Unraveling the Mystery of Phase Differences in Passive Circuits

[carlgrace]

IM NOT SAYING we can work out everything without maths. NO. OF COURSE you need maths.

So your telling me if you told someone

I = C d(Vc)/dt

they will be able to see what it means and just come up with the long paragraph explanation which you just told me? about the electrons moving, more there are the more push is needed etc..

I sure don't know anyone that does...

I can do the maths, I can get the end result and know what it means.

But I want a LONG PARAGRAPH that generally explains what is causing the OVERALL phase shift in the circuit: total impedance phase.

So...can you tell me?

Sure...

An inductor is a device that creates a magnetic field due to a changing current being forced in loops through it. This magnetic field induces a current on the same wire that creates it, but that induced current opposes the original current. So, like a cat chasing its own tail, the inductor opposes changes in its own current. It takes some time to set up this magnetic field. The faster the current changes, the more the induced magnetic field opposes the current change, so inductors are sort of devices that present a "resistance" frequency proportional to frequency (this is called impedance).

The voltage across this inductor is maximum when the time-rate-of-change of the current through it is maximum. This time-rate-of-change of current is maximum when the current passes through 0. Therefore, we get a 90 degree phase shift. This is qualitative, not "math". OF COURSE the voltage must act this way since we already saw that the inductor acts as kind of a frequency proportional resistor. So, its maximum opposing voltage must occur when the inducing current is changing most quickly. That is 1/4 of a period of the sinusoidal input current.

The capacitor is analogous in its behavior, but acts differently physically. The result is the same... the maximum current "through" a capacitor occurs at the point at which the time-rate-of-change of the voltage is a maximum. For a sinusoidal voltage, these occur shifted by 1/4 of a period.

You get various phase angles in real circuits because of the various contributions of pure resistances (which have no phase difference between maximum current and maximum voltage) and reactances. You can see this intuitively if you just think of Kirchoff's voltage and current laws. It is helpful to think of a single frequency. Algebraically, you just use vector algebra to sort out this situation.

The "overall phase shift" then breaks down like this. At a given frequency, the impedances of the inductors and capacitors can be thought of like resistances. Then, the input current or voltage will be split up between these devices in accordance with KCL and KVL. The amount of phase shift at this particular frequency depends on the relative impedances at that particular frequency. For example if the circuit is dominated by reactance at that particular frequency, you will get a lot of phase shift... if it is dominated by resistance at that frequency you will get a little bit of phase shift.

The relative amounts of resistance and reactance in the circuit will change as the frequency changes. The pure resistance will remain the same but the amount of "effective resistance" presented by the inductors and capacitors will change. This ratio of pure and "effective" resistance gives rise to the overall total impedance phase.

Hope this helps!

Carl

 

[IssacBinary]

Sure...

An inductor is a device that creates a magnetic field due to a changing current being forced in loops through it. This magnetic field induces a current on the same wire that creates it, but that induced current opposes the original current. So, like a cat chasing its own tail, the inductor opposes changes in its own current. It takes some time to set up this magnetic field. The faster the current changes, the more the induced magnetic field opposes the current change, so inductors are sort of devices that present a "resistance" frequency proportional to frequency (this is called impedance).

The voltage across this inductor is maximum when the time-rate-of-change of the current through it is maximum. This time-rate-of-change of current is maximum when the current passes through 0. Therefore, we get a 90 degree phase shift. This is qualitative, not "math". OF COURSE the voltage must act this way since we already saw that the inductor acts as kind of a frequency proportional resistor. So, its maximum opposing voltage must occur when the inducing current is changing most quickly. That is 1/4 of a period of the sinusoidal input current.

The capacitor is analogous in its behavior, but acts differently physically. The result is the same... the maximum current "through" a capacitor occurs at the point at which the time-rate-of-change of the voltage is a maximum. For a sinusoidal voltage, these occur shifted by 1/4 of a period.

You get various phase angles in real circuits because of the various contributions of pure resistances (which have no phase difference between maximum current and maximum voltage) and reactances. You can see this intuitively if you just think of Kirchoff's voltage and current laws. It is helpful to think of a single frequency. Algebraically, you just use vector algebra to sort out this situation.

The "overall phase shift" then breaks down like this. At a given frequency, the impedances of the inductors and capacitors can be thought of like resistances. Then, the input current or voltage will be split up between these devices in accordance with KCL and KVL. The amount of phase shift at this particular frequency depends on the relative impedances at that particular frequency. For example if the circuit is dominated by reactance at that particular frequency, you will get a lot of phase shift... if it is dominated by resistance at that frequency you will get a little bit of phase shift.

