# Electrical circuit

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
I really Cant understand the source of electrons in a circuit. So can help me explain it

#### MrChips

Joined Oct 2, 2009
27,113
Welcome to AAC!

Why are you concerned about the source of electrons?

Electrons are everywhere. Every atom and molecule has electrons that are in constant motion.
In the quantum theory of electrons, an electron is a probability cloud. In other words there is a given probability that an electron exists in a given place and time and we cannot pinpoint where it is at any given moment.

#### crutschow

Joined Mar 14, 2008
30,766
All atoms have electrons around the nucleus.
In insulators the electrons are tightly bound and cannot move, so no current can flow.
In conductors the electrons can move from atom to atom freely so that's the source of the electrons in a circuit.
So there are no additional electrons in a circuit, it's just that the ones already there in a conductor are free to move.
Applying an EMF (voltage) between the ends of a conductor causes the electrons to move from the negative side to the positive side, which is called an electrical (electron) current.

To confuse things, conventional current flow is defined as flowing from positive to negative.
It makes no difference which you assume for most circuit analysis (notable exceptions are the inner working of vacuum tubes and semiconductors), as long as your are consistent in all your calculations.
I prefer conventional current flow, since it goes in the direction of the arrow on diodes and transistors, and from top to bottom on most schematics which have the positive supply voltage rail at the top of the page.

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
All atoms have electrons around the nucleus.
In insulators the electrons are tightly bound and cannot move, so no current can flow.
In conductors the electrons can move from atom to atom freely so that's the source of the electrons in a circuit.
So there are no additional electrons in a circuit, it's just that the ones already there in a conductor are free to move.
Applying an EMF (voltage) between the ends of a conductor causes the electrons to move from the negative side to the positive side, which is called an electrical (electron) current.

To confuse things, conventional current flow is defined as flowing from positive to negative.
It makes no difference which you assume for most circuit analysis (notable exceptions are the inner working of vacuum tubes and semiconductors), as long as your are consistent in all your calculations.
I prefer conventional current flow, since it goes in the direction of the arrow on diodes and transistors, and from top to bottom on most schematics which have the positive supply voltage rail at the top of the page.
Thank sir. I am just a student. So I can know it. But in the below picture. Source of electrons that means the point at positive of source right sir?

#### WBahn

Joined Mar 31, 2012
27,389
If you mean "source of electrons" in the sense of what is the "source" in the phrase "electrons from a voltage or current source" in the above article, then all that is referring to is something that that provides the energy to move electrons around the circuit. So a battery is a voltage source, for instance. So is the wall outlet in a house (with the actual "source" being the electric generator at the power station). The alternator in your car is a source, as is a solar panel.

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
If you mean "source of electrons" in the sense of what is the "source" in the phrase "electrons from a voltage or current source" in the above article, then all that is referring to is something that that provides the energy to move electrons around the circuit. So a battery is a voltage source, for instance. So is the wall outlet in a house (with the actual "source" being the electric generator at the power station). The alternator in your car is a source, as is a solar panel.
I know but you can see the picture .

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
If you mean "source of electrons" in the sense of what is the "source" in the phrase "electrons from a voltage or current source" in the above article, then all that is referring to is something that that provides the energy to move electrons around the circuit. So a battery is a voltage source, for instance. So is the wall outlet in a house (with the actual "source" being the electric generator at the power station). The alternator in your car is a source, as is a solar panel.
The point where electrons enter the circuit. I don't know that you can simu for me.

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
If you mean "source of electrons" in the sense of what is the "source" in the phrase "electrons from a voltage or current source" in the above article, then all that is referring to is something that that provides the energy to move electrons around the circuit. So a battery is a voltage source, for instance. So is the wall outlet in a house (with the actual "source" being the electric generator at the power station). The alternator in your car is a source, as is a solar panel.
That means where are electrons from? Just from source but I don't know that is right

#### bidrohini

Joined Jul 29, 2022
93
This video may help a bit :

#### MrSalts

Joined Apr 2, 2020
2,350
That means where are electrons from? Just from source but I don't know that is right
A circuit typically has a voltage source that is an electrochemical source (battery) or from an electro(magneto)mechanical source (generator).
In a battery, there are two parts (an oxidizing agent and a reducing agent. These are connected by a "salt bridge", a membrane or simply packed/stacked in layers to keep them apart to some degree ( depending on the physical form of oxidizer and reducing agent). These electrochemical cells form a natural voltage potential (electromotive force) that you can measure with a volt meter. In an ideal volt meter, no current flows, you would, in theory (idea conditions) just measure the voltage potential. IF and WHEN you connect any conductive materials from the positive side if the battery to the negative side, the electromotive force pushes "current" through the circuit.
Background
There is some historical strangeness in discussing what "current" actually is. There must be space for each electron in the quantum mechanical "cloud" in the fermi band ( conductive band) of any material that is expected to conduct and the first scientists looking at this assumed the charge on the flowing material was positive. It was later discovered that electrons(with negative change) flow in a circuit. Therefore, two possible measurements exist. One called "electron flow" that physicists like to think about, and traditional "current flow" that nearly every electrical engineer cares about and every volt meter is set up to measure. The current flow is essentially measuring the apparent "flow" of unoccupied "holes" created as electrons are pushed in the opposite direction by the electrochemical or electromagnetomechanical voltage.
End background

