Unraveling the Mystery of Phase Differences in Passive Circuits

[IssacBinary]

I know this is basic stuff but honestly your question just seems to have been overlooked... maybe I'm 100% out to lunch but this is the best i can do without math..

Exactly! And what you just explained was not my orignal question. I have said many times I UNDERSTAND the phase inside the capacitor.

Also many times I have explained my question and what I do want explained.

OVERALL PHASE SHIFT.

This post explains it the best.
https://www.physicsforums.com/showpost.php?p=3377063&postcount=19
Then read this post
https://www.physicsforums.com/showpost.php?p=3381323&postcount=48
It has a nice big picture

 

[Evil Bunny]

Moreover, I've seen plenty of students trying to "understand" things in electronics without the math and I therefore know how pointless it is in the long run.

I'm sure you're very smart and a good teacher. Obviously there are plenty of folks on this board who know much on the subject, but you're still missing the point.

He's not trying to understand things without the math... He has repeated himself over and over again that he understands the math. He has proven it with his examples.

He wants more explanation in addition to the math... I have pondered this very question and never bothered to ask about it. The math is easy here.

Some of us like to just think about things like this a little more. Spraying people with equations when they ask questions like this (and tell you repeatedly that they understand the math part of it) isn't helping anything. Telling them they're wasting their time and it's pointless to try to think of it that way comes across as being a little... well... arrogant.

Now maybe to you this is a waste of time, and that's fine... but try to understand everyone is not "wired" the same way, so to speak. There is more to understanding electricity than throwing math books at people. Just sayin...

 

[es1]

He wants more explanation in addition to the math... I have pondered this very question and never bothered to ask about it. The math is easy here.

I am not sure such an explanation exists.

Here is an analogy.

What if we defined phase as the length of a shadow (it is just an arbitrary definition of a physical quantity anyway). And then you said, "ok, but why is the shadow 10m long." And then we replied, "if you know the dimensions of the object and the light source position etc. you can use geometry to figure it out."

And then you said, "ok. I get how you worked that out. But explain why it is 10m without geometry."

How could someone do that? I don't think you could say why the shadow is 10m, and not say 9.8m, without geometry.

 

[es1]

Keep in mind the question was why is the phase shift X. An explanation that can't answer why the phase shift is X and not Y isn't going to work.

 

[sophiecentaur]

Exactly! And what you just explained was not my orignal question. I have said many times I UNDERSTAND the phase inside the capacitor.

But "Phase inside the capacitor" is a meaningless concept so I have a feeling that you don't in fact understand.

 

[IssacBinary]

I am not sure such an explanation exists.

Here is an analogy.

What if we defined phase as the length of a shadow (it is just an arbitrary definition of a physical quantity anyway). And then you said, "ok, but why is the shadow 10m long." And then we replied, "if you know the dimensions of the object and the light source position etc. you can use geometry to figure it out."

And then you said, "ok. I get how you worked that out. But explain why it is 10m without geometry."

How could someone do that? I don't think you could say why the shadow is 10m, and not say 9.8m, without geometry.

Im not asking for an explanation to give an exact answer for an exact value for an exact situation.

But more of a general explanation.

i.e The harder you push something the faster it moves because for you to push harder you need more energy thus you have transferred more energy so the object is moving faster.

F = ma

...So for your example.

The closer the sun is to 90 degrees over your head the less the suns rays have to go from going past your head to the ground. So it makes a smaller length shadow.
If the sun is less than 90 degrees, say its almost straight on, then the sun rays have to go a lot further to get to the ground after touching your head because its more of a horizontal line of sight, so once it does reach the ground it makes a longer shadow as its had to go a lot further.
The space before where the sun rays touch the ground is the shadow.