The relative amounts of resistance and reactance in the circuit will change as the frequency changes. The pure resistance will remain the same but the amount of "effective resistance" presented by the inductors and capacitors will change. This ratio of pure and "effective" resistance gives rise to the overall total impedance phase.

Hope this helps!

Carl

WOW, Finally we are on the right track.

So sophiecentaur, I guess you think what carl said is a waste of time? As its doesn't have any maths.

Anyway.

Like I said I am fine with the shifts inside the capacitor or inductor, that's cool.

I can see how they act like a resistor. If its got 5Volts going one way and 8Volts still pushing against it, its going to look like an overall of 3Volts.

So like I said Carl, I think we are close.

Could you rewrite the explanation in terms of what's pushing and pulling / opposing one way and the other way etc to create the overal phase shift, without using terms, kind of like break them down as well. Does that make sense?. If you look back a few posts at my picture and example you should see.

 

[carlgrace]

WOW, Finally we are on the right track.

So sophiecentaur, I guess you think what carl said is a waste of time? As its doesn't have any maths.

Anyway.

Like I said I am fine with the shifts inside the capacitor or inductor, that's cool.

I can see how they act like a resistor. If its got 5Volts going one way and 8Volts still pushing against it, its going to look like an overall of 3Volts.

So like I said Carl, I think we are close.

Could you rewrite the explanation in terms of what's pushing and pulling / opposing one way and the other way etc to create the overal phase shift, without using terms, kind of like break them down as well. Does that make sense?. If you look back a few posts at my picture and example you should see.

OK, I will give this a shot (and I did have a look at your pictures).

I will focus on inductors because I believe they are a bit more intuitive (you don't have to deal with dielectrics or charges flying off in opposing directions).

So imagine you have an inductor. Its goal in life is to oppose changes in its current. Now, imagine you really slowly increase the current through it (you can do this by attaching it to a variable battery and very, very slowly increasing the output voltage of the battery).

What you will see is: pretty much nothing. The voltage at the output of the inductor will be the same as the input of the inductor. We say the inductor is a short at dc. Remember an inductor works by setting up a magnetic field (in accordance with Ampere's Law) due to a changing current. If the voltage is pretty much constant, the current is pretty much constant, so the magnetic field is zero. Then, the opposing current is pretty much zero as well (in accordance with Faraday's Law) and we can consider the inductor to be a really, really small resistance that doesn't do much of anything.

Now, imagine we start changing the voltage on the battery more quickly. Then, the current initially changes in step with the voltage, but now we have a more substantial magnetic field due to Ampere's Law. This magnetic field generates a voltage that pushes against the changing current from the battery. You can think of this as kind of like "inductor inertia". Now, how hard this inducted voltage pushes back depends on the strength of the magnetic field, which depends on a number of things. For example, to get a stronger magnetic field for a given input current change you need to increase the "inductance" of the inductor. The inductance is just a number that relates how much opposing force (electromotive force, or voltage) you get for a given input current. You can increase the amount of opposing force by increasing the number of current loops (inductance is proportional to the number of loops since each loop contributes some magnetic field due to Ampere) or by wrapping the loops around something ferroelectric instead of just air or plastic. This increases the inductance by concentrating the magnetic field lines and this property is called permeability.

OK, so as you increase the frequency, you are also increasing the opposing force. In fact, the force is proportional to both the inductance of the inductor and the rate of change of the current. So, to double the opposing force, you could either: 1. double the rate-of-change of the input current, or 2. double the number of turns in the inductor.

Now, how does all this relate to phase? Well, we established before that the maximum magnetic field and therefore maximum opposing force occurs when the rate-of-change of the current is a maximum. But because the maximum opposing force, or voltage, occurs when the current is passing through zero, we have a phase difference of 1/4th of the input period.

Now, if there are pure resistances lurking around, things can change a bit. We have seen how hard an inductor can push back (based on its inductance and on the rate-of-change of the input current). A resistor, however, is different, and it always pushes back against (or resists) a current by the same amount. That is called the resistance and it depends on certain details of its construction. Now, current is analogous to water in that is always tries to follow the path of least resistance. This is important, and is the key to understanding phase. At some low frequency, the effective resistance of the inductor is low compared to whatever resistor is around. Then, the current will mostly go through the inductor and the phase shift will be about 90 degrees. As you increase the frequency, the inductor starts pushing back harder and harder, while the resistor pushes back the same as always. At some point, the effective resistance equals the physical resistance of the resistor. At this point, half the current goes through the inductor and the other half goes through the resistor in accordance with Kirchoff's Voltage Law. In this case, the current that goes through the inductor has 90 degree phase shift, but the current through the resistor has zero phase shift. Through the principle of superposition, we can combine these and we end up with a 45 degree overall phase shift.

