voltage difference between two nodes) and the resistance between those two nodes, then you know where and how much current flows.

#### mvas

Joined Jun 19, 2017
539
From our large scale perspective all electrons are identical.
Electrons are a Unit Electrical Charge Carrier.
Electrons are repelled by the (more) negative source and are attracted to the (more) positive source.
We can measure the flow rate ( ie velocity ) of the electrons, as they pass by any given point.
When we measure a flow rate of 6.24 x 10^18 electrons per second, we call that 1 ampere.
All the electrons flowing into, and out of, a junction follow Kirchoff's Law ( KCL ) ...
"... The algebraic sum of all currents entering and exiting a node must equal zero ...”
The electron flow will divide, or combine, based upon the relative Voltage and Resistance in each branch circuit.

Kirchoff's Law ...
https://www.electronics-tutorials.ws/dccircuits/kirchhoffs-current-law.html

Electrons have potential energy, where the Voltage is the electromotive force between the (more) positive and (more) negative points
Resistors "restrict" the flow of electrons through each branch of the circuit.
Voltage and Resistance are typically the independent variables.
Current, the flow rate in Amps, is typically the dependent variable, where AMPS = VOLTS / OHMS.

#### cmartinez

Joined Jan 17, 2007
8,285
Your question, although a valid one, is akin to asking where each water molecule goes when the flow of water meets a "Y" in a network of pipes ... a horribly detailed physics simulation is needed to answer such a thing, and in the end it would mostly be a waste of time ... unless you're trying to dive deep into the quantum realm, which again is a waste of resources if what you want is to learn real-world electronics.

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#### nsaspook

Joined Aug 27, 2009
13,552
The above isn't proving a breakdown in Kirchoff's Laws, but is rather demonstrating a pure lack of understanding of what KCL and KVL actually mean as well as the restatement by Thevenin with his theorem (which should actually be a law because it is essentially corroborated by Kirchoff). Consider this your matrix moment- either you will bend around the spoon or it will bend around you.

Just remember, there is only the electron, and that which impedes it's movement. Grasp this grashopper, and you will finally understand Kirchoff.

This grasshopper learned fundamental EM theory in the 70's and redesigns RF acceleration chambers for fun today. The popular electron fixation is a pox on learning electrical science beyond circuit theory (point connections and infinite signal propagation) where Kirchoff's Laws are useful for a true grasshopper. Yes, it's important to know about electrons in the abstract but it's the property of charge and electromagnetic forces that matter, not the particle.

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#### Dolmetscher007

Joined Mar 21, 2019
36
Okay... I can definitely switch my brain off on the whole "electron flow" part. But no matter what you call it... if you have a transformer taking 120 V AC and converting it down to 18V AC @300mA, when "the electricity" coming down the wire that comes out of the secondary winding of the transformer, and it hits a new wire that has been connected to it perpendicularly, the electricity has two directions it can now go; let's call it left or right. To the left along the newly connected wire is a 1k resistor. To the right is a 10k resistor. Knowing that electricity always takes the path of least resistance, does 100% of the electricity go to the left, through the smaller 1k resistor, or does some go left, and some go right through the 10k resistor?

If you use a water and pipe analogy... a 3 inch diam. pipe connects with a T-connector to another 3 inch pipe. To the left the 3 inch pipe constricts down to a 2-inch pip for 6 inches or so. To the right, the pipe chokes down to 1/2 in pip[e for 6 inches. MORE water will head left through the pipe of less resistance... but SOME of the water will go through the 1/2 inch constrictive pipe.

Is electricity the same in a schematic diagram, or does the water analogy break down?

#### BobaMosfet

Joined Jul 1, 2009
2,123
@Dolmetscher007
Electrons flow in every direction away from the source point in proportion to the resistance/impedance along the way. They don't choose one or the other, they take all. This is a little lesson hard-learned by people who died thinking it only took the path of least resistance.

One of the reasons I know this stuff better than most is I work in bleeding edge technology where current is commonly run through people's bodies in various ways. We can't get wrong. Ever.

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#### djsfantasi

Joined Apr 11, 2010
9,186
Yes.