Its general. But still an explanation, and then it matches in with the "why is it 10m?" "because of geometry"..."but why"..."because of the sun / light".....
Then once you understand the concept, the maths makes more sense. The maths sits on top of the non math explanation and your able to see why and what things now mean.
"Ohhh so that's why it gets smaller, and this is the formula / maths to calculate exact " Like 1 over big angle (between 0 - 90) = smaller shadow (1/x...bigger x is the smaller the 1/x is)...(that was just an example)

 

[wbeaty]

So all I am saying is. When we work out impedance I can see how the resistance / real part of impedance can be explained, like i just did, tug of war expample. But I am asking about the total impedance PHASE part. How is that created?

Note that the phase of current is *leading,* not lagging. I mean, when you apply a sinewave to your R and C in series, the current doesn't lag behind the voltage. It leads.

Your explanation has to end up with a leading phase. Negative degrees relative to the voltage driving the circuit. I think your present explanation predicts a phase lag, right?

Could this be a major sticking point?Going further: if we connect a capacitor directly to a sine-wave voltage generator, what happens? Remember, NO SERIES RESISTOR. The sinewave generator creates a voltage, not a current. The voltage gets applied directly across the capacitor terminals. The capacitor draws a current from the supply, similar to the way a 120V light bulb draws a current from distant 60Hz generators. What will the capacitor current look like? (Again, it's not 'the capacitor-charging current creates a voltage.' That concept can't help us.)

Instead we have to go backwards, starting with voltage.

 

[wbeaty]

I am not sure such an explanation exists.

Definitely right: these types of explanations simply don't appear in textbooks. Search forever, and you won't find this one.

So we'll have to sit down right here and construct it ourselves.


(Or, did you mean that such an explanation is impossible? Nah, I created this style of explanation all the time back in science museum work. "Teach physics to little kids and grandmothers." In physics exhibits you can't fall back on any math whatsoever. The general public education level is ~grade 6-7, and they're all math-hating/phobic. You can't rely on "Latin," you'll have to translate it into "gutter-slang" i.e. words and pictures. Animations. Animations help lots!)

 

[wbeaty]

But "Phase inside the capacitor" is a meaningless concept so I have a feeling that you don't in fact understand.

Phase of the current wrt the applied voltage. The current is inside the capacitor, so the phase is "inside."

But yes, he might be missing something big: that a capacitor's current doesn't only charge the capacitor. That's only half the concept.

The other half: a changing capacitor voltage will draw a current from a power supply, and the faster the capacitor voltage is changing, the higher is the current.

 

[IssacBinary]

Note that the phase of current is *leading,* not lagging. I mean, when you apply a sinewave to your R and C in series, the current doesn't lag behind the voltage. It leads.

Your explanation has to end up with a leading phase. Negative degrees relative to the voltage driving the circuit. I think your present explanation predicts a phase lag, right?

Could THIS be a major sticking point?

No not really. If I get my lagging and leading mixed up it wouldn't really matter too much (some say it does)

But yes. The -56.3 from my early post would mean voltage is -56.3 behind current.

But again the way I see it, but just is never said this way is that I like to think of it as resultant voltages / currents.

As there's multiple things happening to create an overall end result.
In this case, TOTAL impedance is the overall "resistance" the circuit "seems" to have due to the effects of the capacitor and resistor together. Along with a phase shift.

The phase shifted current isn't coming out of thin air, its being created by a voltage that is in phase with it, but its a "resultant" voltage because its made up of different "happenings" in the circuit. So my guess why its never explained this way is because this "resultant" voltage is neither the source voltage or voltage across the capacitor.

 

[wbeaty]

No not really. If I get my lagging and leading mixed up it wouldn't really matter too much (some say it does)

Not mixed up.

If your explanation predicts a lagging current, but the real current is actually leading, then your explanation has a huge flaw.

In physics-speak: your explanation is disproved by simple experiment.

The phase shifted current isn't coming out of thin air, its being created by a voltage that is in phase with it, but its a "resultant" voltage because its made up of different "happenings" in the circuit. So my guess why its never explained this way is because this "resultant" voltage is neither the source voltage or voltage across the capacitor.