Now, imagine we continue to increase the frequency at which we change the battery output. The inductor's magnetic field keeps getting stronger and eventually it will push back much harder than the physical resistor, which doesn't change. In this case, almost all of the current will go through the resistor, and almost none through the inductor. While the very tiny bit of current going through the inductor still has 90 degrees of phase shift, the large amount of current going through the resistor dominates and we have an overall phase shift of about zero degrees by superposition.

And that's it. The physical principles underlying capacitors are different, but they turn out to be pretty much the inverse of inductors and the logic to understand how they generate phase is the same.

 

[sophiecentaur]

I can see how they act like a resistor. If its got 5Volts going one way and 8Volts still pushing against it, its going to look like an overall of 3Volts.

.

Well, if you are happy with that then the job's done. But if you think that inductors and capacitors are "like resistors" then you may not quite have got the full picture.
PS Without the Maths, what is a "sinewave"??

 

[carlgrace]

Well, if you are happy with that then the job's done. But if you think that inductors and capacitors are "like resistors" then you may not quite have got the full picture.
PS Without the Maths, what is a "sinewave"??

In ac circuits I would say thinking of inductors and capacitors as frequency dependent resistors is very powerful, and can provide a lot of intuition into the operation of different circuits such as filters and power supplies. I guess it depends on how far down the rabbit hole Issac wants to go. If he wants to understand and build discrete circuits it is probably enough for now.

 

[sophiecentaur]

In ac circuits I would say thinking of inductors and capacitors as frequency dependent resistors is very powerful, and can provide a lot of intuition into the operation of different circuits such as filters and power supplies. I guess it depends on how far down the rabbit hole Issac wants to go. If he wants to understand and build discrete circuits it is probably enough for now.

Isn't there a serious snag with that argument? If they are just like resistors, then where does the phase shift come from? (Which was what his earlier questions have all been about)
I have a serious problem with any discussion of phase relationship and its 'causes' that doesn't either involve complex numbers or trigonometry - or both.

 

[carlgrace]

Isn't there a serious snag with that argument? If they are just like resistors, then where does the phase shift come from? (Which was what his earlier questions have all been about)
I have a serious problem with any discussion of phase relationship and its 'causes' that doesn't either involve complex numbers or trigonometry - or both.

My explanation of phase implicitly used complex numbers. If you read what I wrote, the inductor is like an "effective resistor" the also provides 90 degrees of phase shift. The resistor adds no phase shift. You then use trigonometry to determine the overall phase shift. I think you can go a long, long way thinking like this. It isn't strictly "correct" but 99% of the time it's fine.

As a matter of comparison, digital designers often think of transistors as ideal switches. That is fine for most of their work, but when they have to worry about power or speed they need to add non-idealities to the model. Same deal here.

 

[sophiecentaur]

My explanation of phase implicitly used complex numbers. If you read what I wrote, the inductor is like an "effective resistor" the also provides 90 degrees of phase shift. The resistor adds no phase shift. You then use trigonometry to determine the overall phase shift. I think you can go a long, long way thinking like this. It isn't strictly "correct" but 99% of the time it's fine.

As a matter of comparison, digital designers often think of transistors as ideal switches. That is fine for most of their work, but when they have to worry about power or speed they need to add non-idealities to the model. Same deal here.

Actually, I was quite happy with your post (with its implications). My response was against IssacBinary's interpretation of what you wrote - which ignored the phase shift:- " If its got 5Volts going one way and 8Volts still pushing against it, its going to look like an overall of 3Volts.". That really does miss the whole point, I suggest. And demonstrates the risks of oversimplification.

 

[carlgrace]

Actually, I was quite happy with your post (with its implications). My response was against IssacBinary's interpretation of what you wrote - which ignored the phase shift:- " If its got 5Volts going one way and 8Volts still pushing against it, its going to look like an overall of 3Volts.". That really does miss the whole point, I suggest. And demonstrates the risks of oversimplification.

Oh, I understand now. I agree with you 100% about that.

 

[sophiecentaur]

Oh, I understand now. I agree with you 100% about that.

Did you think I was rattling your cage dear boy? ;-)
As if I could be arguing against such good sense.

 

[I_am_learning]

So your telling me if you told someone

I = C d(Vc)/dt

they will be able to see what it means and just come up with the long paragraph explanation which you just told me? about the electrons moving, more there are the more push is needed etc..

I sure don't know anyone that does...

Yes. I also don't find people doing that. But my point is, it should be possible.
What I actually mean is -> the equation is actually a logical build-up of physical things we know.
We first developed columbs law, that's pretty physical isn't it? Two particles try to pull/push each other with force... . Then we use that law to develop the concept of Potential. Then Current. Then we establish that if we place them on the plates of a capacitor a voltage is developed, which is logically established to be Q / C (where C, capacitance is geometry thing). Are you following?

