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# AC analysis intro 2

Here's a preview of how AC analysis is going to work. To get ready we need to review some of the ideas from trig and complex numbers. Created by Willy McAllister.

## Want to join the conversation?

• what is s? In the previous videos it is -1/RC, -R/L, -alpha-sqrt(alpha squared - omega squared), in this video i tried to substitute s by trial and error using those formula above to get Vin but somehow i cant justify my equation.
• The solution for s is different for every circuit. You can't "borrow" a solution from another circuit, unless it has the same components and same connections. You CAN use the same solution if the only difference is the component values.
• It is almost like electricity has its own kind of math, which uses all the regular math combined.
• Because you constrained the input to sinusoidal form, Does the voltage of a voltage input source usually take the form of a sinusoidal wave?
(1 vote)
• Yes. We use sine sources when doing AC analysis of a system.

When you go to build a real useful system, the inputs are necessarily sines, but there's beautiful theory (Fourier Series and Fourier Transforms and Laplace Transforms) that lets us view real-world signals as a collection of sines.
• at after all the signs to all circuit elements have been added, it ended-up with like signs in the node between the voltage source and the inductor, and like signs in the node between the voltage source and the capacitor.

Is this correct?
(1 vote)
• Yes, and no. Technically, the polarity should be the other way, but it really doesn't matter. I'll explain why:

If you notice, at he stated when putting the signs across the voltage he's giving it simply "that polarity", which is to say that he is giving an arbitrary polarity for now as a place holder.

This is still correct because the voltage source may very well be that polarity, or it may be the other way around. It is sinusoidal so it will change with time. As will the polarities of the components. This change is not completely instantaneous, (I believe it may be at the speed of light) so it may very well be the polarity at the instant that the source has changed voltages!

:) Hope I did not confuse anything.