Ah, that's the sticking point.

The phase-shifted current *is* coming out of thin air.

It's not being created by a voltage that is in phase with it.

This "resultant" voltage isn't in the circuit. It's not part of the intuitive/picture/verbal description.

It doesn't exist.

Let's get down to details. Why do you see a need for the "resultant" voltage? Is it because voltage causes current?

 

[sophiecentaur]

"is it because voltage causes current?"
Be careful not to make voltage into some kind of 'push'. Remember the actual definition of a volt. 1 Joule per Coulomb.

 

[sophiecentaur]

Definitely right: these types of explanations simply don't appear in textbooks. Search forever, and you won't find this one.

So we'll have to sit down right here and construct it ourselves.


(Or, did you mean that such an explanation is impossible? Nah, I created this style of explanation all the time back in science museum work. "Teach physics to little kids and grandmothers." In physics exhibits you can't fall back on any math whatsoever. The general public education level is ~grade 6-7, and they're all math-hating/phobic. You can't rely on "Latin," you'll have to translate it into "gutter-slang" i.e. words and pictures. Animations. Animations help lots!)

This is very creditable stuff as long as you don't expect the kids and grandmothers to go out and be able to use these scientific nuggets for advancing Science any further. Great to get them into the subject (and for them to approve of taxes being spent on Science) but they'll still need the rigour and the Maths to build on what they've been told / shown.

 

[Evil Bunny]

Thinking of voltage as a 'push' is a perfectly reasonable and logical way of analyzing circuits... In a non-math-hand-wavy sort of way. Makes perfect sense to us!

 

[sophiecentaur]

You carry on that way. Don't mind me. However, you risk falling on your face sometime if you do. "Hand waving" doesn't allow you to design or analyse even a simple electrical circuit, you'll find.
I guess you might also want to describe resistance as 'a sort of force stopping the current from flowing' (even worse!).

 

[DragonPetter]

Thinking of voltage as a push is not so far-fetched in the right context. Remember we can model entire mechanical systems with electrical circuit, because the same math applies to both systems.

 

[sophiecentaur]

The bottom line of all this discussion is to ask the question - how many people who design circuits for a living and who make Engineering Systems work do it without Maths and by waving their arms about? There is no shame in finding Maths, at some level, difficult / impossible. But to dismiss Maths on the grounds that it doesn't reveal what goes on in complicated systems as well as analogies and hand waving, is totally missing the point. That just reads like inverted snobbery.
I totally take my hat off to people who's Maths ability is better than mine and wouldn't have the nerve to say that what they do with their maths is somehow lacking - which has been the flavour of several of the posts on this thread.
Throughout my technical life I have found that learning the 'next step' in Maths has given me an increased understanding of stuff that I may have felt, initially, I already had sussed.

 

[DragonPetter]

No one is arguing that you can explain this completely without math or that you shouldn't use math to describe the process. Of course, math and logic trump any intuitive explanation. But there is more to an understanding of physics concepts and especially engineering than pure math. There are a lot of artistic/creative aspects to circuit design and the math isn't always the first answer even if you end up using math in your final engineered design. Personal conceptions of how circuits work can help you use the right math tools.

Every physical thing should be explainable with math, but do we get out our calculators every time we want to know how much force we should use in our legs to sit in a chair or how many degrees to turn our steering wheel to make a turn? When we start to abstract everything with math, its easy to hide the physical processes.

And besides, most electrical engineering concepts are abstract models and don't necessarily represent what is really happening even if the math accurately describes the process. We don't derive maxwell's laws every time we calculate the current through a resistor, but we all know there is much more to the physical process and the mathematics than an equation that says V = I*R.

 

[NascentOxygen]

What is the mechanics that's causing the current at any point in the circuit to be out of phase with the source current.

This thread is too long for me to read through before midnight, but I hope someone has sorted you out on what I've quoted.