We also establish that due to repulsive nature of electrons, when we try to put more and more charges into a capacitor, it becomes more and more difficult, so when tried by a constant voltage, the current decreases exponentially. This physical phenomenum is written short-hand as
I = Imax(1-exp(-t/RC)).
Then, we establish that, if instead of applying constant voltage, if we apply a voltage that is also varrying (sinusodial) so its not trying to constantly push in more charge but also pulling out, then we can logically derive (which I can't do for the moment, but I think you would be able to do if you try), that although the frequency of current and the applied voltage match the phase wont.

In short, I mean to say that there is no short-cut explanation. If you try and find one, then it may not model all aspect of the real situation. You need to go back to basic theory and then build and accumulate the logical conclusions step by step. Thats why there are chapter1, chapter2 that talks about atoms,electrons,attraction,voltage etc before talking about the power-factor, phase-lag-lead in later chapters.

If you agree with rest of my view and are stuck at only the 'RED' part above, then I may try.

 

[IssacBinary]

Isn't there a serious snag with that argument? If they are just like resistors, then where does the phase shift come from? (Which was what his earlier questions have all been about)
I have a serious problem with any discussion of phase relationship and its 'causes' that doesn't either involve complex numbers or trigonometry - or both.

LIKE resitstors...LIKEEEEE. Like doesn't mean that are 100% the same.

And many posts back... several times I explained the LIKE.

Charges flowing off in one way (capacitor) or flowing in the opposite direction from induced voltages (inductor) and they are having to oppose the source current.

So 5V pushing one way, 8V from source opposite leaves a resultant in original direction of 3V.

THIS IS THE RESISTANCE part of impedance. Impedance is complex and that is the REAL PART.

I said that many times.

So we are working with the imaginary side.

Through the principle of superposition, we can combine these and we end up with a 45 degree overall phase shift.

GREAT! I remember someone saying what does superposition have to do with any of this when I mentioned superposition.

Now, imagine we continue to increase the frequency at which we change the battery output. The inductor's magnetic field keeps getting stronger and eventually it will push back much harder than the physical resistor, which doesn't change. In this case, almost all of the current will go through the resistor, and almost none through the inductor. While the very tiny bit of current going through the inductor still has 90 degrees of phase shift, the large amount of current going through the resistor dominates and we have an overall phase shift of about zero degrees by superposition.

While there is a almost zero phase the current will be almost zero as well right!?.
As a higher frequency increases the inductors reactance (resistance part) so the higher the reactance the less total current flowing in the circuit...so at very high f almost 0 current?..Just want to clear that up.

In this case, the current that goes through the inductor has 90 degree phase shift, but the current through the resistor has zero phase shift. Through the principle of superposition, we can combine these and we end up with a 45 degree overall phase shift

This was my main problem. I thought I understood what was happening 100%. Your post is very similar to my original attempt at explaining it on the FIRST page. But the main part that got me was the surer position part. Take a look at this graph

http://www.wolframalpha.com/input/?i=10sin+x,+8sin+(x+++(pi/2)),+10sin+x+++8sin+(x+++(pi/2))

The BLUE line with amplitude 10 represents source.
The RED line with amplitude 8 represents the opposing discharge from a cap or inductor. Its 90 phase shift. Just like my original thought and what you just said.

The BROWN line is the resultant of the 2 added together.

RED at 8 represents a quite high frequency, as its pushing back almost as much as source, so its high reactance.

I EXPECTED to see a bit of a phase shift..WHICH IS THERE. I am happy...BUTTT
The resultant is BIGGER than either red or blue...But it should be less? In my example amplitude of 2?

Changing the amplitudes you can see the resultant wave shifting left and right.
I THOUGHT THAT WAS IT!
But because the resultant amplitude is bigger that's why I thought there might be something else...

OR...have I just set up the graph wrongly?

But thanks Carl for your help. I can't believe its taken 7 pages to get here! ha

Again I am not looking for something to replace maths. I am looking something to compliment the formulas so we know PHYSICALLY what is going on to roughly cause what's happening.

We have non maths explanations for capacitors, inductors, reactance, current, resistors. So why not overall impedance?

We have already worked out the real part...how the cap or inductor acts like a resistor to effect the AMPLITUDES...due to the pushing back and resultants...but the overall phase?...why has a lot of people started jumping up and down and waving their arms saying "YOU CANT DO IT".

