# Introduction to circuits and Ohm’s law | Circuits | Physics | Khan Academy

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– [Instructor] What we

will introduce ourselves to in this video is the

notion of electric circuits and Ohm’s law, which you can view as the most fundamental law or the most basic law or simplest law when we are dealing with circuits. And it connects the ideas of voltage, which we will get more

of a intuitive idea for in a second, and current, which is denoted by capital letter I, I guess to avoid confusion

if they used a capital C with the coulomb. And what connects these two

is the notion of resistance. Resistance, that is denoted with the capital letter R. And just to cut to the chase, the relationship between these is a pretty simple mathematical one. It is that voltage is equal to current times resistance or

another way to view it, if you divide both sides by resistance, you get that current is equal to voltage divided by resistance. Voltage divided by resistance. But intuitively, what is voltage? What is current? And what is resistance? And what are the units for them so that we can make sense of this? So to get an intuition

for what these things are and how they relate, let’s

build a metaphor using the flow of water, which

isn’t a perfect metaphor, but it helps me at least

understand the relationship between voltage, current, and resistance. So let’s say I have this

vertical pipe of water, it’s closed at the bottom right now, and it’s all full of water. There’s water above here as well. So the water in the pipe, so let’s say the water right over here, it’s gonna have some potential energy. And this potential energy, as we will see, it is analogous to voltage. Voltage is electric

potential, electric potential. Now it isn’t straight up potential energy, it’s actually potential

energy per unit charge. So let me write that. Potential energy per unit, unit charge. You could think of it as joules,

which is potential energy, or units of energy per coulomb. That is our unit charge. And the units for voltage

in general is volts. Now, let’s think about what would happen if we now open the bottom of this pipe. So we open this up. What’s gonna happen? Well, the water’s immediately

gonna drop straight down. That potential energy

is gonna be converted to kinetic energy. And you could look at a

certain part of the pipe right over here, right over here. And you could say, well, how much water is flowing per unit time? And that amount of water that is flowing through the pipe at that point in a specific amount of time, that is analogous to current. Current is the amount of charge, so we could say charge per unit time. Q for charge, and t for time. And intuitively you could say, how much, how much charge flowing, flowing past a point in a circuit, a point in circuit in a, let’s say, unit of time, we could think of it as a second. And so you could also think

about it as coulombs per second, charge per unit time. And the idea of resistance

is something could just keep that charge from flowing at

an arbitrarily high rate. And if we want to go back

to our water metaphor, what we could do is, we

could introduce something that would impede the water, and that could be a narrowing of the pipe. And that narrowing of the pipe would be analogous to resistance. So in this situation, once again, I have my vertical water pipe, I have opened it up, and you still would have

that potential energy, which is analogous to voltage, and it would be converted

to kinetic energy, and you would have a flow

of water through that pipe, but now at every point in this pipe, the amount of water that’s flowing past at a given moment of

time is gonna be lower, because you have literally this

bottleneck right over here. So this narrowing is

analogous to resistance. How much charge flow impeded, impeded. And the unit here is the ohm, is the ohm, which is denoted with

the Greek letter omega. So now that we’ve defined these things and we have our metaphor, let’s actually look at

an electric circuit. So first, let me construct a battery. So this is my battery. And the convention is my negative terminal is the shorter line here. So I could say that’s

the negative terminal, that is the positive terminal. Associated with that battery, I could have some voltage. And just to make this tangible, let’s say the voltage is equal to 16 volts across this battery. And so one way to think about it is the potential energy per unit charge, let’s say we have electrons here at the negative terminal, the potential energy per

coulomb here is 16 volts. These electrons, if they have a path, would go to the positive terminal. And so we can provide a path. Let me draw it like this. At first, I’m gonna not

make the path available to the electrons, I’m gonna

have an open circuit here. I’m gonna make this path for the electrons. And so as long as our

circuit is open like this, this is actually analogous

to the closed pipe. The electrons, there is

no way for them to get to the positive terminal. But if we were to close the

circuit right over here, if we were to close it,

then all of a sudden, the electrons could begin

to flow through this circuit in an analogous way to the way that the water would flow down this pipe. Now when you see a

schematic diagram like this, when you just see these lines, those usually denote something

that has no resistance. But that’s very theoretical. In practice, even a very simple

wire that’s a good conductor would have some resistance. And the way that we denote

resistance is with a jagged line. And so let me draw resistance here. So that is how we denote

it in a circuit diagram. Now let’s say the resistance

here is eight ohms. So my question to you is, given the voltage and

given the resistance, what will be the current

through this circuit? What is the rate at which

charge will flow past a point in this circuit? Pause this video and try to figure it out. Well, to answer that question, you just have to go to Ohm’s law. We wanna solve for current,

we know the voltage, we know the resistance. So the current in this example

is going to be our voltage which is 16 volts,

divided by our resistance which is eight ohms. And so this is going to be 16 divided by eight is equal to two and the units for our current, which is charge per unit