If for simplicity we consider a series circuit, then the current through every element is equal and exactly in phase with the current drawn from the power source. It's a series circuit, so the current at any moment in time is identical in every element. There is no phase shift in current anywhere in a series circuit.

In a series circuit, i_source(t) = i_R(t) = i_C(t) = i_L(t)[/color]

 

[Evil Bunny]

I guess you might also want to describe resistance as 'a sort of force stopping the current from flowing' (even worse!).

Yeah... now you're getting it! It is kind of like that, isn't it? Not so much a "force" but more like a constriction or a road block type of thing getting in the way of those electron race cars that are zooming down the tracks

The bottom line of all this discussion is to ask the question - how many people who design circuits for a living and who make Engineering Systems work do it without Maths and by waving their arms about? .

Actually that's not the bottom line of this discussion at all... You have completely missed the point, even after it has been explained repeatedly.

 

[wbeaty]

"is it because voltage causes current?"
Be careful not to make voltage into some kind of 'push'. Remember the actual definition of a volt. 1 Joule per Coulomb.

The question is, is IsaacBinary basing his reasoning on the idea that "voltage causes current?"

 

[sophiecentaur]

I don't think it has been explained adequately that it's somehow better to hobble oneself by refusing to use a more powerful language. Why not try Latin with Roman Numerals at the same time?

 

[sophiecentaur]

This thread is too long for me to read through before midnight, but I hope someone has sorted you out on what I've quoted.

If for simplicity we consider a series circuit, then the current through every element is equal and exactly in phase with the current drawn from the power source. It's a series circuit, so the current at any moment in time is identical in every element. There is no phase shift in current anywhere in a series circuit.

In a series circuit, i_source(t) = i_R(t) = i_C(t) = i_L(t)[/color]

Sorry. Maths alert. You aren't allowed to state it in that succinct way. ;-)
Wave your arms about a bit, if you want to be accepted.

 

[IssacBinary]

sophiecentaur, This is the last time I am going to say this point.

Am am NOT saying I CANT do the maths. I am not saying you DONT need maths. I am NOT saying I am stuck with the maths.

I am saying that ONLY using maths is not the way to get a full understanding.

As I've already demonstrated, I CAN use the maths.

I completely agree you CAN NOT design systems without maths.

Im saying that MATHS ON ITS OWN IS NOT going to explain what is happening.

The MATHS compliments an explanation of what's happening.

es1. I answered your question of explaining the geometry problem without maths.
https://www.physicsforums.com/showpost.php?p=3381555&postcount=56

But no one has commented to say either they see my point or disagree. It just seems like its been skipped even though that's what you asked me to do...

As per my orignal question and my example on my previous post. I am not asking for an explanation why the "total phase" is -56.3 and not -51.2. I am also not asking for an explanation for why its exactly them numbers.

Im ASKING for an analogy / explanation of what's going on to cause this effect. Then you apply the maths.

This thread is too long for me to read through before midnight, but I hope someone has sorted you out on what I've quoted.

This seems to be the main problem. It seems like most people who are posting really are just posting what they think they have read.

Sometimes it seems like no one is actually reading my posts and just skipping big chunks which is why we are going round in circles sometimes.

So for everyone whos just jumped to this page.

This post explains it the best.
https://www.physicsforums.com/showpos...3&postcount=19
Then read this post
https://www.physicsforums.com/showpos...3&postcount=48
It has a nice big picture.

So, in my big picture...my first drawing. Is that acceptable?

The question is, is IsaacBinary basing his reasoning on the idea that "voltage causes current?"

wbeaty, from all my learning and what I've been taught. I would have to say yes.

You can't have a current without a voltage can you?

When I ask that and people say to me "phase shifts". I say, but the phase shifted current is created out of an overall effect of different components voltages doing different things. But when we plot this on a graph we plot voltage X or voltage Y and then the phase shifted current which is caused by X and Y working together. "Resultant" / "superposition" / "tug of war" how ever you call it. While it may look like on the graph that there could be an instance where there is a current and no voltage. Its just no voltage at X or Y. Thats why we say, WITH RESPECT TO SOURCE VOLAGE. But there are other voltages in the circuit which are changing how it behaves.

