Strange questions about electrical circuits

[Guidestone]

The soldier analogy is really out there in my book. Think of a funnel. Fill it with water and watch what happens. Are the water molecules 'looking ahead' as they have to be going slower at the top of the funnel than the bottom? The problem I have with the 'look ahead' approach is that it implies a faster than light signaling scheme. If you are in a crowded hallway that has a lot of twists and turns so it is impossible to see that up ahead there is a narrow door, no one needs to 'look ahead'. The flow in the hallway just slows down naturally. Yep, there are a lot of analogies in this post.
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Incidentally, electrons don't generally march single file, 2, 4, 6, 8, etc. wide. It is more of a mushy fluidish almost random looking flow along the conductor. Lots of bumping around. Look at the screen of an analog TV not tuned to a station. Often it is referred to as 'the ant race'. Lots of random motion. I would imagine this would be similar to electrons in a conductor.

Well, I made the whole soldier analogy up, I didn't find it on any book, nor I believe electrons actually organize themselves in rows and look ahead for doors in the way; I just imagined it to describe what I'm understanding about current flow, what it seems to be to me. The issue here is the before and the after. Electrons before the resistor and electrons after the resistor. The people that go towards the door should be more numerous than the people coming out of it at the other side, but it just seems the number is always the Same before and after the door and this is already determined only by the presence of the door.

 

[Averagesupernova]

Concentrate right at the door. Are there more people going in than out the other side? I don't think so. Any reason to believe that more people are heading towards the door than actually go into it? Again, don't think so. There are no more free electrons in a copper wire before the resistor than there are after the resistor.

 

[Nidum]

An ideal battery has constant terminal voltage whatever current is drawn from it . Oversimplified I know but say it could supply any value of current from zero to very high without changing the terminal voltage .

There is no resistance in your ideal wires so the only current controlling device in the entire circuit is your actual resistor . Hence the entire battery terminal voltage is seen across the resistor . The current flowing through the resistor is set by Ohms Law . Hence current flowing through entire circuit is determined and circuit is in equilibrium .

This condition of equilibrium does not get set up absolutely instantaneously in a real circuit . For a very brief initial period current rises from nothing up to the equilibrium value . In your simple circuit settling time is probably less than a microsecond but in more complicated circuits with reactive components it can be much longer .

The principle of a settling down period before reaching equilibrium applies to all systems . Sometimes called the rise time .

Not all systems are stable and they may oscillate for a period or forever after switch on .

 

[davenn]

Well, I made the whole soldier analogy up, I didn't find it on any book, nor I believe electrons actually organize themselves in rows and look ahead for doors in the way; I just imagined it to describe what I'm understanding about current flow, what it seems to be to me.

this is going to lead you to a heap of misunderstanding

electron movement in a conductor is random in all directions. With an applied potential difference (voltage) there is still a randomness of movement, but with an underlying movement in one direction ... electron drift

Dave

 

[Guidestone]

Concentrate right at the door. Are there more people going in than out the other side? I don't think so. Any reason to believe that more people are heading towards the door than actually go into it? Again, don't think so. There are no more free electrons in a copper wire before the resistor than there are after the resistor.

Yes, you're right. I expressed myself wrong. What I meant is the amount of people passing by per cross section, like if you placed some kind of detectors in some part of the hallway both before and after the door, I think we would notice that before the door the detector would count more people by section than after it, but the total amount of people doesn't change at all, I agree with you.

An ideal battery has constant terminal voltage whatever current is drawn from it . Oversimplified I know but say it could supply any value of current from zero to very high without changing the terminal voltage .

There is no resistance in your ideal wires so the only current controlling device in the entire circuit is your actual resistor . Hence the entire battery terminal voltage is seen across the resistor . The current flowing through the resistor is set by Ohms Law . Hence current flowing through entire circuit is determined and circuit is in equilibrium .

This condition of equilibrium does not get set up absolutely instantaneously in a real circuit . For a very brief initial period current rises from nothing up to the equilibrium value . In your simple circuit settling time is probably less than a microsecond but in more complicated circuits with reactive components it can be much longer .

The principle of a settling down period before reaching equilibrium applies to all systems . Sometimes called the rise time .

Not all systems are stable and they may oscillate for a period or forever after switch on .

Ok, you're nailing it. Equilibrium condition, never heard of that in circuits theory. So at first current rises and then it settles down. And what happens with the amount of electrons when they reach the battery? I'm sure it doesn't increase but why?

this is going to lead you to a heap of misunderstanding

electron movement in a conductor is random in all directions. With an applied potential difference (voltage) there is still a randomness of movement, but with an underlying movement in one direction ... electron drift

Dave

It looks like you are not satisfied with high level models but want to know what's really happening in the wire.
Many basic concepts like voltage, resistance, Ohm's law and Kirchhoff's laws are models that can be derived from more fundamental laws.
You really only need two equations to answer most of your questions.