 

[sophiecentaur]

I thought Vectors Were Maths. Was I wrong? Isn't complex arithmetic Maths? If all that about imaginary and real isn't just a stealth way of getting some maths into the description then what is it?
I can see that you really don't want to drop this but can you really justify, not just one paragraph but page after page of 'explanation' which involves all sorts of Implied Maths - which may be beyond some people - and try to say that makes anything clearer or easier to understand? Whilst you say you can add volts across reactances to volts across resistance with simple arithmetic then there is no hope for you. Please don't try to confuse the uninitiated further than you have to. Do you not see that many of the readers of your posts may be taking such stuff as gospel and trying to use it to build up their world view of electronics?
I appreciate that you may want a 'pet' view of things. I have said that I have my own. But you do seem to be after a very 'special' and selective form of explanation which has to 'appear' not to be mathematical - when it is, in fact, loaded with implied Maths. I can't see what you are really after, here.

 

[IssacBinary]

Who said vectors where not maths?
You seem to be missing the point big time.

But, all I am going to say right now is this. If you see me saying wrong things and see my understanding as being so wrong and I've got no hope. Maybe try to help out and fix some of my understandings?...After all you are a teacher right?

 

[sophiecentaur]

OK
Don't subtract the voltages across resistors and capacitors by simple arithmetic. Do you understand that's not correct?

 

[IssacBinary]

Yes however,

The voltages I was referring to is the voltage that is coming out of the capacitor due to the charge stored in it, or voltage induced in the opposite direction in an inductor .

Does that make sense?

So your saying that is not right? You can't see it as the first box in my picture?

If that's the case what explanation can you give me to make me understand / fix it.

And also what could I do to adjust that graph if anything?

 

[sophiecentaur]

I see very little point in continuing this. You clearly have a very specific requirement for how you want to be 'taught' this and it is just not on my wavelength. It would seem that there aren't many others prepared to go along with you, either.
Teaching and learning involve two parties and, if someone wants to learn something then they may have to consider shedding some preconceptions if they want a result. As I'very said before, you seem to be demanding a very specific mix of maths and non-maths before you will be satisfied. I think you may need to compromise about that. I am not sure that you even know what it is that you want.

 

[IssacBinary]

What are you asking me to compromise?

I said I can do the maths, I can do the equations, I can do the questions. I can get to the end result.

It sounds like your compromise is, ... if you can do it, just get on with it, and stop trying to work out what it all means and why. If you can do it that's all you need.

So what preconceptions are you saying I need to get rid of?

Im not demanding anything, I am just asking for a physical "long paragraph" explanation on how an overall phase shift is created in a circuit. Not an explanation of the phase in a cap or inductor, those I know and people have said a few times, but when we put them in the cicuit and work out the overall phase shift. That is what I would like explained.

Thats all I am asking.

I can do the maths, I can do the equations, but them alone arnt going to make you understand what's happening...and if you say they can then please tell me.
Please explain to me a long paragraph explanation on what the formula means / how overall phase shift is created but general theory such as "this gets bigger so this gets smaller, this goes that way then it goes this way".

Thats all I am asking.

I don't want to have to just blindly accept that overal phase is -tan(Xc/R). Yes it works, but what is happening

 

[I_am_learning]

As I'very said before, you seem to be demanding a very specific mix of maths and non-maths before you will be satisfied.

Thats how I was feeling too.
He accepts talking about phasor sums but denies talking about differentials.

 

[I_am_learning]

Hi, IssacBinary,
I am finding it interesting to talk to you.
For me the complete explanation of any physical phenomenum is when everything is eplained down to the basic laws i.e. everything is explained in terms of Newton's law of motion, Max-well's equations and Lorentz force law. Sometimes, its easier to explain things interms of other equivalet law such as the Columb's law and gauss law and I accept that.
Even more, I even accept things when they are explained in terms of 'pseudo law' (like, in Capacitor Current leads voltage by 90 degree) bacause 'pseudo law' has already been logically derived from the basic law.
Whats your criteria for complete explanation?
You must accept some basic laws, there is no other way? Which laws are you ready to accept?

 

[sophiecentaur]

What are you asking me to compromise?

I said I can do the maths, I can do the equations, I can do the questions. I can get to the end result.

It sounds like your compromise is, ... if you can do it, just get on with it, and stop trying to work out what it all means and why. If you can do it that's all you need.

So what preconceptions are you saying I need to get rid of?

Im not demanding anything, I am just asking for a physical "long paragraph" explanation on how an overall phase shift is created in a circuit. Not an explanation of the phase in a cap or inductor, those I know and people have said a few times, but when we put them in the cicuit and work out the overall phase shift. That is what I would like explained.

Thats all I am asking.

I can do the maths, I can do the equations, but them alone arnt going to make you understand what's happening...and if you say they can then please tell me.
Please explain to me a long paragraph explanation on what the formula means / how overall phase shift is created but general theory such as "this gets bigger so this gets smaller, this goes that way then it goes this way".

