# Homework Help: Series-Parallel Capacitive Circuit

1. Dec 9, 2012

### freshbox

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

Thanks.

Last edited: Dec 9, 2012
2. Dec 9, 2012

### Staff: Mentor

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

3. Dec 9, 2012

### freshbox

lawl.. phew.. i thought i was wrong, i have been doing this question for the whole day..

4. Dec 9, 2012

5. Dec 9, 2012

### freshbox

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 alot 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: Dec 9, 2012
6. Dec 9, 2012

### Staff: Mentor

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.

7. Dec 9, 2012

### freshbox

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

Last edited: Dec 9, 2012
8. Dec 9, 2012

### Staff: Mentor

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.
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).

9. Dec 9, 2012

### freshbox

How do i make the dt go away?

10. Dec 9, 2012

### Staff: Mentor

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. Dec 9, 2012

### freshbox

ah.. i see. thanks for the explanation :)