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### Course: Electrical engineering > Unit 2

Lesson 5: AC circuit analysis- AC analysis intro 1
- AC analysis intro 2
- Trigonometry review
- Sine and cosine come from circles
- Sine of time
- Sine and cosine from rotating vector
- Lead Lag
- Complex numbers
- Multiplying by j is rotation
- Complex rotation
- Euler's formula
- Complex exponential magnitude
- Complex exponentials spin
- Euler's sine wave
- Euler's cosine wave
- Negative frequency
- AC analysis superposition
- Impedance
- Impedance vs frequency
- ELI the ICE man
- Impedance of simple networks
- KVL in the frequency domain

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# Euler's formula

Euler's formula relates the complex exponential to the cosine and sine functions. This formula is the most important tool in AC analysis. It is why electrical engineers need to understand complex numbers. Created by Willy McAllister.

## Want to join the conversation?

- I was wondering how did Euler come up with this formula?(6 votes)
- The reasoning behind the formula requires calculus to understand.(4 votes)

- So if i want to be a electric and electronic egineering i have to know this by heart?(3 votes)
- Hello Miguel,

Yes and no. First, forget about this laundry list and ask yourself why you are interested in electronics.

There are many fun and easy ways to get involved. You could say it depends on how you view the world. Some folks like to learn all the theory first. Others (myself included) start out slow building and experimenting. The maths and scarry stuff can come later.

Please leave a comment below if you would like some tips on getting started.

Regards,

APD(6 votes)

- So, all these variables that he is using, are they interchangeable for other letters? For example, at3:07, where he says e ^ jx , could he as easily have said, e^ nr?(2 votes)
- The "x" variable name is free to choose. The "j" variable is not. "j" is the imaginary unit, the square root of -1. (In engineering we use "j" instead of "i" for the imaginary unit.) So you have to keep the "j", but you can use anything in place of "x".(4 votes)

- At3:30, he mentions a video of Sal, can anybody give me link of that video, I'm unable to find? Thank you(2 votes)
- what is the class code of this topic, AC circuit analysis?.....Like if we want to add it in our google classroom.(2 votes)
- Each class is created by a teacher and gets a unique class code, which they share with students. Students enter the code and join the class. The individual assignments don't have their own code -- there is not a universal "AC circuit analysis" code.

Here is an article on how teachers make assignments: https://www.khanacademy.org/resources/k-12-teachers-1/assignments/a/creating-assignments-for-your-students(2 votes)

- 7:51Why can you treat "j" as a variable and manipulate it in the equation? Shouldn't "j" be a vector notation in dimension of imaginary number?(1 vote)
- "j" is the variable name in engineering for the "square root of -1". It is treated like any other variable name in algebra. It is not a vector notation or "decoration".(2 votes)

- i couldnt understand 5 minutes into the video , can someone recommend a video about what exactly is d/dx thingy?(1 vote)
- We use the notation dy/dx(where y is a function) to represent the rate of change of y with respect to x. This can be calculated using calculus.(1 vote)

- I got a hard problem"prove that the sum of n nth roots of any complex number is zero."

I(1 vote) - Where does : "So Euler's Formula says that e to the jx equals cosine X plus j times sine x. Sal has a really nice video where he actually proves that this is true. "?(1 vote)
- What would "sin(x) + jcos(x)" and "sin(x) - jcos(x)" equal?(1 vote)
- Hmm. Perhaps e^j(x-pi/2)? It's some shifted way of wandering around the unit circle.

After checking with some expansions, the second one is indeed e^j(x-pi/2), I suspect the first one would then be e^-j(x+pi/2)(1 vote)

