Lesson 1 - What Is Alternating Current? (AC Circuit Analysis)

Summary

In this AC circuit analysis tutorial, host Jason introduces the concept of Alternating Current (AC), emphasizing its ubiquity in household appliances. He explains that unlike Direct Current (DC), AC's voltage and current constantly change direction, flipping 60 times per second in the US. Jason stresses the importance of understanding trigonometry and sinusoidal functions to analyze AC circuits, promising a clear explanation of complex terms like frequency and phase angle in upcoming lessons.

Takeaways

• 📚 The lesson is an introduction to AC (Alternating Current) circuit analysis, presented by Jason, the tutor.
• 🔌 AC is fundamentally different from DC (Direct Current) as it involves changing voltage and current directions, unlike the constant flow in DC circuits.
• 🔁 The term 'sinusoid' refers to the wave shape of sine and cosine functions, which are crucial in understanding AC behavior.
• 👨‍🏫 Jason emphasizes the importance of prior knowledge of voltage, current, resistance, capacitors, inductors, and circuit analysis techniques for understanding AC circuits.
• 🔄 AC is used in power grids due to its efficiency in long-distance transmission, and is the standard for most home and industrial electrical systems.
• ⚙️ Generators produce AC through the rotation of a coil within a magnetic field, resulting in a cyclic recurring voltage.
• 📈 In the United States, the standard frequency for AC is 60 Hz, meaning the voltage and current switch directions 60 times per second.
• 🌐 Different countries may have different frequencies for their AC power systems, but the concept of alternating remains the same.
• 📊 The voltage and current in AC circuits are represented graphically as sinusoids, which are mathematical functions that describe their periodic nature.
• 📚 A fundamental understanding of trigonometry is necessary for analyzing AC circuits, as it helps in understanding the sine and cosine wave patterns.
• 📈 The next steps in the course will involve writing down mathematical functions for voltage and current in AC circuits and understanding terms like frequency and phase angle.

Q & A

What is the primary focus of the AC circuit analysis tutor lessons?

The primary focus of the AC circuit analysis tutor lessons is to teach the principles and skills needed to analyze and understand alternating current circuits.

Why is it important to have a strong foundation in DC circuit analysis before studying AC circuits?

It is important to have a strong foundation in DC circuit analysis because the techniques learned, such as Ohm's law, node voltage, and Norton equivalent circuits, are directly applicable to AC circuits once the theory behind alternating current is understood.

What is the significance of the term 'phasor' mentioned in the script?

The term 'phasor' refers to a concept that will be introduced later in the lessons, which is essential for understanding and analyzing AC circuits, particularly in terms of how voltages and currents behave over time in relation to each other.

Why is trigonometry important for understanding AC circuits?

Trigonometry is important for understanding AC circuits because the voltage and current waveforms in AC circuits are sinusoidal, and knowledge of sine and cosine functions is necessary to analyze and represent these waveforms mathematically.

What is the difference between direct current (DC) and alternating current (AC)?

Direct current (DC) is characterized by a constant flow of electric charge in one direction, while alternating current (AC) involves a periodic reversal of the direction of the electric charge flow, typically 60 times per second in the United States.

Why is AC more efficient for transmitting electricity over long distances?

AC is more efficient for transmitting electricity over long distances because it can be easily transformed to higher or lower voltages using transformers, which reduces power loss during transmission and allows for more efficient use of the power grid.

What is the frequency of the AC voltage in the United States, and how does it relate to the concept of sinusoids?

The frequency of the AC voltage in the United States is 60 hertz, which means it switches direction 60 times per second. This rapid switching is represented by sinusoidal waveforms, which are the graphical representation of sine and cosine functions.

What does the term 'sinusoid' refer to in the context of AC circuits?

In the context of AC circuits, the term 'sinusoid' refers to the wave-like pattern of voltage and current that changes periodically, which is the same shape as that of sine and cosine functions in trigonometry.

Why is it necessary to review trigonometry before diving into AC circuit analysis?

It is necessary to review trigonometry before diving into AC circuit analysis because understanding the characteristics of sine and cosine functions is crucial for analyzing the behavior of voltage and current in AC circuits, which are represented by sinusoidal waveforms.

What is the significance of the wall socket example used in the script to explain AC?

The wall socket example is used to illustrate the concept of AC by showing how the voltage between the prongs of a wall socket alternates direction, representing the alternating nature of AC and how it differs from the constant voltage of a DC source.