So is that understanding wrong?

Im happy to accept I am wrong, that's not the problem but I would like it to be explained the correct way. Just throwing formulas out there does not help one bit.

Im not trying to design a new CPU by throwing my arms about or using anologys and no maths. I can DO the maths and I say that you 200% NEED maths. YOU DO NEED MATHS NO QUESTION ABOUT IT. But maths alone will not tell you what is happening inside.

As for a resistor.

You can think of it that it is a dense material or a material with higher friction so engery is lost by the electrons to get through it.

Actually, sophiecentaur, where has anyone "waved their arms" about? Can you show me?

It seems like most of the frustration is coming because people arnt really reading my posts and question.

Also wbeaty, I've seems a few places here and there about "current drawing" , "loads" etc but I've never come across any learning material about these effects / properties or anything explained in those ways. So maybe I am missing something?

But even if I am missing something, my question still stands.

So can we work together to come up with an explanation / analogy on how and what causes a total phase shift with RESPECT to source voltage.

 

[FOIWATER]

Exactly! And what you just explained was not my orignal question. I have said many times I UNDERSTAND the phase inside the capacitor.

Also many times I have explained my question and what I do want explained.

OVERALL PHASE SHIFT.

This post explains it the best.
https://www.physicsforums.com/showpost.php?p=3377063&postcount=19
Then read this post
https://www.physicsforums.com/showpost.php?p=3381323&postcount=48
It has a nice big picture

You mean if you had an RC circuit you want to know why the Phase is shifted at a certain angle (other than 90 degrees) and how that circuits resistance capacitance values account for a shift in overall phase?... but you want a purely physical explanation of what's happening?... I don't think you will find one.

 

[sophiecentaur]

This has gone on far too long. We do agree that Maths is a good thing. I can even agree that you need to enter and exit the Maths bit in a valid way (you can prove anything if you don't use the right Maths).

But, when you ask "what's going on inside", I just couldn't start on that question without calling on Maxwell's equations. How, without Maths, would I be able to describe the equivalent of Curl??
The next best thing is at the 'component' level, to describe how current and volts are related for a Capacitor and how current and volts are related in a resistor and then set up the boundary conditions at all the points in the circuit. Why would you Want to do all that without Maths? It's like trying to run with your shoelaces tied together.

We all have little 'private models' in our heads to get us round some of the quirky bits of Science but trying to communicate these highly personal ideas is loaded with the danger of misunderstanding. I have been there with two students, each of whom wants his own model used for the explanation. I may manage to see their two points of view but they never see how the other person's model applies. That's one of the best reasons for using Maths as your common language. It's already been vetted by the scientific World and is, for good reason, used as the means for communication as well as the means for working stuff out.

The answer to the final (repeated) question above is that the simple application of the network equations will tell you the current and volts everywhere in a circuit, however complicated. The non-maths explanation really does fall down if you try to discuss more than a single RC combination - there would be just too many 'parentheses' in the description of what happens to what and why and your audience would be lost / go to sleep during the explanation.

There's a very good reason for all singing from the same hymnbook.

 

[IssacBinary]

sophiecentaur, again it seems like you really havnt been reading my posts. Or atleast not fully.

As there several things in my posts, some for you, but just isn't replied to. Instead people are just replying "generalised" posts but no reply to what I might have said.

Im not looking for an explanation which will allow me to go and develop a complex circuit. I am just looking for an overview / general concept of why the phase is shfted at a certain angle which is what foiwater picked up on.

No one seems to have problems with the phsyical models or anologys or explanations of things that are already out there. Capacitors etc. So what is different about what I am asking.