Thats all I am asking.

I don't want to have to just blindly accept that overal phase is -tan(Xc/R). Yes it works, but what is happening

You are, actually being Very Demanding because you require an answer that Exactly fits some picture, in your head, of how the answer should read. The only answer of that kind would be one that you invent for yourself.

I suggest that you make two lists, one with all the maths (implied or explicit) that you are prepared to admit into an 'acceptable' answer and the other with the maths that you are not prepared to be in there. You will see that you are already allowing loads of maths in there and that the few bits you don't want involve the consequences of complex arithmetic and the Argand diagram and a few basic definitions.

I have scanned through some of your past posts and find a number of inexact terms used.These are not helping you in finding a better understanding.
Here are a few:
"electrons moving":- you must be aware that the electrons may only be 'moving' (that is the mean drift) less than an atomic radius during a cycle of AC. This model has little use in these discussions and need not be involved at all.

"I have said many times I UNDERSTAND the phase inside the capacitor":- this is a meaningless statement and doesn't make it clear whether you mean the phase relationship between the current and volts or something else.

"THIS IS THE RESISTANCE part of impedance. Impedance is complex and that is the REAL PART.
I said that many times.
So we are working with the imaginary side .:- what 'we' are dealing with is a complex value - not a real or imaginary part.

the voltage that is coming out of the capacitor :- volts don't 'come out of' a component. They appear across it - they are a Potential Difference between two points.

What "blind acceptance' is involved with calculating the phase angle in terms of X and R? You seem happy that differentials (rates of change of one variable relating to the value of another variable) are involved. Going from that to the result of differentiating as sine function and a bit of vector addition is no worse than working out the speed of an object if you know the distance and the time. Would THAT be too mathematical and would you need motion to be described in verbal terms of 'how fast' you go and how far?

Could you allow yourself the luxury of a "paragraph of non-mathematical explanation" if you were to have any hope of getting to grips with QM? (I am not talking of Pub-Conversation level understanding here)

"just get on with it, and stop trying to work out what it all means' is not my message at all. My message is keep at it but don't expect anyone else to make that particular conceptual leap for you; it is far too specific a problem. All the tools are there for you to do this yourself; it's a personal thing. There are no 'really's in Science.

 

[NascentOxygen]

The voltages I was referring to is the voltage that is coming out of the capacitor due to the charge stored in it, or voltage induced in the opposite direction in an inductor .

Does that make sense?

Could I suggest that you get into good habits right from the word "go" in your physics studies? Adopting correct terminology is vitally important. Others have pointed out your use of the word "resistance" where referring to reactance. Because the word "resistance" is precisely defined in electronics, feel free to come up with something synonymous but not already taken, perhaps "resisting effect" would suit your purposes?

You also speak of "the voltage coming out of the capacitor". In electronics, we refer to current flow into or out of a capacitor. The voltage is described as being "on the capacitor" or "across the capacitor" (more precisely, across the capacitor plates). If you are meticulous in how you apply these key phrases, observing how one applies to current and the other to voltage, then others will be able to better appreciate your explanations.

As a student, that means you will be best placed to earn the marks you deserve for endeavoring to understand the circuit's operation.

 

[wbeaty]

@I am learning
Well put. The suggestion in a previous post that you can look upon a circuit as a flywheel would never have been made if Maths had been used. Maths involves actual numbers and would have revealed the sort of value for the angular momentum involved with 1/4000 of the mass of the wire and speeds of a few mm/second being involved. A Flywheel??

Yes of course a flywheel. I've seen this analogy in several undergrad physics and EE texts. Capacitors are analogous to springs, inductors are analogous to moving mass (to flywheels,) and if you hook a spring up to a flywheel you obtain an oscillator with a known frequency.

A superconductor energy-storage ring connected in a closed circuit? It's much like flywheel energy storage. Breaking the circuit of an SC ring will give you a violent "inductive discharge," and it's analogous to sticking a piece of wood into the spokes of a spinning bicycle wheel. BANG!

But of course with circuits, the "flywheel" is extremely massive, and is spinning very slowly. (The analogous "mass" is part of the b-field, while the electrons' mass is tiny and irrelevant.)

And because of the usual resistivity of copper, this electron-flywheel is sitting on a pile of sand, or perhaps molasses! If spun, it grinds to a halt almost instantly. Stick a magnet pole into the center of a copper ring. The induced current starts spinning in a circuit, but it halts within microseconds. Add an iron core and you can get this time constant up to a second or so (and there's an Exploratorium exhibit which demonstrates this by using a laminated split-core and a massive aluminum ring made of 1" rod.)