Outlines

00:00

📚 Introduction to AC Circuit Analysis

In this introductory lesson, Jason, the host, expresses excitement about teaching AC (alternating current) circuit analysis, emphasizing that the skills learned in previous lessons are crucial for understanding AC. He mentions that all techniques such as Ohm's law, node voltage, and Norton equivalent circuits will be applicable to AC circuits once the basic theory of AC and the use of phasors are understood. Jason also stresses the importance of reviewing trigonometry, especially sine and cosine functions, as they are fundamental to grasping AC concepts. He introduces the term 'sinusoid' as interchangeable with AC, highlighting its efficiency in long-distance transmission and its generation through the rotation of coils in magnetic fields.

05:00

🔌 Characteristics of Alternating Current

This paragraph delves into the characteristics of AC, explaining how it differs from DC (direct current) by constantly changing direction, which is visualized through the alternating voltage in a wall socket. Jason instructs viewers to observe a wall socket to understand this concept. He explains the frequency of AC in the United States, which is 60 Hz, meaning the voltage direction changes 60 times per second. This rapid flipping is a key feature of AC, which is used in most household appliances that plug into a wall. The paragraph also contrasts AC with DC, where the current flows in one direction, whereas in AC, the current switches direction in sync with the voltage changes.

10:01

🌀 Understanding Sinusoidal Functions in AC

The final paragraph provides a basic overview of sinusoidal functions, which are the graphical representation of AC voltages and currents. Jason describes how these sinusoids, or sine waves, are fundamental to AC circuits, alternating 60 times per second in the US. He prepares learners for the next section, where mathematical expressions for these sinusoids will be introduced, including concepts like frequency and phase angle. The paragraph ends with an encouragement to review trigonometry to ensure a solid foundation before proceeding with AC circuit analysis.

Keywords

💡Alternating Current (AC)

Alternating Current, often abbreviated as AC, is a type of electrical current where the flow of electrons periodically reverses direction. In the context of the video, AC is the main theme as it discusses the nature of AC circuits and their analysis. The script mentions that AC is fundamentally different from Direct Current (DC), where in a DC circuit, the current flows in one direction, whereas in an AC circuit, the current changes direction periodically, typically 60 times per second in the United States.

💡Circuit Analysis

Circuit analysis is the process of understanding the behavior of electrical circuits. The video script emphasizes that the skills learned in circuit analysis, such as voltage, current, resistance, and the use of Ohm's law, are crucial for understanding AC circuits. The host, Jason, mentions that all the techniques learned previously will be directly applicable to AC analysis, highlighting the importance of these foundational concepts.

💡Sinusoid

A sinusoid refers to a mathematical function that describes a smooth, periodic oscillation. In the video, sinusoids are described as the wave-like pattern seen in AC signals, which can be represented by sine or cosine functions. The script explains that the voltage and current in AC circuits change in a sinusoidal manner, alternating back and forth to form a wave pattern.

💡Trigonometry

Trigonometry is a branch of mathematics that deals with the relationships between the sides and angles of triangles, and it is essential for understanding AC circuits. The script suggests that viewers should have a basic understanding of trigonometric functions like sine and cosine, as they are integral to analyzing the wave patterns of AC circuits. Jason, the host, plans to review these concepts to ensure that viewers can follow the AC analysis.

💡Ohm's Law

Ohm's Law is a fundamental principle in electrical engineering that states the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = IR. The video script mentions Ohm's Law as one of the crucial techniques that will be used in AC analysis, indicating that understanding how voltage, current, and resistance interact is vital for analyzing AC circuits.

💡

💡Frequency

Frequency in the context of AC refers to the number of oscillations or cycles that occur per unit of time, typically measured in Hertz (Hz). The script explains that in the United States, the standard AC frequency is 60 Hz, meaning the voltage and current switch directions 60 times per second. This frequency is a key characteristic of AC circuits and is essential for understanding their behavior.

💡Phase Angle

Phase angle is the measure of the difference in phase between two waves. Although not explicitly defined in the script, the concept is implied when discussing how AC signals can be out of phase with one another. Understanding phase angles is important for analyzing the interactions between different AC circuits and their components.

💡Voltage

Voltage, also known as electric potential difference, is the force that pushes electric charge through a conductor. In the video, voltage is described as the force that causes the flow of current in an AC circuit. The script emphasizes that in an AC circuit, the voltage changes direction periodically, which is a fundamental characteristic of AC as opposed to DC.