Yes, this thread has gone on very far but I have NOT seen ANYONE try to work with me / together to fix my understanding / come up with an anology / explanation together.
Apart from wbeaty and a few others, All I see is people claiming how it can't be done and how maths is the only thing needed. Just like your students, perhaps you can't accept this can be done / you don't understand yourself?

I ask you, please read my posts fully.

 

[sophiecentaur]

Yes, this thread has gone on very far but I have NOT seen ANYONE try to work with me / together to fix my understanding / come up with an anology / explanation together.
Apart from wbeaty and a few others, All I see is people claiming how it can't be done and how maths is the only thing needed. Just like your students, perhaps you can't accept this can be done / you don't understand yourself?
.

Perhaps you can take this as a hint that you may be on a hiding to nothing. As was mentioned earlier, by someone, there are all levels of ability on these forums (a real luxury, I'm sure you'll agree) so perhaps this general opinion you are getting back is the answer to your request.
Firstly, the Maths does it so much better and, secondly, no one wants to stick a head above the parapet and give us, publicly, their own pet models, for fear of looking daft. But I am sure we all use them 'in our heads'.
I, for example, always tend to draw (or even re-draw) circuit schematics, when it's practicable with the higher potential bits at the top and the lower (-) bits near the bottom. 'Downhill' and 'Uphill' help me with things. But it can let you down terribly - particularly where electrolytic capacitors in DC feedback loops are concerned.

 

[wbeaty]

The bottom line of all this discussion is to ask the question - how many people who design circuits for a living and who make Engineering Systems work do it without Maths and by waving their arms about?

If you don't mind my asking, what's your background? Physics student? Degree? Done any analog design professionally?

 

[wbeaty]

The question is, is IsaacBinary basing his reasoning on the idea that "voltage causes current?"

wbeaty, from all my learning and what I've been taught. I would have to say yes.

Then that's another conceptual sticking point. Yes, in resistors, voltage DOES essentially cause current (meaning that e-fields accelerate the resistor's mobile charges.)

It also works the same way for real-world metal conductors. The current in a wire is always caused by a (tiny) voltage applied lengthwise.

But inductors and capacitors are different. In these, the applied voltage doesn't directly cause current. L and C devices ...they're very weird.

You can't have a current without a voltage can you?

Sure you can: a bunch of charges flowing in a ring-shaped conductor are analogous to a flywheel.

If you get this 'flywheel' going at high amperes, then disconnect it from everything, the currents persists for a time. If you used ideal zero-ohm wire, the current will continue forever, a flow without a voltage. If you used actual realworld superconductor, the current will keep going, and only decay over billions of years. It's a kind of electromagnetic inertia, fast moving charges, zero drive volts.

So is that understanding wrong?

Yes.

And also no. In capacitors and inductors, the currents and voltages are divorced from each other. It's easy to produce currents without voltage, and voltages without currents.

It's similar to flywheels: they can keep spinning without any drive force. It's similar to balloons, they can remain pressurized without needing any flow. But still you'd need a brief force to initially accelerate your flywheel up to speed. And still you'd need a brief flow of air in order to inflate your balloon initially. The V and the I are separated in time, and don't occur together.

Also wbeaty, I've seems a few places here and there about "current drawing" , "loads" etc but I've never come across any learning material about these effects / properties or anything explained in those ways. So maybe I am missing something?

Bingo, another sticking point. Resistors are associated with two separate concepts:

1. If you have a constant current in an ideal conductor, and then you cut the conductor and insert a resistor, a voltage drop will appear. You started out with a pure current and no voltage. The resistor opposes the flow and causes the voltage-drop to arise. It's like sticking your hand in a rushing creek and experiencing a force.

2. If you have a constant voltage between two conductors, and then connect a resistor across them, the resistor "draws a current." You started out with a pure voltage and zero current. By adding the resistor, you provided a leakage path between the conductors, causing a current to arise. It's like puncturing a balloon and experiencing an air jet.

