Strange questions about electrical circuits

[davenn]

There is plenty of time in the future for the questioner to "crack it".

but unfortunately by that future time, they have become so screwed up in there misunderstanding that it will take years to get them out of that
bad rut and into the real understanding of the science

Teach the real thing from the start and build on the basic concepts

The only "right" way is to read the math and intuit what it means. I = E/R : what does that really say. Kirchoffs law, etc etc

I'm a big believer in good teachers developing an intuitive explanation and then embarking on a rigorous one. But that is different than using analogies.
Analogies are a last resort for those that just can't get the picture from the math.

yup exactly :)Dave

 

[Nidum]

As most of you know it is very difficult to gain any real understanding of concepts in science and engineering without a good grounding in maths .

Anyone studying these subjects formally usually gets taught adequate maths in parallel with the technical subject itself .

There are many people though with no maths who just want to understand some of these concepts at a descriptive level .

Some of you will remember programmes like 'Tomorrows World' where presenters had the knack of describing sometimes complicated subjects in a very simple way . Some science teachers could do this as well even if only as a prelude to formal studies .

Perhaps formal maths based teaching should be reserved for those people doing in depth studies and more informal non maths based teaching used for people just seeking general knowledge .

 

[sophiecentaur]

Some of you will remember programmes like 'Tomorrows World' where presenters had the knack of describing sometimes complicated subjects in a very simple way

The presenters were mostly Arts graduates; very articulate and they could probably sell ice cream to the Eskimos (as they say). TW was a 'must' to watch when it was on and there were many fascinating gadgets and pictures plus some excellent entertainment. The actual Science content was often very questionable. I had some dealings with the production team and I remember a lovely howler, involving Red, Green and Blue Electrons, producing a colour TV picture. I got a very petulant reception when I pointed out that it was rubbish.
The secret of good Science Entertainment is to leave people feeling they understood what they have just heard. But it ain't Science Education. PF tries to get things far nearer to the currently accepted Science than TV Science does.
I received some very good advice when I was doing my Teacher training year. I was told that kids don't get metaphor (and that's what an analogy is). You give them a metaphor and, unless you are incredibly careful, they will take that as the FACT that the lesson is about. This can largely be because they weren't actually listening to most of the lesson and only got the bit about water squirting out of a hose. That doesn't only happen with kids either.

 

[Guidestone]

Whoa! I haven't read Every post. The only thing I can tell so far is that I wasn't satisfied with the water analogy. I'm the kind of obsessive-fascinated-passionate guy who tries to understand the things the closest possible as the way they really behave no matter how hard they seem to be. However, the analogy was first useful to give me a little idea of what the behavior of electricity looks a bit like. I may not be as smart as Ohm or Maxwell and I may never be, but if someone made those laws, then that person understood better than anyone the behavior of electricity, this can only mean that our understanding can improve, I believe. I'll promess I'll read each one of your posts slowly and check out the links you all have provided. I'll reply soon.
By the way, this post was rated as one of the most fascinating discussions from last week OMG!
Thanks a lot again!

 

[sophiecentaur]

By the way, this post was rated as one of the most fascinating discussions from last week OMG!

You are a star, then!
(Oh dear, is that an analogy? - Or at least, it;s a metaphor)

 

[Guidestone]

This advice may be tiresome but it would be really worth your while to take it (in the long run) - if you want to understand the subject and not just get on with it for the purposes of making electrical gadgets work or repairing them. Water analogies are sometimes used on technician courses and in the armed services but they will turn out to be a snare and a delusion if you want a deep knowledge

I'm studying mechatronic engineering. The guy who gave me the course of electromagnetism is an engineer as well and he is like a God to me, his class was not only about making circuits on a breadboard, most of the time he explained the theory, he made very complicated equation deductions and he obliged us to prove they were truth in the lab. Every day at the end of his classes I felt like if my brain had been liquified or pulverized . At the end of the course I was never going to take anything for granted anymore. It's been a year since the course ended and I'm still trying to understand his teachings. Sometimes I can't allow myself to build electronic stuff just because I haven't understood enough of its laws.

 

[Guidestone]

For me its a last resort, but the OP just was not trying or not getting it after many good explanations. So, I dragged it out (against my better judgement). I'd rather debate the water analogy than deal with the OP's continual misunderstandings. He was hardly trying. I doubt he ever read a basic electronics book.

What's an OP?

 

[Guidestone]

Just curious - Is there any good visual teaching software for base level electrical circuit theory ??

I've been looking for something like that but it would rely on only analogies as well don't you think?

 

[Guidestone]

I totally disagree.

Analogies are an excellent way to get a conceptual grip on a new idea.

At the beginning they may help but then comes a time when they satisfy no longer

 

[Guidestone]

If you use analogies (and I'm guilty of introducing the water analogy in this thread) you will achieve a sophisticated level of misunderstanding.

The only "right" way is to read the math and intuit what it means. I = E/R : what does that really say. Kirchoffs law, etc etc

I'm a big believer in good teachers developing an intuitive explanation and then embarking on a rigorous one. But that is different than using analogies.
Analogies are a last resort for those that just can't get the picture from the math.
Simplifications are different than analogies. You don't need to understand maxwell's equations to develop a decent understanding of ohms law.
Few engineers understand the relationship between relativity and the appearance of E and H fields as manifestations of EMF, but they can still design antennas.