💡Current

Electric current is the flow of electric charge, typically through a conductor. The script explains that in an AC circuit, the current changes direction periodically, which is a key difference from DC circuits where the current flows in one direction. The current in an AC circuit is said to 'alternate,' hence the term 'alternating current.'

💡Resistors, Capacitors, and Inductors

Resistors, capacitors, and inductors are components of electrical circuits that affect the flow of current. The script mentions that the understanding of these components and their behavior in circuits is essential for AC analysis. Resistors oppose current flow, capacitors store energy in an electric field, and inductors store energy in a magnetic field. Their interaction with AC signals is different from their behavior in DC circuits.

💡Thevenin and Norton Equivalent Circuits

Thevenin and Norton equivalent circuits are methods used to simplify complex circuits for analysis. The script mentions these techniques as important tools for analyzing AC circuits, indicating that they will be used to simplify the analysis of AC circuits by representing them with simpler equivalent circuits that have the same input-output relationship.

Highlights

Introduction to AC circuit analysis by Jason, the tutor, emphasizing the culmination of previous circuit analysis skills.

Brief review of what alternating current (AC) is and its graphical representation.

Importance of understanding the skills learned in previous lessons for AC analysis.

Relevance of Ohm's law, node voltage, and Norton equivalent circuits to AC circuits.

The necessity of understanding the theory behind AC and the use of phasors.

Request for students to review prior circuit analysis material for a solid foundation in AC analysis.

Recommendation to refresh trigonometry knowledge, especially sine and cosine functions, for AC analysis.

Explanation of the term 'sinusoid' and its interchangeability with AC in certain contexts.

The historical and practical reasons behind the use of AC, including its efficiency in long-distance transmission.

Description of how generators produce alternating currents through the rotation of coils in a magnetic field.

The prevalence of AC in household appliances and its distinction from DC sources.

Clarification of the sinusoidal wave nature of AC and its characteristics.

Visualization of voltage alternating between two prongs of a wall socket in an AC circuit.

The frequency of AC in the United States, mentioned as 60 Hertz, and its rapid switching of voltage directions.

Difference between DC and AC in terms of current direction and voltage stability.

The sinusoidal nature of voltage and current in AC circuits, visualized as sine or cosine waves.

Assurance that understanding of AC and its mathematical representation is not difficult and will be clearly explained.

Encouragement for students to review trigonometry before proceeding with AC circuit analysis.

Preview of the next lesson's focus on writing sinusoidal functions for voltage and current in AC circuits.

