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All physics textbooks wrong in the setup and derivation of the RLC series circuit equation?

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DerAlbi

Tue Jun 16 2015, 03:27PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

My problem is currently, that with Kirchhoff i can clearly describe a RLC circuit, its eigenfrequency and its amplitude decay over time given a certain starting condition by solving the differential equations.. And all this can be measured and validated with an oscilloscope..
And you actually tell me that Kirchhoff is wrong in the RLC case... thats hard to agree on. Kirchhoff is used regularly in circuits with inductive components and it allways works.

by assuming that the closed path integral Eâ€¢dl = 0 (it is actually -L di/dt) and that the integration Eâ€¢dl along only the inductor is -L di/dt (it is actually zero).

How? Who? Where? What?

Could you please make a proper cicuit diagram, and apply kirchhoff and tell me what you think along the way?

What would be specially interesting is when you write all the equations like
Vr = I*R
VL = L*dI/dt
Vc = I/C*dt
for Kirchhoff at the individual components, where your ring- and area-integrals come into mind...
(i honestly dont see why: Kirchhoff is circuit analysis, your integrals over E and B and stuff is NOT circuit-level)

And this is really not about disagreement. I actually can not follow your mixup like the E- and B-field in relation to a circuit. In a circuit an inductor is just an impedance where VL = L*dI/dt holds. Like the capacitor is an impedance where Vc=I/C*dt applies.

Signification

Tue Jun 16 2015, 06:53PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

...like I said DerAlbi, I don't know what else to say to you. You are simply mis-applying Kirchhoff's law You can find a "good" physics text, study what I have said as many ways as I know how, or find a youtube video that properly applies Faraday's law to a circuit containing an inductor. Since you can't seem to accept anything I send, I spent ten minutes and found this:

I have bent over backward for you on this subject--I write what I know, how I know. You are just running me in circles here--and I don't have time for that. So, I just hope someone will chime in and do better.

Uspring

Tue Jun 16 2015, 07:10PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Signification has a point, but a minor one. If you consider a circuit of say a 10mH inductance a 10uF capacitance and 1kohm in series, arranged in a loop of say 10cm diameter and driven e.g. by a sine generator, the loop made out of these components will itself have an inductance of a few 100nH. To calculate the circuits correct behaviour, one would have to replace the 10mH by 10.0003mH.

There are many textbooks out there explaining Newtonian mechanics. Newtonian mechanics is wrong, as known since Einstein. The point is, that Newtonian mechanics works very well in many cases, but is much more simple.

DerAlbi

Tue Jun 16 2015, 08:51PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Uspring, thats exactly what i say but that does not satisfy signifiaction.
I tell him, if the loop is an inductor you have to put that (coupled) inductor in the schematic but thats somehow not enough. Hes fine with adding ESR, parasitic capacitances, and he can even consider ESL of capacitors and stuff.. but the parasitic inductance in the circuit seems a no-go to add.
This is perfecly my short circuited resistor above. if the short circuit wire is considered a loop and there is coupling happening, then the resistor needs to be short circuited by a transformer, not a simple wire anymoe - the model/circuit would be wrong. But somehow this seems wrong for signification...
I really do not know what we can add to this...

Signification

Tue Jun 16 2015, 09:16PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Uspring wrote ...

There are many textbooks out there explaining Newtonian mechanics. Newtonian mechanics is wrong, as known since Einstein. The point is, that Newtonian mechanics works very well in many cases, but is much more simple.

Of course! It would be ridiculous to do this in any other way for "everyday" applications. However, I have found it just as easy in most "everyday" circuit problems like we are discussing to use Faraday as opposed to Kirchhoff, even where Kirchhoff holds true. Since I learned the proper use and "respect the way", I tended to not use an "invalid" analysis, even though this established 'incorrect' method does give the proper answer--at the expense of thinking wrongly. Strangely, for Kirchhoff, this has become (or was always) accepted, even in the texts. I am noticing, recently, that the proper method and its realization are slowly emerging. Of course it would be insane to use relativistic-corrected equations in an classical application of the motion equations.

IMHO, a full understanding of the Faraday / Kirchhoff laws is very valuable in the E&M fields

DerAlbi

Tue Jun 16 2015, 09:39PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

...and that youtube video... what is your problem with it? I dont see where Kirchhoff does not apply.
In the end (the RL-circuit) he actually comes to the statement that the voltages add up to 0.