Analogies like resistance vs. friction and current vs. water flow are more than simplifications. They are downright distortions. But, they have their uses.

There's this weird philosophy in guys studying engineering, they think they need to be practical and understanding has a second place if it ever has one, it's so so sad. :(

 

[Guidestone]

True or false doesn't really apply to an analogy. Wikipedia says "Electrical resistance shares some conceptual parallels with the notion of mechanical friction." https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance.

The hydraulic analogy would say it is more like an orifice causing a pressure drop.

It's kind of hard to visualize a material with fewer free electrons (therefore higher resistance) as exhibiting friction.

It's weird but it's easier for me to imagine a material with fewer free electrons than water loosing pressure. Thank you, it's been helpful.

 

[Guidestone]

the number of electrons leaving the battery will equal the number of electrons returning to the battery
No electrons were lost or harmed in the process

Yep, I get that, but do you notice that the presence of the resistance already determines the current? I may be misunderstanding this but this sounds to me like if there were 10 electrons in the battery ready to go at the same time, with the presence of the resistance there would be only five to get out of it per second instead of ten. Here goes a terrible analogy of what I understand of this: there's an army of four rows marching towards a door 30 meters ahead, this door can only have two people or two rows going through it at the same time so the captain sees the door 30 meters before they reach it and he orders the soldiers to make two rows, so way before they reach the door and 25 meters before the door there are already two rows marching towards it. The fact that the captain saw the door ahead determined the number of rows waaaaay before they were even there!

 

[Guidestone]

@ Guidestone

Please review the hydraulic analogy in https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance. Then you can ask questions about how that analogy breaks down. (read about the hydraulic analogy in other places also).

Note that when you start a hydraulic flow, (in a system with no air gaps) that water comes out of the end immediately upon applying pressure, not just after the input water reaches the output. The water flow is the same all along the system (but the pressure can change). No more can go in than comes out (unless there are storage elements)

It doesn't seem like you are really thinking about or digesting what others are telling you. Maybe reread some basic electrical theory. You are totally missing the basic points.

Man, I'm asking here because electrical theory doesn't answer lots of my questions. I'm aware I'm missing points, but you also know most of times answers only lead to more questions. I'm going to read what you sent me I promess.

 

[Nidum]

There's this weird philosophy in guys studying engineering, they think they need to be practical and understanding has a second place if it ever has one, it's so so sad. :(

Many engineers have an in depth understanding of the underlying theory of whatever field they work in . In fact they often have to extend the boundaries of known science .

 

[Nidum]

It is sometimes more useful to think about energy circulating in a physical system than to get bogged down with detail consideration of voltage , current , pressure , displacement et al .

In your idealised circuit battery supplies an amount of energy and resistor dissipates the same amount . Circuit is in equilibrium .

 

[Averagesupernova]

Yep, I get that, but do you notice that the presence of the resistance already determines the current? I may be misunderstanding this but this sounds to me like if there were 10 electrons in the battery ready to go at the same time, with the presence of the resistance there would be only five to get out of it per second instead of ten. Here goes a terrible analogy of what I understand of this: there's an army of four rows marching towards a door 30 meters ahead, this door can only have two people or two rows going through it at the same time so the captain sees the door 30 meters before they reach it and he orders the soldiers to make two rows, so way before they reach the door and 25 meters before the door there are already two rows marching towards it. The fact that the captain saw the door ahead determined the number of rows waaaaay before they were even there!

-
That is quite a stretch. I am not one to always rip on analogies but that one I would stay away from. With the ten electrons ready to go bit and the 5 vs. 10 electrons per second are not really the same thing. One is a rate and the other is not. In electronics I believe it is often best to blindly believe the facts (real facts, not assumed ones), apply an analogy only if necessary but revisit this and study it until the analogy is no longer necessary.

 

[Guidestone]

It is sometimes more useful to think about energy circulating in a physical system than to get bogged down with detail consideration of voltage , current , pressure , displacement et al .

In your idealised circuit battery supplies an amount of energy and resistor dissipates the same amount . Circuit is in equilibrium .

You mean power?

-
That is quite a stretch. I am not one to always rip on analogies but that one I would stay away from. With the ten electrons ready to go bit and the 5 vs. 10 electrons per second are not really the same thing. One is a rate and the other is not. In electronics I believe it is often best to blindly believe the facts (real facts, not assumed ones), apply an analogy only if necessary but revisit this and study it until the analogy is no longer necessary.

And what do you think about the soldier analogy?
I'm bad at believing blindly in this case. I have faith In lots of things but applying critical thought it's more important to me in this case. I know electricity can't almost compare to anything else but still I have to compare it with the little I know before passing on to another subject.

 

[sophiecentaur]

What's an OP?

OP is Original Post or Original Poster - so he is referring to you.
Your tutor sounds a guy worth following.
We could keep on for ever about the strengths and weaknesses of analogies but there will always be a risk when you try to explain something to someone else with an analogy. That risk is that they will take it far too literally. It even happens with an idea like the photon. They hear the word "particle" and immediately think of a little bullet. Wrong, and responsible for more misconceptions than I have had hot dinners. Just browse through the threads with the word "photon" in them and you will see what I mean.

 

[Averagesupernova]

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.
-
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.

 

[DrZoidberg]

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.

 

[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.
-
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?