Try some calculations. The transformer and rectifier actually have little to do with your problem, so I’m going to ignore them.

Start with a basic fact. Current in a series circuit (one wire with two resistors) remains the same for all components through the path. Voltage however, is divided amongst the components in the path. Compare that to a parallel circuit. The wires split at a junction into two paths, each with a resistor. Voltage is the same in both paths. Current divides between the two paths. The latter situation is what you’re inquiring about.

So knowing this, in your example how do we calculate the current in each path?

That’s where Ohms law comes into play.

In the first path, the one with the 1K resistor, voltage is 18V. Applying Ohms Law, we can calculate the current with I=V/R. Hence the current is 0.018A or 18mA.

Now the second path, where the resistor is 10K. Voltage is still 18V. Current is 18/10,000 or 0.0018A or 1.8mA.

We can see that the current in the path of the 1K ohm is 10 times greater than the other path.

#### BobaMosfet

Joined Jul 1, 2009
2,123

This grasshopper learned fundamental EM theory in the 70's and redesigns RF acceleration chambers for fun today. The popular electron fixation is a pox on learning electrical science beyond circuit theory (point connections and infinite signal propagation) where Kirchoff's Laws are useful for a true grasshopper. Yes, it's important to know about electrons in the abstract but it's the property of charge and electromagnetic forces that matter, not the particle.

That's OK. Maybe one day you'll understand. Right now, you're just regurgitating what you've been told. This is how it is with most people in electronics. If you backed up 20 or 30 years before the 70's, you'd find they taught electronics by explaining electron flow before the popular models you rely on were developed.

#### nsaspook

Joined Aug 27, 2009
13,552
Okay... I can definitely switch my brain off on the whole "electron flow" part. But no matter what you call it... if you have a transformer taking 120 V AC and converting it down to 18V AC @300mA, when "the electricity" coming down the wire that comes out of the secondary winding of the transformer, and it hits a new wire that has been connected to it perpendicularly, the electricity has two directions it can now go; let's call it left or right. To the left along the newly connected wire is a 1k resistor. To the right is a 10k resistor. Knowing that electricity always takes the path of least resistance, does 100% of the electricity go to the left, through the smaller 1k resistor, or does some go left, and some go right through the 10k resistor?

If you use a water and pipe analogy... a 3 inch diam. pipe connects with a T-connector to another 3 inch pipe. To the left the 3 inch pipe constricts down to a 2-inch pip for 6 inches or so. To the right, the pipe chokes down to 1/2 in pip[e for 6 inches. MORE water will head left through the pipe of less resistance... but SOME of the water will go through the 1/2 inch constrictive pipe.

Is electricity the same in a schematic diagram, or does the water analogy break down?
If you have an AC current 'the electricity' (in physics this is the movement of charge, not a form of energy) goes nowhere averaged over time. It just vibrates over a very short distance in the conductor. The energy of the circuit does down each path (left right in your example) unidirectional from potential source to load down both sides of the circuit loop.
http://amasci.com/miscon/energ1.html

http://amasci.com/elect/poynt/poynt.html

#### WBahn

Joined Mar 31, 2012
30,295
Okay... I can definitely switch my brain off on the whole "electron flow" part. But no matter what you call it... if you have a transformer taking 120 V AC and converting it down to 18V AC @300mA, when "the electricity" coming down the wire that comes out of the secondary winding of the transformer, and it hits a new wire that has been connected to it perpendicularly, the electricity has two directions it can now go; let's call it left or right. To the left along the newly connected wire is a 1k resistor. To the right is a 10k resistor. Knowing that electricity always takes the path of least resistance, does 100% of the electricity go to the left, through the smaller 1k resistor, or does some go left, and some go right through the 10k resistor?

If you use a water and pipe analogy... a 3 inch diam. pipe connects with a T-connector to another 3 inch pipe. To the left the 3 inch pipe constricts down to a 2-inch pip for 6 inches or so. To the right, the pipe chokes down to 1/2 in pip[e for 6 inches. MORE water will head left through the pipe of less resistance... but SOME of the water will go through the 1/2 inch constrictive pipe.

Is electricity the same in a schematic diagram, or does the water analogy break down?