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Come on you EE people

K

kevinthenerd

Platinum Member
I'm sure there are a LOT of people on here who can help me.

Can you briefly describe for me how complex numbers can be used for circuit analysis? I'm a mechanical engineering major having a hell of a time in Principles of Electrical Engineering, and when this A+jB crap shows up I'm lost. I know complex numbers, but I'm having a hard time realizing just how they work for this and what the physical significance of them is.

Thanks. I already googled for quite a while now. The final is Thursday. Right now, I'm off to study for tomorrow's exam in a different subject.
 
you have a final on AC analysis using sinusoids and phasors and you don't know what they are used for? I don't really think it's something that someone can explain to you really quickly. However, it's really not that bad at all if you need to learn it on your own.
 
the short answer is that complex numbers are really practical for math involving sinusoids of different phases. Just the kinds of thing you would need to analyze RLC networks.
 
Do you have a good calculator? Makes solving those RLC circuits quite easy.

Now, how about someone explain to me the modeling and operation of semiconductor devices?😛 Damn me for picking the class that fit my schedule! Oh well, at least the prof told me yesterday I'd pass😀

ONE MORE FINAL!!!!
 
Complex numbers come from the Laplace/Fourier transforms. It's a hell of a lot easier to work with jwL than L*di/dt.

Okay, it's like this, I could turn a bolt with my hand, but it's a lot easier to use a wrench. 🙂 The complex numbers make the math really easy.
 
Transistors, MOS devices, Schottkey devices, etc. I struggled through the required course and "wisely" took the senior level elective😛

And I agree with JohnCU with using the 1/jwC and jwL. Makes infinitely easier.
 
Originally posted by: NutBucket
Transistors, MOS devices, Schottkey devices, etc. I struggled through the required course and "wisely" took the senior level elective😛

And I agree with JohnCU with using the 1/jwC and jwL. Makes infinitely easier.
Click to expand...

Sucker! Hand calculations of bulk-charge modelled MOS devices were the bane of my existance last semester.

Although things aren't much better calculating field distributions in optical fibers this semester. Three cheers for Bessel functions!
 
Originally posted by: NutBucket
Do you have a good calculator? Makes solving those RLC circuits quite easy.

Now, how about someone explain to me the modeling and operation of semiconductor devices?😛 Damn me for picking the class that fit my schedule! Oh well, at least the prof told me yesterday I'd pass😀

ONE MORE FINAL!!!!
Click to expand...

NOoooo I have my semiconductor final tommorow. My last core class! Only 3 more electives remaining and they're all computer 😀

Anyway, phasors let you look at the steady state response of a system easily, without laplace transformations or differential equations.
 
Blow me computers!!😀

The only "computer" course I liked was a CPU design course I took. Programming can suck it!😀
 
Originally posted by: RaynorWolfcastle
Originally posted by: NutBucket
Transistors, MOS devices, Schottkey devices, etc. I struggled through the required course and "wisely" took the senior level elective😛

And I agree with JohnCU with using the 1/jwC and jwL. Makes infinitely easier.
Click to expand...

Sucker! Hand calculations of bulk-charge modelled MOS devices were the bane of my existance last semester.

Although things aren't much better calculating field distributions in optical fibers this semester. Three cheers for Bessel functions!
Click to expand...

Heat transfer is loaded with Bessel functions. It's fun stuff.
 
Ok, nothing of substance so far. I know how to do the stuff with differential equations, but that gets tedious. He's looking for the complex analysis anyway.
 
Complex numbers are just a transformation. It's hard to add cosines and sines, but not hard to add 1+j2 and 3-j4.

cosx+j*sinx=e^(jx)

(e^(jx)+e^(-jx))/2 = cosx
 
Originally posted by: kevinthenerd
Can you briefly describe for me how complex numbers can be used for circuit analysis?
Click to expand...

Short answer: because they work.

Long answer: You'll find that a lot of the equations in EE topics are derived from experimentation. People look at things, circuits, materials, and run many tests, many experiments, and try to come up with equations to define how they work. What's the proof behind their results? Only that the equations have never failed for anyone anywhere, and any consequences of those equations or definations have always held up.


EDIT - misunderstood the question, I thought it was asking how complex numbers can relate to circuit analysis, not the procedural question of how is it done.
 
Why don't you ask a specific question, like a problem or something?

Physical significance of a capacitor... it's impedance is defined as 1/jwC, w is the radian frequency. As it increases, the impedance decreases, showing that the capacitor is a high pass filter.
 
Originally posted by: kevinthenerd
I = C * dv/dt
V = L * di/dt
V = IR

then there's KVL, KCL, etc.

but what about this complex stuff?
Click to expand...

take the fourier transform to get complex impedances.
Z(resistor) = R
Z(inductor) = jwL
Z(capacitor) = 1/jwC

Simplify circuits in the frequency domain; it makes things much simpler. Note, these calculations are only valid for steady-state response, you still need the differential eqns if you want the transient response
 
Originally posted by: JohnCU
Why don't you ask a specific question, like a problem or something?

Physical significance of a capacitor... it's impedance is defined as 1/jwC, w is the radian frequency. As it increases, the impedance decreases, showing that the capacitor is a high pass filter.
Click to expand...

by w you mean omega, right?

good start. Inductors?
 
Originally posted by: kevinthenerd
Originally posted by: JohnCU
Why don't you ask a specific question, like a problem or something?

Physical significance of a capacitor... it's impedance is defined as 1/jwC, w is the radian frequency. As it increases, the impedance decreases, showing that the capacitor is a high pass filter.
Click to expand...

by w you mean omega, right?

good start. Inductors?
Click to expand...

yeah, omega, and inductors are jwL so as omega increases, the impedance increases, which results in less current, which means that at high frequencies, very little current (ie the signal) is getting through, meaning it's a low pass filter.
 
Originally posted by: JohnCU
www.allaboutcircuits.com
Click to expand...

:thumbsup:

I just had my ECE final few hours ago and some of the material on allaboutcircuits helped clairfy things. Regardless, I don't know if I passed the class. 🙁
 
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