Transcripts

hello welcome to this lesson of the AC

circuit analysis tutor I'm Jason I'll be

your host with this uh set of lessons

with this course here and I'm very

excited to teach uh alternating current

AC analysis to you because I've been

building the skills with you uh as we've

been working on our circuit analysis

skills up till now and everything we've

been doing all of the things that we

have learned up until this point really

is culminating in us being able to

tackle AC analysis so what we're going

to do in this lesson is just kind of

briefly review what is alternating

current what is AC uh kind of give you a

little bit of a graphical outline as to

what it exactly it is I think you'll

probably learn a few things even if you

kind of know a little bit about AC

already uh and then what I really want

you to to know though as we go through

this class here is that the skills that

we have learned up until this

point just think back to everything

we've learned we've learned about

voltage current resistance learned about

capacitors and inductors we've learned

about uh the current voltage

relationships OHS law node voltage thin

and Norton all of that stuff that we've

done it's all extremely crucial uh and

I'll give you a little bit of a preview

the reason why all that stuff's so

important is because really for AC

analysis once I show you how to set

everything up all of those techniques

that we have learned before are going to

be directly applicable to all AC

circuits and I'll say that again because

it's really important all of those

techniques we've learned before th and

Norton equivalent circuits series and

parallel Arrangements node volt all the

stuff that we've done the techniques of

analysis they're all applicable to

alternating current circuits but you

have to know a little bit of theory

behind alternating current um and how to

use what we're going to call phasers

which will be a little bit later you

have to know how to use all that stuff

and be very comfortable with it so I

don't want to just bulldoze into it I

want to make sure you understand what AC

is I want to make sure you understand a

little bit later how we're going to

handle these uh these sources and all

the capacitors and everything in a an

alternate current so that whenever we do

the problems using Fant or

Norton or node voltage or mesh current

or whatever you'll be very comfortable

with the new material the AC part and

you all already should be pretty

comfortable with the pr prior material

so if you haven't already done so I

strongly encourage in fact I require you

to go back and learn

everything that I've taught in circuit

analysis up until this point uh and the

second thing that I want to request of

you I'm going to give you a good primer

on on trig function here but if you're

Rusty in s and cosine what they look

like characteristics you know basic

ideas that you learn in trigonometry I

strongly recommend that you go and

review some trigonometry if you're Rusty

in that I'm going to go through it with

you of course like I always do but I

really want to make sure that when you

come into this and I talk about sign and

cosine you at least kind of know what

they look like I'm going to draw it for

you but you know you still there's a lot

of mathematical background that we get

that's going to be useful in this

class so we're going to we're going to

throw around some big terms in this

class first one we've already

encountered alternating current we're

also going to throw around uh a term

called sinusoid all right so those are

kind of interchangeable basically uh if

you could if I could boil it down for

you alternating current is exactly what

it sounds like you see when you compare

that to what we've done before which was

direct current and direct current

circuits which we already covered

everything is is basically constant you

have a battery uh or some other fixed

voltage source or current Source it

doesn't change when you switch the

circuit on you might have some transient

stuff but once it all settles out the

currents and the voltages all across the

components in a DC circuit they never

really change it's like a battery

operated toy or something and so as long

as the thing is supplying power you can

measure all over the circuit the

different currents and the voltages and

everything is staying the same nothing

is changing but in alternating current

it's fundamentally different all right

in alternating current current the

current or the voltage and I'll we'll

talk a little bit more about how to know

later but basically everything in the

circuit is going to be constantly

alternating or changing changing

directions literally it's going to be

actually changing directions and you

might say well why is there alternating

current why do we use alternating

current there's a lot of history between

DC and AC and it turns out that AC is

very very efficient for transmitting

over long distances so that's where we

have our whole power grid set up as

alternating current and when you go dig

back into physics you'll

realize that uh when you generate

electricity usually what you're doing is

you have a generator which is just a

coil of wire and some magnets and you're

rotating the coil of wire uh around the

mag around the magnetic field and that's

generating an electric current and

you'll see when you cut apart a motor

and see how they're actually constructed

the way they're built is they generate

these alternating currents because of

the rotation you go half a circle and

then you'll come back the other half the

circle and you go half the circle the

other half the circle so that's why it

leads to these cyclic recurring back and

forth kinds of voltages that are

generated and that's why you know we end

up using alternating current so much so

almost everything that you uh use at

home that's bigger than battery operated

device is going to deal with alternating

current anything that you plug into a

wall outlet is using alternating current

so think about your televisions think

about your refrigerators think about

your um you know stereo systems anything

that plugs into a wall is using

alternating current so when I say

alternating current I'm going to explain

what that is in a minute on the board um

but I'm going to throw in um another

term here called a sinusoid see I've

talked about s functions you know just

briefly you should learn those in trig

I'll talk about cosine functions when

you talk about sinusoid as a as a word

it just means that shape you know that

shape of a sign or a cosine in fact if

you remember back from rig s and cosine

have the same shape they have exactly

the same shape if you shift them over

and line them up they're the same thing

so I'll explain the differences here in

a minute between s and cosine as a

review but the shape is what we're

talking about so that up and down wave

nature that's called a sinusoid so

sometimes you'll hear me talk about

sinusoidal functions or your book you

might hear about that sinusoid over

there that just means that nice shape it