Now back to your question.
When an electron leaves the cathode of the battery and flow to the anode in an "electron flow" view of the circuit (like a physicist would look at it since you asked about electrons), this event would seem to cause the battery to become positively charged on the anode side and negatively charged at the cathode after the circuit is connected. This does not happen because other secondary chemical reactions occur to keep both sides of the battery neutrally charged overall. Look up current flow in a lead-acid battery, for a well-documented system with a liquid/solid electrochemical cell.
For a generator, look up Lenz's Law that shows how current flow is induced when a conductor (best viewed as a loop of conductive wire) is pushed (into/through) a magnetic field. The flow simply flows in a circle through the circuit.

#### MrChips

Joined Oct 2, 2009
27,113
Electrons do not enter a circuit. They are already in the circuit.

Using a common analogy of electron flow, imagine a closed loop of tubing filled with marbles. The loop is closed, hence the term "circuit".

When an electric current flows, all the marbles have to move at the same time. Energy is transferred by the movement of the electrons. The electrons do not move very far nor do they move very fast.

In DC, a typical drift velocity of electrons is about 0.02 mm/s.
For 60Hz AC, electrons don't go anywhere. A typical maximum displacement is about 0.0002 mm.

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
A circuit typically has a voltage source that is an electrochemical source (battery) or from an electro(magneto)mechanical source (generator).
In a battery, there are two parts (an oxidizing agent and a reducing agent. These are connected by a "salt bridge", a membrane or simply packed/stacked in layers to keep them apart to some degree ( depending on the physical form of oxidizer and reducing agent). These electrochemical cells form a natural voltage potential (electromotive force) that you can measure with a volt meter. In an ideal volt meter, no current flows, you would, in theory (idea conditions) just measure the voltage potential. IF and WHEN you connect any conductive materials from the positive side if the battery to the negative side, the electromotive force pushes "current" through the circuit.
Background
There is some historical strangeness in discussing what "current" actually is. There must be space for each electron in the quantum mechanical "cloud" in the fermi band ( conductive band) of any material that is expected to conduct and the first scientists looking at this assumed the charge on the flowing material was positive. It was later discovered that electrons(with negative change) flow in a circuit. Therefore, two possible measurements exist. One called "electron flow" that physicists like to think about, and traditional "current flow" that nearly every electrical engineer cares about and every volt meter is set up to measure. The current flow is essentially measuring the apparent "flow" of unoccupied "holes" created as electrons are pushed in the opposite direction by the electrochemical or electromagnetomechanical voltage.
End background

Now back to your question.
When an electron leaves the cathode of the battery and flow to the anode in an "electron flow" view of the circuit (like a physicist would look at it since you asked about electrons), this event would seem to cause the battery to become positively charged on the anode side and negatively charged at the cathode after the circuit is connected. This does not happen because other secondary chemical reactions occur to keep both sides of the battery neutrally charged overall. Look up current flow in a lead-acid battery, for a well-documented system with a liquid/solid electrochemical cell.
For a generator, look up Lenz's Law that shows how current flow is induced when a conductor (best viewed as a loop of conductive wire) is pushed (into/through) a magnetic field. The flow simply flows in a circle through the circuit.
Detail sir. Thank so much

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
Electrons do not enter a circuit. They are already in the circuit.

Using a common analogy of electron flow, imagine a closed loop of tubing filled with marbles. The loop is closed, hence the term "circuit".

View attachment 275541
When an electric current flows, all the marbles have to move at the same time. Energy is transferred by the movement of the electrons. The electrons do not move very far nor do they move very fast.

In DC, a typical drift velocity of electrons is about 0.02 mm/s.
For 60Hz AC, electrons don't go anywhere. A typical maximum displacement is about 0.0002 mm.
It's easy to imagine with this explanation. So we can choose any point where the electron starts moving right, sir?

#### MrChips

Joined Oct 2, 2009
27,113
It's easy to imagine with this explanation. So we can choose any point where the electron starts moving right, sir?
No. All electrons start moving at the same time.
Think of a closed loop. The whole loop moves as one unit.

#### MrAl

Joined Jun 17, 2014
9,533
I really Cant understand the source of electrons in a circuit. So can help me explain it
Hello there,

This gets pretty complicated especially since we always use analogies throughout the entire act of analyzing a circuit. That means we hardly ever use any definitions that actually deal directly with electrons. It's interesting to think about what happens here but to completely understand it or at least to almost completely understand it you have to wait for future courses in physics because it's not just the electrons themselves that work to make current flow it is also the electromagnetic field. So for now you have to and actually should be content with using an analogy.

