Series-Parallel Capacitive Circuit

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Discussion Overview

The discussion revolves around a series-parallel capacitive circuit problem, specifically focusing on the calculations related to charge and voltage in the circuit. Participants are checking their answers for specific parts of a problem and seeking clarification on related concepts, including the rate of change of influx and the use of formulas from their lecture notes.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant reports a calculated charge of 7.26uC for part (iv) and seeks confirmation of its correctness.
  • Another participant agrees with the charge result but suggests a slight correction to the voltage answer for part (iii), proposing it should be 4.24V instead of 4.26V.
  • Several participants express confusion regarding the formulas for the rate of change of influx, noting discrepancies between their textbook and formula sheet.
  • One participant asks for clarification on which formulas to use for specific calculations, indicating uncertainty about ignoring certain formulas from the book.
  • Another participant explains the significance of two specific formulas related to voltage and current in circuits, emphasizing their practical application.
  • There is a discussion about the phase angle in relation to the induced voltage, with one participant questioning whether differentiation is necessary.
  • Clarifications are provided regarding the notation for flux and the rate of change of flux, with emphasis on the correct representation in formulas.
  • Participants inquire about how to manipulate the expression for the rate of change of flux with respect to time.

Areas of Agreement / Disagreement

Participants generally agree on the correctness of the charge calculation but express differing views on the voltage calculation. There is ongoing confusion and debate regarding the appropriate formulas and concepts related to the rate of change of influx, indicating that multiple competing views remain unresolved.

Contextual Notes

Participants reference various formulas and their derivations, indicating potential limitations in understanding how these concepts interrelate. There are also unresolved questions about the application of certain formulas and the interpretation of phase angles.

freshbox
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Hi there, can someone help me check if the answer for part (iv) is correct? I got 7.26uC.


safari.jpg



Thanks.
 
Last edited:
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Yes, your result looks correct. Also, the "ANSWER" for part (iii) looks slightly off, too. Should be more like 4.24V rather than 4.26V.

So the Answer key guy gets 6 marks out of 11 from me :smile:
 
lawl.. phew.. i thought i was wrong, i have been doing this question for the whole day..
 
Glad I could help!
 
gneill thanks for your help so far, can I ask you some questions on the rate of change of influx? I have been reading through my lecture notes but I can't seem to understand how should I go about it.

I am confused. My book is showing a lot formula, whereas the formula sheet is showing a different one. Can you explain to me which formula is for finding which value?Thank you.

http://i303.photobucket.com/albums/nn129/icefrogftw/Picture10.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/Picture11.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/Picture12.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/Picture13-1.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/for.jpg

I know some part of the book is showing how the formula is being derived. So should i ignore formula (4),(5),(6) and just go with (1),(2) from the formula sheet?
 
Last edited:
freshbox said:
gneill thanks for your help so far, can I ask you some questions on the rate of change of influx? I have been reading through my lecture notes but I can't seem to understand how should I go about it.

I am confused. My book is showing a lot formula, whereas the formula sheet is showing a different one. Can you explain to me which formula is for finding which value?


Thank you.

http://i303.photobucket.com/albums/nn129/icefrogftw/Picture10.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/Picture11.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/Picture12.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/Picture13-1.jpg
http://i303.photobucket.com/albums/nn129/icefrogftw/for.jpg

I know some part of the book is showing how the formula is being derived. So should i ignore formula (4),(5),(6) and just go with (1),(2) from the formula sheet?

The two formulas on your Formula Sheet summarize the important results of the derivations in your book; they are the two formulas that you will be likely to use most often.

The first one, ##V_L = L\frac{di}{dt}## relates the voltage induced across an inductance of value L to a change of current through it. This will be handy when writing the differential equations for circuits where the variables of interest are voltage and current (think Kichhoff's laws).

The second formula, ##V_L = N\frac{d\Phi}{dt}##, will be handy when you're looking at voltages induced in an inductor by external magnetic fields. This comes up when looking at solenoids, transformers, loops of wire in changing magnetic fields, and so on.

The book's derivations show how these items are interrelated, so that you aren't left thinking that they are mutually exclusive concepts. You may come across situations where understanding this interrelationship will be important, but mostly you'll use these two formulas in their given form.
 
For this question, I would use VL=N∅/dt.

VL=200
N=500

How do I find the phase angle? Do i need to differentiate the phase angle? ∅/dt

ap.jpg
 
Last edited:
freshbox said:
For this question, I would use VL=N∅/dt.
Make that a capitol ##\Phi##, the standard symbol representing flux, and it should be ##\frac{d\Phi}{dt}## in that formula, the rate of change of flux with respect to time.
VL=200
N=500

How do I find the phase angle? Do i need to differentiate the phase angle? ∅/dt

There's no phase angle. ##\Phi## represents the flux. ##\frac{d\Phi}{dt}## is the rate of change of flux, and happens to be what the question is looking for (the value that ##\frac{d\Phi}{dt}## takes on with the given number of turns and the resulting induced voltage).
 
How do i make the dt go away?
 
  • #10
freshbox said:
How do i make the dt go away?

You don't have to. ##\frac{d\Phi}{dt}## as a whole is what you're looking for; it's the rate of change of the flux with respect to time. Replace it with ##\dot{\Phi}## (##\Phi## with a dot over it) if you wish to give it a variable name...
 
  • #11
ah.. i see. thanks for the explanation :)
 

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