turns out that sin swords signs and

cosines they're extremely important in

all of engineering and science

um they pop up everywhere and sinus

swords are very very special and they're

they're so very common we use them all

the time because of that so let's talk

about some characteristics of sinusoids

or AC alternating current number one it

comes out of our wall and what do I mean

by that exactly I want you to visualize

this a little bit so imagine a wall

socket right so let's just go over here

let's go over here for now let's say we

have a wall socket usually you have a

plate like this and you have you know

one or two plugs there so usually you

have uh two guys like that with one of

this dots down there and you might have

something that looks kind of like this

so this is a a plug right don't worry

about the uh prong down here just kind

of forget about that for a minute so

I'll erase it even though you may have

that in your wall focus on the two uh

flat ones up here what's happening in

your wall socket okay what I actually

kind of want you to do while you're

watching this video is find a wall

socket in the room and look at it

nothing's plugged into it find one that

doesn't have anything plugged into just

kind of look at it and just listen to me

while we talk before your very eyes if

you look at these holes here if you had

invisible Vision if you had some kind of

vision that could

visualize um uh voltage without a meter

if you could see it then you would see

the voltage between these two prongs

alternating and switching Direction in

other words you know how we used to talk

about the voltage source right we have

to label it so in one instance of time

um

this prong has a plus and a minus right

that would be the voltage drop going

from there to there right but then if

you could watch it a short time later it

will flip around so that this one is

negative and this one is positive see

how they've alternated right so

literally it's plus minus and then it's

plus minus the other way so they flipped

around so this would be like your

voltage and if you had some kind of a

meter you could don't do this unless you

know what you're doing but you could

measure the voltage in the wall and you

would see that it literally switches

Direction how fast does it switch

directions well in the United States

it's 50 60 hertz right that's 60 times

per second now around the world there

are different frequencies of the

switching going on but in the United

States it's 60 htz 60 times per second

so it's very fast me think about 60

times of anything per second

that's that's pretty fast so the

flipping around here is literally

flipping around just really really

really fast 60 times every single second

um but that's what's going on the reason

that 60 HZ goes into into the history of

of of the of the um you know of the

transmission and and generation system

that we have here different frequencies

around the world they have their own

standards but the idea is it's flipping

back and forth now in a DC circuit right

remember our good old DC circuits that

we we used to talk about a lot simple

ones like this if this is a DC Source

then we say that the

current is basically always going One

Direction it comes out of the positive

terminal back around forms a circuit

comes back to the other side now if it's

an

AC uh guy so you'll see this drawn

different ways but uh you might see a

little sine wave inside of there may not

always say that but um a lot of books

will will'll show it like that what this

means is that the voltage across this

guy sometimes as I've said before it's

plus minus but 60 * a second it flips

back where it's minus plus and because

it flips back and forth like that that

means sometimes the current is literally

flowing through the circuit this way but

then other times the circuit switches

direction or the current switches

Direction and goes back through the

source the other way and then it flips

back and goes the other way and then it

goes the other way and then it goes the

other way so literally the current in an

alternating current circuit that's why

it says alternating current right the

voltage is switching directions and also

in in in line with that the current is

switching directions so again look at

that wall socket you can visualize that

wall socket uh pretend that you've

plugged a vacuum cleaner or something

into it in one instance of time the

current may be coming out of the plug

into your load and back in to the other

prong but then just a short time later

you know a few milliseconds later

everything's flipped around and then

you've got the current coming out of the

right hand prong going this way and then

it flips around and it goes like this so

literally the current is coming out to

in out to in out to in N Out to n and

that's flipping back and forth 60 times

per second in the United States very

slight different variations around the

world that is the basic idea of what the

heck alternating current really is it

literally is changing 60 times per

second right back and forth back and

forth the voltage is switching Direction

and because of that because of ohms law

the current is switching directions also

at the same frequency 60 times per

second in this

case that is a very um

broad overview of what's going on so if

you could plot the voltage plot the

strength of the voltage you would see

that it's alternating 60 times per

second but you would see it alternating

as a sign or a cosine we call those

sinusoids and also you would see that

the current is also alternating as a

sinusoid right so the currents and the

voltages in the circuits they're all

going to look like sinusoids they're all

going to look like some kind of cosine

function that we can write down math

mathematically so that's a general

overview without any math we haven't

done anything really mathematical yet

but I wanted to give an introductory

lesson to show you what we're doing give

you a little bit of motivation explain

to you what alternating current is

before you do anything mathematical what

I like to do now is for you to follow me

on to the next section where we'll put

pencil to paper and we'll start to write

down how you would look at these

sinusoids how you would write down the

function for a voltage for instance um

mathematically and lots of different

terms we're going to be talking about

frequency and phase angle and all of

these things that sound complicated but

just remember and repeat after me none

of this stuff is hard all right none of

it's hard I can explain it to you so

it's so clear that you'll be very very

comfortable with it but you do need to

have some fundamental understanding of

trig so I'm going to take you through

that but if you have no idea what a

sinusoid is please stop now and go get

some of my lessons on trigonometry so

you're comfortable with that uh signs

and cosiness and angles and things like

that uh and then meet me back here and

we'll pick it up in the next lesson with

uh alternating current circuit

analysis