time, coulombs per second, you could say two coulombs per second, or you could say amperes. And we can denote

amperes with a capital A. We talked about these electrons flowing, and you’re gonna have two coulombs worth of electrons flowing per second past any point on this circuit. And it’s true at any point, same reason that we saw over here. Even though it’s wider up

here and it’s narrower here, because of this bottleneck,

the same amount of water that flows through this

part of the pipe in a second would have to be the same

amount that flows through that part of the pipe in a second. And that’s why for this circuit, for this very simple circuit, the current that you would

measure at that point, this point, and this point,

would all be the same. But there is a quirk. Pause this video and think

about what do you think would be the direction for the current? Well, if you knew about electrons and what was going on, you would say, well, the

electrons are flowing in this direction. And so for this electric current, I would say that it was flowing in, I would denote the

current going like that. Well, it turns out that

the convention we use is the opposite of that. And that’s really a historical quirk. When Benjamin Franklin was

first studying circuits, he did not know about electrons. They would be discovered

roughly 150 years later. He just knew that what he

was labeling as charge, and he arbitrarily labeled

positive and negative, he just knew they were opposites, he knew something like charge was flowing. And so, in his studies of electricity, he denoted current as going from the positive to

the negative terminal. And so we still use that convention today, even though that is the

opposite of the direction of the flow of electrons. And as we will see later on, current doesn’t always involve electrons. And so this current here is going to be a two ampere current.

Dev BekzatPost authorAs always, perfect ☺

T 25Post authorKapow

Hunain ShujaPost authorThanks

jonathan sanchezPost authorI love khan Academy!

hope gizzyyunPost authorHiii

Kaedo -Post authorGreat explanation!

Mohammed RashidPost authorNice

greg leePost authorThis 8 min video is worth more that 3 years of my dumb school… Teachers never bothered to explain those things, they just use A, V, Ohms like it's obvious, and nobody gets it…

R.L. H.Post authorAny other vapers here? 😅

isaak hPost authorVery quality explanation. Will this series eventually get to how electromagnetic radiation is produced by the the change in electric and magnetic fields?

Sohel SohelPost authornothing 2 say…in 1 word…awesome

CH QaisarPost authorPlease upload npn and pnp transistor…

HudsonAPost authorJust in time for my electricity and magnetism course! more please 🙂

PirschoPost authorSorry but isn't Voltage = U ?

waleed eterPost authorplease if you can give to me the name of program you used it in explain the information

PC Gamer DttVPost authorHoly shit I have an exam on this day after tommorow thanks

How did u know

SnitselPost authorU

Gaming EpochsPost authorWhen will be this on khanacademy.org

Smit PatelPost authorThis video worth a billion views!

Tactical PandaPost authorHow Smart must you be after all these videos!

Autumn SimlerPost authorthis is helpful for me

Qin Shi HuangPost authorthxs so much

L boratePost authorBeautiful sal!! Kirchoffs law next! What is more dangerous 1A or 1kV

Clarissa NeldaPost authorur voice reminds me of “HEY ARNOLD”

Shahid ShaikhPost authorBest Teaching ever seen

Katharine TraversPost authorThank you, Knan Academy. Good review of the basics. I didn't know Franklin instituted flow direction. Facts should change conventions.

The Stikbot ChannelPost authorYay

JKN ProductionsPost authorI love your videos. really great. any chance of getting 1080p or 4k video?

Shane MullanPost authorno sound?

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SANTHOSH KONDRAJUPost authorPlease explain about microprocessors

Ikbal HossainPost authorWe need more vdo like this

Bruh...Post authorThis is a life saver, my teacher doesn't explain any of this and then gets mad we the class doesn't do well.

snazzynavaPost authorTest is in two days, thank for for this.

Kristofi HansPost authorSo what happens when the electrons reach + from – , what happens then ?

animations for youPost authortest is tomorrow, thank you for this.

kgosi fanyanaPost authorthanks for the video bro

Rushik SriramojuPost authorThank you,

This helped me a lot. I have a test tomorrow on this

Jose JimenezPost authorWow. This whole time I've been messing with microcontrollers I believed that the flow of electrons was from the (+) terminal to the (-) terminal. What a misconception! Thank you Sal!

Abdelmalek HalawaniPost authorHi,

I am trying the record videos about circuits. Please advise how to write this neatly on the screen and draw clear circuits.

thanks

Chirag CKPost authorPLEASE COME TO MY SCHOOL AND BE MY TEACHER. Ive learnt more from this 9 min video then my hours of science, Thank you. I can take my science test with ease

Minju LeePost authorThank you always

CHOOSE MEPost authorAmazing

Zeo ArtistPost authorYou are a good drawer !

FEDERICO MOLINA SUAZO JRPost authorto confusing

Ben MillerPost authorIs this Marc Gunn narrating? Irish and celtic music podcast

Faizan Ul HaqPost authorExcellent…cleared all my doubt…nice job sal

Mohamed HasanPost authorWhy current is the same after passing the resistor? the resistor will decrease the flow of charges and the current should be decreased as well.. isnt it?