Resistors can be leakage paths on constant voltage supplies. Or they can be opposers-of-current in a constant-current power supply. (All these concepts are hidden in intro DC engineering texts on "Thevenin equivalent," and CC and CV supplies.)

Mechanical analogies:

1. if a flywheel is spinning, and you let it rub against your finger, this creates a force which makes the flywheel speed start slowing constantly.

2. If you have a pressurized container, and you drill a small hole in it, this creates a flow which makes the pressure start falling constantly.

In education research, the first one is called "current based reasoning," and most of us learn this version in grade school. "Batteries create current electricity." "Light bulbs consume the current." In physics class, the challenge for the teachers then becomes this: get all the students' minds loose from no. 1 above. They have to be freed up so they can learn "voltage reasoning" as in no. 2.

Your question about phase lies within the domain of no. 2, because capacitors have two functions as well.

1. When inserted into a constant current, a capacitor opposes the current and creates a rising voltage-drop across the capacitor.

But here I think is the key you've been missing:

2. when a capacitor is connected across a voltage-based power supply, it "draws a current." Or said more conventionally: the capacitor's current is proportional to the slope of changing supply voltage.

Suppose you connect a capacitor to a triangle wave voltage generator. When the triangle voltage is smoothly rising, the capacitor draws a perfectly constant current. (The current will be larger for a high-value capacitor.) And, when the triangle voltage is dropping, the capacitor draws a negative current. The upshot: apply a triangle-wave voltage across a capacitor, and the capacitor draws a square wave current from the power supply.

Analogy: if you connect a balloon to a regulated supply of air pressure, then slowly adjust the supply pressure upwards, the balloon will draw a constant air flow as it inflates. Next, adjust the regulated pressure downwards, and the balloon will deflate as it pumps a constant unchanging air flow back into the supply.

 

[sophiecentaur]

If you don't mind my asking, what's your background? Physics student? Degree? Done any analog design professionally?

Physics degree.
24 yrs research engineering with some analogue design and 're-design' / fault finding.
20 yrs teaching GCSE / A level Physics
Fwiw :-)

 

[sophiecentaur]

@wbeaty
Great last post btw. It shows that the shorthand statements people make are full of misconceptions and often only deal with the specific.

 

[IssacBinary]

Wbeaty, you know, you might have just done something right there.
Its late at the moment, 1:30AM, so I am not going to reply very big, ill save that for tomorrow. But I definitely think something is there. Just going to see if I can piece it together and come up with an explanation.

sophiecentaur, again, you seem to be completely ignoring my posts. Also, if your so so qualified as you say you are (im not saying your not) and you see I've got problems in my understanding why arnt you helping me out to fix it and perhaps work together to come up with an explanation?
You wouldn't be just helping me but anyone else who might be looking.

Again, this knowledge I am asking for, along with how a capacitor works and the reactance explanation...of course you don't need to know them to be able to do circuits. If you can use the formulas your good to go. So even if you have 100 years building and design experience you still might not know what's going on.

But I am just asking so when I help explain it to other people and for myself its something to show why there's a lag.

 

[sophiecentaur]

I think the problem is yours, I'm afraid. Why do you not see the Maths (including graphs, if you like) as giving the best possible description of what's happening?
As I said before, what would be the point in insisting in having an explanation given to you using Latin which is a language which just doesn't have all the necessary words? Maths actually gives you a way of stating relationships in a very concise way. Why do you think it's used so universally? How could you say "Q=CV" in a less complicated way? Why would you want to? It requires that you know the vocabulary, of course, but that goes for the spoken version too.
Perhaps your problem (and I do believe it is a real problem that many people have) is that you believe that somehow there is an ultimate answer to all these questions which can be stated in familiar terms. The fact is that years ago, Scientists realized that words are not enough to describe the processes we see. Since Newton, the Maths has been a basic requirement for understanding things. Why do you not feel that it is adequate. You seem to be saying that it is an optional bolt-on to Science.