The analogy is to think of electrons as tiny balls that migrate though the wire, and the are the cause of the current flow. The tiny balls are already in the wire before you apply any voltage. The voltage pushes the tiny balls though the wire almost like a pump pumps water though a pipe, except the circuit almost always has a definite return path. So it's like a pipe that runs around in a circle and come back and connects back to itself, with the start end connecting to the finish end. The pump pumps the tiny balls around this circular path.

That's not the end of it though unfortunately because we also have the 'charge' analogy. This is where we think of the electrons as being very small points of charge. That means when we say we are charging a battery it seems as though we are pumping electrons into the battery and they are somehow staying there filling up the battery with 'charge'. That's not the way it happens, but it is also convenient to think of it that way in many cases.

I'll leave it at that for now, except to say that in light of the above a better question would be to ask "What makes electrons move in a wire". The electrons are always there, they just start to move when a power source is applied.
The actual reason they move is because the electromagnetic field pushes them but we can get more into that at a later time.

With all this in mind we usually divide problems up into two main groups. The first is circuit analysis, and the second is pure physics. In physics we ask questions about things like the fields, but in circuit analysis we usually just ask about the components and what is happening with the current often without regard for what the field is actually doing.
Regular circuit analysis is much simpler because we assume that the components are 'ideal' in that their behavior is well defined without fields and the components have very restricted physical definitions called "models".

One more little thing. The terms "electrical" and "electronic" are not as well defined as the Wikipedia article suggests. For some components like diodes it can usually be called electronic or electrical. This would actually be the case with resistors also, and voltage sources and the like. It may be that transistors belong to the electronic class, but i dont think we should burn these class definitions too deep into those components either.

Good luck

Thread Starter

#### nguyengiathinh

Joined Jul 4, 2022
14
Hello there,

This gets pretty complicated especially since we always use analogies throughout the entire act of analyzing a circuit. That means we hardly ever use any definitions that actually deal directly with electrons. It's interesting to think about what happens here but to completely understand it or at least to almost completely understand it you have to wait for future courses in physics because it's not just the electrons themselves that work to make current flow it is also the electromagnetic field. So for now you have to and actually should be content with using an analogy.

The analogy is to think of electrons as tiny balls that migrate though the wire, and the are the cause of the current flow. The tiny balls are already in the wire before you apply any voltage. The voltage pushes the tiny balls though the wire almost like a pump pumps water though a pipe, except the circuit almost always has a definite return path. So it's like a pipe that runs around in a circle and come back and connects back to itself, with the start end connecting to the finish end. The pump pumps the tiny balls around this circular path.

That's not the end of it though unfortunately because we also have the 'charge' analogy. This is where we think of the electrons as being very small points of charge. That means when we say we are charging a battery it seems as though we are pumping electrons into the battery and they are somehow staying there filling up the battery with 'charge'. That's not the way it happens, but it is also convenient to think of it that way in many cases.

I'll leave it at that for now, except to say that in light of the above a better question would be to ask "What makes electrons move in a wire". The electrons are always there, they just start to move when a power source is applied.
The actual reason they move is because the electromagnetic field pushes them but we can get more into that at a later time.

With all this in mind we usually divide problems up into two main groups. The first is circuit analysis, and the second is pure physics. In physics we ask questions about things like the fields, but in circuit analysis we usually just ask about the components and what is happening with the current often without regard for what the field is actually doing.
Regular circuit analysis is much simpler because we assume that the components are 'ideal' in that their behavior is well defined without fields and the components have very restricted physical definitions called "models".

One more little thing. The terms "electrical" and "electronic" are not as well defined as the Wikipedia article suggests. For some components like diodes it can usually be called electronic or electrical. This would actually be the case with resistors also, and voltage sources and the like. It may be that transistors belong to the electronic class, but i dont think we should burn these class definitions too deep into those components either.

Good luck
amazing sir. in my school. Mostly I will get exams to solve with numbers or formulas circuits. No nature sir. Rigidly

#### dcbingaman

Joined Jun 30, 2021
720
Welcome to AAC!

Why are you concerned about the source of electrons?

Electrons are everywhere. Every atom and molecule has electrons that are in constant motion.
In the quantum theory of electrons, an electron is a probability cloud. In other words there is a given probability that an electron exists in a given place and time and we cannot pinpoint where it is at any given moment.
An important distinction:
An 'electron' is not actually at any given location before we look at it. The act of looking fixes its position but technically it was not at that location before we looked for it. The double slit experiment with electrons verifies this. It is a wave of probability before we looked at it. But if we know what slit it went through it acts again like a particle. It is not that we 'interfered' with it that caused it to take on a given position but merely the knowledge of which slit it went through. This is backed up by the delayed choice quantum erasure experiment. It does not matter how you acquire that knowledge, the act of knowing influences the system.

#### WBahn

Joined Mar 31, 2012
27,389
I'm pretty sure that quantum mechanics and statistical probability densities and the uncertainty principle are well beyond the scope of what the TS is trying to understand.