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 Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help.

#1
10-20-2013, 11:25 AM
 Sgt.Incontro Junior Member Join Date: Dec 2012 Location: (GMT + 0) Posts: 47
RLC Circuit Current

Hi all,

I need help with the first question from this problem sheet I am doing. Of course I am not asking anyone to just post the answer, but some guidance in the right direction would be appreciated. That way I can attempt it, and post my answer for scrutinizing ...

Question:

Thank you.
#2
10-20-2013, 12:32 PM
 WBahn Senior Member Join Date: Apr 2012 Location: Larkspur, Colorado Posts: 8,068 Blog Entries: 9

Let's do it the other way around. Post YOUR best attempt to solve YOUR homework problem -- it doesn't have to be complete or correct, but it does have to be present. We can then use that to see where you are and then provide feedback to help move you in the right direction.
#3
10-20-2013, 12:59 PM
 Sgt.Incontro Junior Member Join Date: Dec 2012 Location: (GMT + 0) Posts: 47

Quote:
 Originally Posted by WBahn Let's do it the other way around. Post YOUR best attempt to solve YOUR homework problem -- it doesn't have to be complete or correct, but it does have to be present. We can then use that to see where you are and then provide feedback to help move you in the right direction.
Hi WBahn,

I think the lecturer only wants a brief description of what happens, and not a complex mathematical answer.

Of course I am able to provide you with what I think:

To begin with, for part a), I wrote:

When voltage is first applied after closing the switch (at t=0), the inductor impedes current flow in the circuit, so the current = 0. After time t=0, the current will follow a second order natural response characteristic as shown below

((P.S: I may be also interested in the mathematical answer later on.))
#4
10-20-2013, 10:48 PM
 WBahn Senior Member Join Date: Apr 2012 Location: Larkspur, Colorado Posts: 8,068 Blog Entries: 9

Quote:
 Originally Posted by Sgt.Incontro Hi WBahn, Thanks for your reply. I think the lecturer only wants a brief description of what happens, and not a complex mathematical answer. Of course I am able to provide you with what I think: To begin with, for part a), I wrote: When voltage is first applied after closing the switch (at t=0), the inductor impedes current flow in the circuit, so the current = 0. After time t=0, the current will follow a second order natural response characteristic as shown below ((P.S: I may be also interested in the mathematical answer later on.))
You answer is basically correct, but it would be good to indicate if the initial current (after t=0+) will be positive or negative.

How about a shot at part b)?
#5
10-21-2013, 12:35 PM
 Sgt.Incontro Junior Member Join Date: Dec 2012 Location: (GMT + 0) Posts: 47

Quote:
 Originally Posted by WBahn You answer is basically correct, but it would be good to indicate if the initial current (after t=0+) will be positive or negative. How about a shot at part b)?
Hi WBahn,

I will post my answers for b) once I get home, are you able to just write the characteristic equation for part A or describe it so I know whether my answer is correct? Thanks.
#6
10-22-2013, 12:28 AM
 WBahn Senior Member Join Date: Apr 2012 Location: Larkspur, Colorado Posts: 8,068 Blog Entries: 9

Quote:
 Originally Posted by Sgt.Incontro Hi WBahn, Thanks for your reply. I will post my answers for b) once I get home, are you able to just write the characteristic equation for part A or describe it so I know whether my answer is correct? Thanks.
Ah, no. Post your answer and we will look it over and let you know if it is correct and, if not, give you hints to help you find what is wrong with it and how to proceed. And again, we don't require full-up solutions. Just get as far as you can and show your work up to that point.
#7
10-22-2013, 10:23 AM
 Sgt.Incontro Junior Member Join Date: Dec 2012 Location: (GMT + 0) Posts: 47

Quote:
 Originally Posted by WBahn Ah, no. Post your answer and we will look it over and let you know if it is correct and, if not, give you hints to help you find what is wrong with it and how to proceed. And again, we don't require full-up solutions. Just get as far as you can and show your work up to that point.
Thanks. I only asked if you could do that so it would save time - as this is due very soon. (If I were to cheat with this question, I would only be cheating myself.)

Anyway, I have scanned my (mathematical) solutions for part a) and part b).

I will also upload the written explanation for part b) as I said yesterday.
#8
10-22-2013, 10:30 AM
 Sgt.Incontro Junior Member Join Date: Dec 2012 Location: (GMT + 0) Posts: 47

Solutions:

As soon as both s1 and s2 are closed (at t=0), the current will be equal to V(0)/R, as the infinite instantaneous impedance of the inductor will act as an open circuit.

Part a) & b) mathematical answers:

http://postimg.org/image/somif17fl/

EDIT: Realized the silly mistake in part b) where I did the parallel impedance wrong. (Ignore that.)

Last edited by Sgt.Incontro; 10-23-2013 at 06:17 AM.
#9
10-26-2013, 02:21 PM
 Sgt.Incontro Junior Member Join Date: Dec 2012 Location: (GMT + 0) Posts: 47

Bump...... (>10char)

 Tags circuit, current, rlc

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