If you were describing the motion of an object under the influence of a force, would you insist on not using the basic equations of motion. How could you even start to explain how a simple trajectory is parabolic without involving Maths?

Has anyone else stepped into help you, seeing that I have proved to be so inadequate? Why would that be? I think you may just have to 'join the club' on this one. If you can do the maths then just go along with what it tells you. That's as near to understanding as we can hope for.

 

[wbeaty]

Physics degree.
24 yrs research engineering with some analogue design and 're-design' / fault finding.
20 yrs teaching GCSE / A level Physics
Fwiw :-)

If teaching, are you on PHYS-L physics teacher forum? How about TAP-L?

 

[sophiecentaur]

If teaching, are you on PHYS-L physics teacher forum? How about TAP-L?

Just this forum aamof. I must do some visiting.

 

[carlgrace]

Sure you can: a bunch of charges flowing in a ring-shaped conductor are analogous to a flywheel.

If you get this 'flywheel' going at high amperes, then disconnect it from everything, the currents persists for a time. If you used ideal zero-ohm wire, the current will continue forever, a flow without a voltage. If you used actual realworld superconductor, the current will keep going, and only decay over billions of years. It's a kind of electromagnetic inertia, fast moving charges, zero drive volts.

You don't even need a 'flywheel' to have a current. If you have an excess concentration of charge carriers in one area relative to another area you will have a diffusion current from the high-density area to the low-density area. This can be important in some contexts, most notably in solid-state image sensors.

 

[I_am_learning]

I want to put my point.
This seems to be battle of maths/no_maths.
But I ask, what's the difference.
If I start by saying " because, I = C d(Vc)/dt "
You would I scream, its math, I want physical explanation. Actually, I = C d(Vc)/dt is a explanation of physical phenomena, just stated concisely.
You can start like, in capacitor we store charges (electrons). They exert forces on each-other in inverse-square law (columbs law). When we try to push a lot of them into a metal plates (capacitor) they try to oppose. If we connect them to a source in series with a resistor (which acts like a bla bla...) then the electrons oppose ...
Bla bla bla... Due to the nature of electrons, capacitor seems to follow a strange behaviour, which can be accurately modeled by, I = C d(Vc)/dt.

So, instead of speaking the long paragraph each time, we use the mathmatical forumla. The funny thing is, not everyone knows or understands, the long paragraph but only the formula. Just like we accept the columbs law (Have you tested this theory? I haven't) as a fact that can be tested by experiment, Some people like myself, accept, I = C d(Vc)/dt as another fact that can be tested. Ofcouse, you don't need to accept both fact, one can be logically derived from other, but it simplifies my life and let's me make machines that do my job.

 

[sophiecentaur]

@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??

 

[IssacBinary]

I want to put my point.
This seems to be battle of maths/no_maths.
But I ask, what's the difference.
If I start by saying " because, I = C d(Vc)/dt "
You would I scream, its math, I want physical explanation. Actually, I = C d(Vc)/dt is a explanation of physical phenomena, just stated concisely.
You can start like, in capacitor we store charges (electrons). They exert forces on each-other in inverse-square law (columbs law). When we try to push a lot of them into a metal plates (capacitor) they try to oppose. If we connect them to a source in series with a resistor (which acts like a bla bla...) then the electrons oppose ...
Bla bla bla... Due to the nature of electrons, capacitor seems to follow a strange behaviour, which can be accurately modeled by, I = C d(Vc)/dt.

So, instead of speaking the long paragraph each time, we use the mathmatical forumla. The funny thing is, not everyone knows or understands, the long paragraph but only the formula. Just like we accept the columbs law (Have you tested this theory? I haven't) as a fact that can be tested by experiment, Some people like myself, accept, I = C d(Vc)/dt as another fact that can be tested. Ofcouse, you don't need to accept both fact, one can be logically derived from other, but it simplifies my life and let's me make machines that do my job.

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?

 

[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.