 

[wbeaty]

I can see how they act like a resistor. If its got 5Volts going one way and 8Volts still pushing against it, its going to look like an overall of 3Volts.

Nope Issac, this is still wrong. If you hook an ideal capacitor up to a voltage source, there is only one voltage. No subtraction. When you change that power supply voltage (by twisting the knob on your ideal regulated supply,) there will be a current in the capacitor. No resistors involved.

Remember the capacitor analogy with the iron sphere? Rubber membrane across the center, with both sides full of water? In that analogy, if you slightly increase the pressure difference, it makes the rubber get stretched more. More pressure, more deflection of the rubber. For every value of voltage, the rubber will get deflected by a different amount. OK?

What happens if you slowly and constantly increase the pressure difference across the water-capacitor?

 

[IssacBinary]

I understand I may be using the wrong terms at the wrong times and places, so ill make sure I do my best to keep everything in correct terms.

Ok I am going to try and go through all my understandings and hopefully we can pick it apart and rebuild what I need to finally get my original question sorted out.

In my subtracting voltages example. This is my thinking and reasoning..

If you connect 2 batteries in series. Let's say 8 Volts and 3 Volts and their polarity's are in the same direction you now effectively have 1 battery / voltage source of 11Volts.
However if you reverse the 3Volts battery so now its opposing the 8Volts you effectively have 1 battery / voltage source of only 5 Volts.

The 11 Volts and 5 Volts is what I am referring to as a "resultant" voltage.

So...

If you hook a capacitor to a battery, the capacitor will charge up to the voltage of the battery.

If you then remove the battery and connect the two ends of the capacitor it will discharge through the side it was charged.
So the voltage across the capacitor is in the opposite direction of the voltage across the battery.

As we know caps block DC. But if you have and LED after the CAP and you connect a battery, for a split second the LED will light, then will very quickly get dim and go off.
Because when you first flick the switch there is no voltage across the cap, so there is no opposition.

But the cap starts building its charge. So as its voltage increases (in the opposite direction) they start to cancel each other out.

Then when the cap is fully charged it has the same voltage across it as the battery.

This is when the circuit has no current flowing.

As there is 10V from left to right (across source) and 10V now across the cap going right to left.
Its like a balancing act, nothing can move as they are perfectly pushing against each other.

So a resultant of 0V and thus that's why there is 0amps flowing in the circuit. = Caps block DC.

This resultant voltage in the circuit can be worked out by using the capacitors reactance which is its opposition measure. In DC which is 0 frequency you effectively have 1/0 = infinite. So basically a very high "opposition" measure in ohms. So you could replace a capacitor with a resistor of the same ohms and the effect in the circuit will be the same.

So,

Lets start with just this for now, then ill move on to the next bits after this is verified and or fixed.

Thanks

 

[wbeaty]

You also speak of "the voltage coming out of the capacitor". In electronics, we refer to current flow into or out of a capacitor.

No, if we're being precise, we refer to charges flowing through a capacitor. Or we refer to the current in the capacitor.

"Since a current is a flow of charge, the common expression
'flow of current' should be avoided, since literally it means
'flow of flow of charge.' "
-Modern College Physics. Sears,Zemanski,Richards,Wher
​

Hah! :)

Actually "flow of current," versus "flow of charge" is a major pet peeve of mine because as a physics student I personally experienced disruption of my entire understanding of simple circuits. Once I finally attained an intuitive grasp of the basics, I could finally see my prior misconceptions. The main offender among these was my belief that "current" could flow, as if there was a substance called "current" which could move around inside of wires. (No, the 'substance' is called charge. Electric current is a flow of charge. Charges can flow along, but currents just appear and vanish, same as in rivers.)

 

[I_am_learning]

I understand I may be using the wrong terms at the wrong times and places, so ill make sure I do my best to keep everything in correct terms.

Ok I am going to try and go through all my understandings and hopefully we can pick it apart and rebuild what I need to finally get my original question sorted out.

In my subtracting voltages example. This is my thinking and reasoning..

If you connect 2 batteries in series. Let's say 8 Volts and 3 Volts and their polarity's are in the same direction you now effectively have 1 battery / voltage source of 11Volts.
However if you reverse the 3Volts battery so now its opposing the 8Volts you effectively have 1 battery / voltage source of only 5 Volts.

The 11 Volts and 5 Volts is what I am referring to as a "resultant" voltage.

So...

If you hook a capacitor to a battery, the capacitor will charge up to the voltage of the battery.

If you then remove the battery and connect the two ends of the capacitor it will discharge through the side it was charged.
So the voltage across the capacitor is in the opposite direction of the voltage across the battery.

