Help with this network analysis question

[Pratik77]

given:
source voltage =120V
voltage across 15 ohm resistor = Va
there is a dependent source 2Va
and a 30 ohm resistor.


I tried applying KVL i.e 120-30I+2Va-Va=0
where Va=15I
and got I=2.66A
whereas my teacher told me that the answer is 8A. how is it so? Is it some basic knowledge that I have overlooked?
Also as a side note, I have difficulty knowing which theorem to apply in which circuit. I do practise problems but even then I have difficulties especially in chapters like 2-port network. Are there any tips regarding that?

 

[CWatters]

I tried applying KVL i.e 120-30I+2Va-Va=0
where Va=15I
and got I=2.66A
whereas my teacher told me that the answer is 8A.

Show your working. I think you just made an arithmetic error. I used your equations and get 8A.

 

[Pratik77]

Umm.. yep you are correct. what would have been the current if the polarity of 2Va was reversed?
Do you have any advice on how to deal with two port network problems. I just don't know how to approach the problems for the asked parameters(z,y,h,etc.)

 

[CWatters]

Umm.. yep you are correct. what would have been the current if the polarity of 2Va was reversed?

I made that 1.6A.

Do you have any advice on how to deal with two port network problems. I just don't know how to approach the problems for the asked parameters(z,y,h,etc.)

Usually it's a matter of remembering if you should short or open circuit a port and then working out the effect of that on the circuit. Perhaps print out some of the slides from here to have as a reference?..

http://www.slideshare.net/PinkyChaudhari/two-port-network

If I've misunderstood perhaps start a new thread with an example.[/quote][/QUOTE]

 

[Pratik77]

Thanks!