## Video transcript

- [Voiceover] So in this
video we're goint to talk about Euler's formula. And one of the things I
want to start out with is why do we want to talk about this rather odd looking formula? What's the big deal about this? And there is a big deal. And the big deal is e. We love e. And I'll underline that twice. Now the reason is, because when we take a derivative of e D DT of e to the x equals e to the x. And D DT of e to the ax so where a is anything equals ae to the ax. And so the property is that
when you take a derivative of the function, the
same function comes out. Or, if you take a
derivative of the function a scaled version of the
same function comes out. And we love this, because why? Because when we do differential equations e to the x is the solution. Almost every time. Whenever we did a circuit e to the x was the answer. If you recall from when we were solving circuit simple circuits
with differential equations that we always said something like well we're gonna guess that V of T is some constant times e to the st. That was a proposed solution. This turned out to work every time. So there's something else we love, too. And that is sinusoids. Or, sines and cosines. Okay. We love these. And that gets two lines. Now why do we love these? Is because they happen in nature. If you whistle the air pressure looks like a sine wave. If you ring a bell the bell moves in a sine wave. In any kind of music if you look at the notes in music the sound they make the pressure waves look like sine waves. And circuits make sine waves, remember? We analyzed this circuit in great detail it was the LC circuit. We looked at the natural response of this and that was a sine wave. Okay. So, electric circuits make sine waves. All these things make sine waves. They occur in nature. And we want to be able to analyze things that happen when
sine waves are present. So, we have two things we love and we want to relate these two things. And these are going to be related through that. Euler's Formula. That's how we connect
these two separate ideas. Let me, let's go do that. So Euler's Formula says that e to the jx equals cosine X plus j times sine x. Sal has a really nice
video where he actually proves that this is true. And he does it by taking the MacLaurin series expansions of e, and cosine, and sine and showing that this expression is true by comparing those series expansions. And I'm not going to repeat that here we're just going to state that as fact. And now we're going to look at
this equation a little bit more. So, this is the expression
that relates exponentials that we love, to sines
and cosines that we love. And part of the price of
doing that is we introduce complex numbers into our world. Here's two complex numbers. Okay. This is where complex numbers come into electrical engineering. So we have to mention the other form of this formula which is e to the, I put a minus sign in here, e to the minus jx. And that equals cosine X minus J sine X. So these two expressions together are Euler's Formula, or Euler's Formulas. And we're gonna exploit this by taking we'll be able to take
the cosines and sines that we find in nature we're going to be able to fashion
them into exponentials. And these exponentials then go into our differential equations
and give us solutions. And we're going to come back
and pull out the cosines and sines. That's the rhythm of how
we're going to use this equation to help us solve circuits. One small point I want to share, notice that in both these
equations the sine comes first? And the sine is over here on the imaginary side. So the cosine is the real side. This is the reason that we
have a preference in the future we're going to have a preference if we're talking about
our real world signals in terms of the cosine function. It's because in this Euler's Formula the cosine comes first, in both cases. So what I want to do now is take a second and I want to see if we had our signal expressed
in these exponentials how do we recover the
cosine and the sine term? How do we flip these equations around so we can solve for the
cosine and the sine? It's a simple bit of algebra here. It's good to see. All right, so if I want to
isolate the cosine term. If I want to isolate the cosine term let me get rid of these guys here. So now to isolate the cosine term what I'm going to do is add
these two equations together and that plus and minus
are going to cancel out this second term here. That's what I'm going for. So, if I add I'll get e to the jx plus e to the minus jx equals, cosine doubles, two cosine X. And the two sine terms cancel out. All right? So then I could write I'll write it over here. I can write, cosine of X equals e to the plus jx plus e to the minus jx over two. All right? So that's the expression that's the expression
for cosine in terms of complex exponentials. Okay, let's go back and
see if we can get sine. So what I'm gonna do with sine is I'm going subtract. And that gives me what that'll do is that'll
get the cosine terms to fall away. And then I get e to the jx subtracting, right? Minus e to the minus jx equals cosine terms, they subtract out. And I get two times j times sine X. All right, and that means I can write sine X equals e to the plus jx minus e to the minus jx over two j. And it's really easy that j term is down there. That's easy to forget sometimes. So I put a square around these guys 'cause that's important. So there's the two expressions. That's the two expressions for if you have complex
exponentials and you want to extract the cosine, this is how you do it. And if you want to extract the sine this is how you do it. Okay? And you can either you can put these in your head or probably easier for me, if you just remember Euler's Formula, this is
a pretty straightforward quick derivation. So the other thing we put a square around is this guy here. So we have Euler's Formula. And basically the cosine
and sine extracted from Euler's Formula.