Vani SampathPost authorKa is very useful to me

lori mcPost authorOmg thanks so much u helped me alot

LORENZO BALLAKISTANPost authorThis is grade 7 electric circuits

Prantik Sutra DharPost authorNice video👍

Faadumo HassanPost authorThanks you so much, this video helped me alot i swear

CassianPost authorSal, this is an awesome video. Thank you ever so much!

Warshala ParveenPost authorselection is tomorrow…….. Thnqeww very much for this!!!!!!!!! awssm❤ video sal

NATTERPost authorwhy does he draw so good with a mouse?

Nickoh667 Historicalnik667Post authorI know this will sound like a dumb question, but with the bottleneck analogy, i cannot help but think that it is the normal reaction of the pipe wall to the container which is contributing to the prevention of flow. How would this "normal reaction" apply to the circuit case? Also, how would we tie in I=navq to this analogy? Disclaimer: I haven't watched all the videos yet.

Dylan moellerPost authorhate physics

Prasad PophalePost authorSal Khan learn in MIT so he is very knowledgeable

koondoogPost authorWhy does the flow rate decrease in the example here (4:22) but in fluids problems the flow rate stays the same but the velocity increases?

Edit: if current is analogous to flow rate, and resistors only increase the velocity (the flow rate stays the same), what is the point of using resistors/narrower or wider pipes?

GASTRO GAMINGPost authorThis Minecraft red stone update is really confusing and overly complicated

Brendon SturgeonPost authorI'm horrendously bad at math and I have no idea where to start. FFS I can't even divide without a calcualtor…

Liela MasucolPost authorOkay this is my topic for science. And I understand it now

122123jPost authorhour of reading text books = i kinda get it

10 min video = i totally get this

Zepherius ColePost author(I) is for current because it stands for inductance not necessarily because (C) was not available.

Elizabeth HernandezPost authorI love Khan Academy. Thanks for this great video, exactly what I needed.

Cesar LoveraPost authorHow many resistivity circuit we can make with n resistence?

blasphemy18Post authorwhy does the same amount of water/sec flow in the big part and the small part of the tube^ i dont get it

Seyitan AjetunmobiPost authorthank u verrrrrrrrrrrrry much my exam is month u've really healped me a lot

Ȟᴇċĸȧĸŷʜ ÂɡᴇĸʙȧᴛᴇʜPost authorThink about electricity as water; and power lines as pipes. Voltage is = water pressure. Current is = water flow speed. Resistance is = pipe resistance.

Sami BogerPost authorTwinkle twinkle little star, I equals V divided by R

Mayank JaatPost authorSir ICAR entrance exam ki Tyari Krni

Thetrucky69Post authorExcellent information.

Nathan HawkinsPost authorHi, I was doing a paper and one of the questions had two batteries in parallel and one in series with those two, how would I find the total voltage if they all supply the same voltage?

Molecule ManPost authorWhy would u want to use a resistor to slow it down?

Cole SmithPost authorFranklin has a 50% chance of getting current right and he messed it all up!

Cole SmithPost authorMan, I wish people would learn basic Newtonian physics before trying to use them to explain stuff like this. This video helped me sort out like 3 or 4 bad descriptions of how current works that people explained using backwards ideas of how water would flow or blocks would fall…. Thank you Khan Academy.

Blee803Post authorLove this

Naomi BeckfordPost authorVery useful

Cicolas NagePost authorthis is exactly why i don't need to go to school/study even when my physics exam is tomorrow. will try to remember to update with what i get on it.

KxnfidentPost authorThis is confusing but ok

chiz antoPost authorIm grade 4 and 10 years old… why am i studying this ?!?!

Sea SnekPost authorHey Khan Academy i can’t thank you enough for all these great physics videos. They have helped me out so much and have made principles of physics much more simpler and easier to understand than what highschool taught me throughout the years. Keep them up!

Emmalie R FotiPost authori have an exam on tuesday and i am this close to having a seizure.

Sizwe Ngwane-MhlunguPost authorThis guy is good I’m telling you.

Even if you literally are not interested in that particular subject but when this guy is explaining you enjoy and understand the subject. That’s what I call it a Good teacher 😄

Peter LemmonPost authorSimplest Ohms Law to remember all 12 formulas in simplicity. P. E. P. E^2

PEPE^2 their is this guy name. I. E. I R. I^2 R. P. R

PEPE eats PIE

PEPE speaks Ebonics and say IR

PEPE stutters and repeats I^2R

PEPE Nationality.?. PR

Two equations with the square ^2. Are transposed. I= Square root of P/R and. E=square root of P.R

Draw a circle and use it as the pie circle you learned around the four sets of equations. Done., you should be able to memorize and write the twelve Ohms Law in approximately 15 seconds.

sukhdeep singhPost authorYou're a good teacher ! Love from Indian students!!!