# Ohms Law, Thevenin and Norton Theory

Discussion in 'Homework Help' started by davegulpilil, Mar 27, 2008.

1. ### davegulpilil Thread Starter New Member

Mar 27, 2008
3
0
I've been having quite a bit of trouble with a uni assignment, mostly due to the fact the supplied notes on the subject are less than sufficient. After a four hour struggle today i decided i needed outside help. Basically i don't really even know where to start, if someone could show me a method for the questions or just the required information to do the questions that would be a massive help. (I've also put the circuits into Pspice in an attempt to reverse engineer the questions but this was to no avail.)

Cheers,

Dave

The Questions are in the attatched file.

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2. ### S_lannan Active Member

Jun 20, 2007
247
2
using the all about circuits textbook and google you should be able to find the techniques to figure out how to solve these.

you need to read network analysis on the DC chapter.

3. ### S_lannan Active Member

Jun 20, 2007
247
2
to find a thevenin network's voltage and resistance
calculate the output voltage of that network unloaded
calculate the resistance by setting the supply voltage to 0. (ie short the supply).

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
170
Hi Dave,

Have a look at our DC Network Analysis section: http://www.allaboutcircuits.com/vol_1/chpt_10/index.html this will cover most of your questions.

Q2/Q3 use the Branch Current method: http://www.allaboutcircuits.com/vol_1/chpt_10/2.html, obviously you need to be aware of KCL, KVL and P = VI = I^2*R = V^2/R

Q6 a) Not sure what how you can derive the resistance based solely on the cable length and cross-section. You would need to know the resistivity.

Q6 b) Is just Ohms law using the above resistance

Dave

5. ### S_lannan Active Member

Jun 20, 2007
247
2
i'd assume the conductor used for microphone cables is copper and calculate it according to copper using the relative conductivity of the mentioned material.

6. ### Dave Retired Moderator

Nov 17, 2003
6,960
170
I wonder if these questions have any preamble (the attached questions start at Q2 so this is a possibility) that can shed any light on this.

Dave

7. ### davegulpilil Thread Starter New Member

Mar 27, 2008
3
0
Thanks for the help. Though i am still having a bit of trouble........

(In reference to Q2)
So using the branch current page you provided I have got to the point where i have three equations, which are as follows:

-I1+I2-I3=0
-25+3.9I2+15I1=0
-3.9I2+12V-10I3=0

I have tried solving these using the substitution method for simultaneous equations but there is always two unknowns left, whichever way i try it.

(I've had a go using this method with the example in the branch current chapter and I got the right answers.)

8. ### davegulpilil Thread Starter New Member

Mar 27, 2008
3
0
I also still don't see what Q5 has to do with Thevenin and Norton circuits. Don't even know where to start with that one.