Question: may i ask what kirchhoff is for you?
For me there are 2 fundamental rules:
-At a circuit junction, the currents add up to 0A.
-In a cicuit loop, the voltages add up to 0V.

and that basic statement is actually shown in the endscreen of the video.
To proove that Kirchhoff is wrong? Sry, i still dont get it.
He adds up the voltage over the resistor, and the timedependend voltage over the inductor... and adds it up to 0. that invalidated kirchhoff how again?? *whooooooot*

Signification

Tue Jun 16 2015, 10:15PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

The video is correct, I thought you would see that--that's why I linked it for you. Then in your reply you say--"he used magnetic field lines that don't connect". I wanted to know what you meant by that statement--but when I went right back to quote that statement you had edited it out and it -now- seems you decided to agree with the video and are asking me what MY problem was with it???????????? I am interested in straight learning and teaching, you play dirty!

DerAlbi

Tue Jun 16 2015, 11:04PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I dont agree with his simplifications.. they were strange but lead to his goal somehow, thats right. i decided that my feelings about the lecturer dont matter and you allready replied, so i added a new post.

But can you now tell me how this invalidates Kirchhoff? You say that the video is correct, but you also say that i am using Kirchhoff wrong and it does not apply...

I wrote the Voltages like this:
Vr = I*R, VL = L*dI/dt, Vc = I/C*dt
And the Video used the same notations, set the sum of all voltages to zero (which implements Kirchhoff) and is correct in the end...
Is me beeing wrong just a matter of personal dislike?

Could you pleeeaase shed some light on this?

Signification

Wed Jun 17 2015, 12:25AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

DerAlbi wrote ...

I wrote the Voltages like this:
Vr = I*R, VL = L*dI/dt, Vc = I/C*dt
And the Video used the same notations, set the sum of all voltages to zero (which implements Kirchhoff) and is correct in the end...
Is me beeing wrong just a matter of personal dislike?

Could you pleeeaase shed some light on this?

DerAlbi:
Use:
1) VL = 0 And
2)integral Eâ€¢dl= -L di/dt
BTW: for the capacitor use VC = q/C, NOT Vc=I/C*dt ...I think you meant to write Vc=I*dt/C above

Now set up the equation using 1) and 2). What do you get? (post it--we can start here)

Are you sure in the video the sum of the voltages was set to zero?? and not -L di/dt ? I'll replay it

DerAlbi

Wed Jun 17 2015, 01:19AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

1) VL = 0

WHY should i use that? That is only true if the current is constant. Its called a steady state. And its actually expressed by VL = L*I/dt. -> if I is steady I/dt == 0. But its simply not true. The full truth is VL = L*I/dt.
If all textbooks are wrong about Kirchhoff, i would like to know any textbook that sais that the voltage over a coil is unconditionally zero....

BTW: for the capacitor use VC = q/C, NOT Vc=I/C*dt ...I think you meant to write Vc=I*dt/C above

Sry, what is the difference between Vc=I/C*dt and Vc = I*dt/C ? 1/2*3 = 1.5. 1*3/2 = 1.5 And NO, even some americans tend to read it wrong, there is not bracket implied by the notation... if i want a bracket there, then i would have written it. (I assume you read it the **** american way like Vc=I/(C*dt). There is no reason for that - multiplication and division do have the same priority (Both over + and - and below the "power-of"-operator)

VC = q/C

...and q is the integral of the current over time, right? so... q = I*dt. you get Vc=I*dt/C ....or I/C*dt for that matter

Now set up the equation using 1) and 2). What do you get? (post it--we can start here)

Sry to deny your request, i wont participate in such BS. 1) is wrong. 2) is true. Unfortunately mixing false and true adds up to false in this case.

Are you sure in the video the sum of the voltages was set to zero??

a few seconds before Endscreen.

And pleeeeas that guy is kind of a dude who should not teach..... In his last example he charges the coil to steady state current so that there is no voltage drop across the inductor and uses that VL=0 as condition for non steady state analysis.
The only thing that is true in that case is that dI/dt equals 0 which implies VL=0, but writing only Zero and neglecting the true mathmatical formula is... i can not tell you that in words. i actuallly can understand your confusion if you watch stuff like that.

Again and forever: The voltage across a coil is determined by its inductance and the rate of current change only. If the current change happens to be zero - fine, coilvoltage is then zero, BUT that is in no way the general case and should NEVER be asumed in a non-steady state.

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