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

– [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.

## 87 thoughts on “Introduction to circuits and Ohm’s law | Circuits | Physics | Khan Academy”

• ### Dev Bekzat Post author

As always, perfect ☺

Kapow

Thanks

Hiii

• ### Kaedo - Post author

Great explanation!

Nice

• ### greg lee Post author

This 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 author

Any other vapers here? 😅

• ### isaak h Post author

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

• ### Sohel Sohel Post author

nothing 2 say…in 1 word…awesome

• ### HudsonA Post author

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

• ### Pirscho Post author

Sorry but isn't Voltage = U ?

• ### waleed eter Post author

please if you can give to me the name of program you used it in explain the information

• ### PC Gamer DttV Post author

Holy shit I have an exam on this day after tommorow thanks
How did u know

U

• ### Gaming Epochs Post author

When will be this on khanacademy.org

• ### Smit Patel Post author

This video worth a billion views!

• ### Tactical Panda Post author

How Smart must you be after all these videos!

thxs so much

• ### L borate Post author

Beautiful sal!! Kirchoffs law next! What is more dangerous 1A or 1kV

• ### Clarissa Nelda Post author

ur voice reminds me of “HEY ARNOLD”

• ### Shahid Shaikh Post author

Best Teaching ever seen

• ### Katharine Travers Post author

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

Yay

• ### JKN Productions Post author

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

no sound?

• ### Vishal Singh Post author

Hello guys, I, Vishal Singh, working as PO in Canara Bank, have started a new way to learn to speak English. I will be helping you in speaking English through Whatsapp video calling. In this way, we will be talking with each other face to face in english. My number is 9756774966. Drop me a message at whatsapp, i will explain you everything there.

• ### Ikbal Hossain Post author

We need more vdo like this

• ### Bruh... Post author

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

• ### snazzynava Post author

Test is in two days, thank for for this.

• ### Kristofi Hans Post author

So what happens when the electrons reach + from – , what happens then ?

• ### animations for you Post author

test is tomorrow, thank you for this.

• ### kgosi fanyana Post author

thanks for the video bro

• ### Rushik Sriramoju Post author

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

• ### Jose Jimenez Post author

Wow. 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 Halawani Post author

Hi,

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

thanks

• ### Chirag CK Post author

PLEASE 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 Lee Post author

Thank you always

Amazing

• ### Zeo Artist Post author

You are a good drawer !

to confusing

• ### Ben Miller Post author

Is this Marc Gunn narrating? Irish and celtic music podcast

• ### Faizan Ul Haq Post author

Excellent…cleared all my doubt…nice job sal

• ### Mohamed Hasan Post author

Why 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 Sampath Post author

Ka is very useful to me

• ### lori mc Post author

Omg thanks so much u helped me alot

• ### LORENZO BALLAKISTAN Post author

This is grade 7 electric circuits

Nice video👍

• ### Faadumo Hassan Post author

Thanks you so much, this video helped me alot i swear

• ### Cassian Post author

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

• ### Warshala Parveen Post author

selection is tomorrow…….. Thnqeww very much for this!!!!!!!!! awssm❤ video sal

• ### NATTER Post author

why does he draw so good with a mouse?

• ### Nickoh667 Historicalnik667 Post author

I 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.

hate physics

• ### Prasad Pophale Post author

Sal Khan learn in MIT so he is very knowledgeable

• ### koondoog Post author

Why 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 GAMING Post author

This Minecraft red stone update is really confusing and overly complicated

• ### Brendon Sturgeon Post author

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

• ### Liela Masucol Post author

Okay this is my topic for science. And I understand it now

• ### 122123j Post author

hour of reading text books = i kinda get it
10 min video = i totally get this

• ### Zepherius Cole Post author

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

• ### Elizabeth Hernandez Post author

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

• ### Cesar Lovera Post author

How many resistivity circuit we can make with n resistence?

• ### blasphemy18 Post author

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

• ### Seyitan Ajetunmobi Post author

thank u verrrrrrrrrrrrry much my exam is month u've really healped me a lot

• ### Ȟᴇċĸȧĸŷʜ Âɡᴇĸʙȧᴛᴇʜ Post author

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

• ### Sami Boger Post author

Twinkle twinkle little star, I equals V divided by R

• ### Mayank Jaat Post author

Sir ICAR entrance exam ki Tyari Krni

• ### Thetrucky69 Post author

Excellent information.

• ### Nathan Hawkins Post author

Hi, 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 Man Post author

Why would u want to use a resistor to slow it down?

• ### Cole Smith Post author

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

• ### Cole Smith Post author

Man, 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.

Love this

Very useful

• ### Cicolas Nage Post author

this 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.

• ### Kxnfident Post author

This is confusing but ok

• ### chiz anto Post author

Im grade 4 and 10 years old… why am i studying this ?!?!

• ### Sea Snek Post author

Hey 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 Foti Post author

i have an exam on tuesday and i am this close to having a seizure.

• ### Sizwe Ngwane-Mhlungu Post author

This 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 Lemmon Post author

Simplest 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 singh Post author

You're a good teacher ! Love from Indian students!!!