As we know caps block DC. But if you have and LED after the CAP and you connect a battery, for a split second the LED will light, then will very quickly get dim and go off.
Because when you first flick the switch there is no voltage across the cap, so there is no opposition.

But the cap starts building its charge. So as its voltage increases (in the opposite direction) they start to cancel each other out.

Then when the cap is fully charged it has the same voltage across it as the battery.

This is when the circuit has no current flowing.

As there is 10V from left to right (across source) and 10V now across the cap going right to left.
Its like a balancing act, nothing can move as they are perfectly pushing against each other.


So a resultant of 0V and thus that's why there is 0amps flowing in the circuit. = Caps block DC.

This resultant voltage in the circuit can be worked out by using the capacitors reactance which is its opposition measure. In DC which is 0 frequency you effectively have 1/0 = infinite. So basically a very high "opposition" measure in ohms. So you could replace a capacitor with a resistor of the same ohms and the effect in the circuit will be the same.

So,

Lets start with just this for now, then ill move on to the next bits after this is verified and or fixed.

Thanks

Ok that's quite fine. But a few points. To be precise its not precise to say - "Caps Block DC". As you have yourself clarified, DC current flows for sometime (or longtime?). So you should be wary of such generalizations.
Also, you can't get complete modelling by replacing the Capacitor by its equivalent reactance. This too you yourself have verified because the reactance comes out to be infinity so no current should flow. But current flows for sometimes initially. So, be wary of the generalization this time also. It works only for 'steady state condition' (Know this term?)

 

[sophiecentaur]

These ideas tend to chase their tails. Someone 'knows' the theory of a complicated phenomenon well. They use a simplification or generalisation in an informal way, to get over the idea quickly to someone who can't get the full theory. Next thing you know, the (attactive) generalisation gets used by someone, who doesn't know basic theory, to explain the original sophisticated idea to someone who knows even less. And thus the Science Myth is born and arguments rage on forums like this one.

e.g. You can replace a capacitor with a resistor if you want to explain phase. Excuse me.

 

[sophiecentaur]

Yes of course a flywheel. I've seen this analogy in several undergrad physics and EE texts. Capacitors are analogous to springs, inductors are analogous to moving mass (to flywheels,) and if you hook a spring up to a flywheel you obtain an oscillator with a known frequency.

A superconductor energy-storage ring connected in a closed circuit? It's much like flywheel energy storage. Breaking the circuit of an SC ring will give you a violent "inductive discharge," and it's analogous to sticking a piece of wood into the spokes of a spinning bicycle wheel. BANG!

But of course with circuits, the "flywheel" is extremely massive, and is spinning very slowly. (The analogous "mass" is part of the b-field, while the electrons' mass is tiny and irrelevant.)

And because of the usual resistivity of copper, this electron-flywheel is sitting on a pile of sand, or perhaps molasses! If spun, it grinds to a halt almost instantly. Stick a magnet pole into the center of a copper ring. The induced current starts spinning in a circuit, but it halts within microseconds. Add an iron core and you can get this time constant up to a second or so (and there's an Exploratorium exhibit which demonstrates this by using a laminated split-core and a massive aluminum ring made of 1" rod.)

This flywheel thing is interesting. It is an absolutely lousy 'physical' analogy, bearing in mind the values of the quantities involved BUT the Maths of both systems are pretty much identical. SO, by Isaac's argument, I think, it shouldn't be used as an explanation because, physically, it's not close enough but the Maths are spot on. The 'flywheel' thing is, in fact, a magnetic field - not electrons buzzing around.

 

[wbeaty]

I understand I may be using the wrong terms at the wrong times and places, so ill make sure I do my best to keep everything in correct terms.

Nope, that's not the problem.

When you connect a capacitor to an ideal 6-volt battery, you get infinite current. It lasts for zero time. The capacitor voltage jumps up to 6v instantly. OK? Then to get rid of the infinities we add a resistor.

BEEEP wrong. Big mistake.

Ok I am going to try and go through all my understandings and hopefully we can pick it apart and rebuild what I need to finally get my original question sorted out.

The problem appears when you add the resistor. Or the LED.

That phase-shift you're having trouble with? I understand it pretty well, no math needed at first. From my perspective I can clearly see that it's impossible to understand this stuff if we add a resistor to the circuit. The resistor totally derails our thinking; sends us up a conceptual dead end. We have to avoid the resistor. Bad, evil, naughty resistor, corrupts our minds and derails our search for the Holy Grail. A SHRUBBERY! Ignore that shrubbery.

A totally different approach is required.

Instead, do it with an ideal capacitor and an ideal variable-DC voltage supply. Or do it with the iron-sphere water capacitor connected to an ideal water pump: a weird special pump which creates constant pressure. No resistances are needed (everything has infinite conductivity, zero friction.)