1. Coulomb's law which gives you the force between two point charges (e.g. electrons)
F = ke * q1*q2/r^2
q1 and q2 are two point charges, r is the distance between them and ke is Coloumbs constant

2. The generalized form of Ohm's law
J = σ*E
J is the current density, E the electric field and σ the conductivity.

If you had a sufficiently fast computer you could create a program that simulates currents flowing through a circuit by simulating a huge number of electrons and protons (which could just be treated as massless point charges).
Even if that program knew only those two equations plus how to do calculations with electric fields, it would be able to simulate the current in a simple electric circuit correctly even though it doesn't know anything about Kirchhoff's laws.
A simple circuit in this case would be one that contains only resistors, wires and capacitors.

So the reason for why the resistance is able to control the current, seemingly from a distance, can be derived by just looking at the way the electrons and protons in the wire and their electric fields interact with each other.

Ok, I will check those out again. I just wasn't able to relate those to what happens inside a conductor. Maybe electromagnetism course form Dr. Walter Levin from MIT will help me out with that.

this is going to lead you to a heap of misunderstanding

electron movement in a conductor is random in all directions. With an applied potential difference (voltage) there is still a randomness of movement, but with an underlying movement in one direction ... electron drift

Dave

Yep, I get that, randomness. Voltage kinda directs electrons in a direction right? Not everyone of them of course.
Where could I find information about the way current behaves atomically speaking?

 

[Nidum]

Ok, you're nailing it. Equilibrium condition, never heard of that in circuits theory. So at first current rises and then it settles down. And what happens with the amount of electrons when they reach the battery? I'm sure it doesn't increase but why?

Not quite with you on latest question ??

 

[Ouabache]

Wow! That was quick! Thank everyone for your replies. But the matter goes on. Damn! Electricity challenges me so bad!Ok, voltage is not consumed, got it.
So a resistor allows a current to flow, what happens when I plug both ends of a cable to the ends of the battery without any resistance? There's an infinite current circulating, and there's still a current. So a resistor just avoids currents from being infinite then?

The cables do have resistance, so you may think of this as a small resistor. The current is not infinite. For example #24 Ga. wire has a resistance of ~25.5 ohms per 1000ft of cable. So if you connected a constant voltage (e.g. 12V) across 1000ft of this wire, the current is not infinite. It would be I = V/R = 12/25.5 = ~470mA. A small length of this same cable would have proportionally less resistance and therefore a higher current will pass through it As the current in the cable increases, the wire heats up. If this current is too high. the insulation on the wire will melt and the wire will fuse open, which is the basis of how fuses work in a circuit.

 

[meBigGuy]

Have you ever read the wiki article on electric current.
https://en.wikipedia.org/wiki/Electric_current

You act like this stuff is really complicated, and in fact make it much more complicated than it is. All your analogies and examples are way off the mark.

JUST READ THE DANG WIKIPEDIA ARTICLE. TWICE, No, THREE TIMES.

It's really very simple. Current flows from positive to negative. ( Electrons flow from negative to positive). Something called resistance can impede that flow and dissipate energy. The rate at which electrons actually move is much slower than the current. In order to develop a potential (voltage) someone had to use energy to move charges into a source, or storage device of some kind. When a path (a circuit network from + to -) is provided, that potential energy forces charge through the resistances in the path provided. All of the potential energy will be dissipated in the network before returning to the source.

The potential energy associated with any given electron depends on how much work was done to get it to where it is. It could be a 5Volts, or it could be a 0 volts.

The tricky part is that electrons leaving the source may not actually arrive at the other side. Their energy is passed to other electrons much faster than the electrons actually move. The current (charge transfer) is at the speed of light. The electrons move at the drift velocity. Drift velocity might be 10 inches per hour. (1A in 1mm wire = 33cm/hour)

 

[sophiecentaur]

It's really very simple.

Oh no it ain't. If you think it is simple then you would need to find QM simple, too. Would you actually claim that?
The only 'simple' approach is to use the few basic mathematical formulae as your model and believe what they tell you. Mathsphobics are actually excluding themselves from what is probably the only reliable way into EE for us mere mortals.

 

[sreeragk1998]

so as all said voltage is not consumed, and battery supplies voltage. so how does battery gets out of charge?

 

[sophiecentaur]

so how does battery gets out of charge

When all the chemicals have been converted and there are no electrons / ions left to be displaced.