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

-[ Instructor] What wewill introduce ourselves to in this video is thenotion of electric circuits and Ohm’s law, which you can view as the most fundamental law or the most basic law or simplest regulation when we are dealing with circuits. And it connects the ideas of voltage, which we will get moreof a intuitive meaning for in a few seconds, and current, which is denoted by capital letter I, I approximate to avoid confusionif “theyre using” a capital C with the coulomb. And what connects these twois the notion of resistance. Resistance, that is meant with the capital letter R. And simply to cut to the chase, the relationship between these is a pretty simple mathematical one.It is that voltage is equal to current duration fighting oranother way to contemplate it, if you divide both sides by resist, you get that current is equal to voltage divided by resistance. Voltage divided by resistance. But instinctively, 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 intuitionfor what these things are and how matters relating, let’sbuild a metaphor using the flow of liquid, whichisn’t a perfect metaphor, but it helps me at leastunderstand the relationship between voltage, current, and fight. So let’s say I have thisvertical hose of water, it’s closed at the bottom right now, and it’s all full of sea. There’s ocean above now as well. So the spray in the pipe, so let’s say the water right over here, it’s gonna have some possible energy.And this potential energy, as we will see, it is analogous to voltage. Voltage is electricpotential, electric potential. Now it isn’t straight up possible intensity, it’s actually potentialenergy per unit freight. So let me write that. Potential energy per unit, legion accusation. You could think of it as joules, which is potential energy, or divisions of energy per coulomb. That is our unit charge. And the human rights unit for voltagein general is volts. Now, let’s “ve been thinking about” what would happen if we now open the bottom of this hose. So we open this up. What’s gonna happen? Well, the water’s immediatelygonna drop straight down. That possible energyis gonna be converted to kinetic energy. And you could look at acertain part of the piping right over here, right over here. And you could say, well, how much irrigate is spurting per unit term? And that extent of water that is flowing through the pipe at that point in a specific amount of day, that is analogous to current.Current is the amount of indict, so we could say indict per unit hour. Q for indictment, and t for epoch. And intuitively you were able to say, how much, how much attack spurting, spurting past a phase in a circuit, a point in circuit in a, let’s say, contingent of period, we could think of it as a few seconds. And so you could also thinkabout it as coulombs per second, bill per unit term. And the idea of resistanceis something could just restrain that bill from flowing atan arbitrarily high rate. And if we want to go backto our sea allegory, what we could do is, wecould initiate something that would clog the ocean, and that could be a narrowing of the pipe.And that narrowing of the pipe would be analogous to resistance. So in such a situation, once again, I have my horizontal water pipe, I have opened it up, and you still would havethat potential energy, which is analogous to voltage, and it would be convertedto kinetic energy, and you would have a flowof spray through that pipe, but now at every point in this pipe, the amount of liquid that’s spurting past at a given moment oftime is gonna be lower, because you have literally thisbottleneck right over here. So this narrowing isanalogous to resistance. How much bill flow obstructed, clogged. And the unit here is the ohm, is the ohm, which is represented withthe Greek letter omega. So now that we’ve defined these things and we have our analogy, let’s actually look atan 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 now. So I could say that’sthe negative terminal, that is the positive terminal.Associated with that battery, I could have some voltage. And simply to make this tangible, let’s say the voltage is equal to 16 volts across this battery. And so one highway to be considered it is the potential energy per unit charge, let’s say “were having” electrons here at the negative terminal, the potential energy percoulomb here is 16 volts. These electrons, if they have a path, would go to the positive terminal. And so we can provide a footpath. Let me extorted it like this. At first, I’m gonna notmake the path available to the electrons, I’m gonnahave an open circuit here. I’m gonna make this path for the electrons. And so as long as ourcircuit is open like this, this is actually analogousto the closed pipe. The electrons, there isno way for them to get to the positive terminal. But if we were to close thecircuit right over here, if we were to close it, then all of a sudden, the electrons could beginto flow through this route in an analogous highway to the way that the liquid would spurt down this pipe.Now when you see aschematic diagram like this, when you just see these lines, those generally represent somethingthat has no resistance. But that’s very theoretical. In practise, even a very simplewire that’s a good conductor would have some fight. And the lane that we denoteresistance is with a jagged cable. And so let me draw resistance now. So that is how we denoteit in a circuit representation. Now let’s say the resistancehere is eight ohms. So my question to you is, given the voltage andgiven the resistance, what will be the currentthrough this tour? What is the rate at whichcharge will spurt past a degree 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 exampleis going to be our voltage which is 16 volts, divided among our resist which is eight ohms. And so this is going to be 16 divided by eight is similar to two and the units for our present, which is charge per unittime, coulombs per second, you could say two coulombs per second, or you could say amperes.And we are able to denoteamperes with a asset A. We talked about these electrons spurting, and you’re gonna have two coulombs worth of electrons flowing per second past any detail on this circuit. And it’s true at any point, same reason that we discovered over here. Even though it’s wider uphere and it’s narrower here, because of this bottleneck, the same amount of ocean that spurts through thispart of the tube in a second would have to be the sameamount that spurts through that one of the purposes of the pipe in a few seconds. And that’s why for this circuit, for this very simple circuit, the present that you wouldmeasure at that point, this site, and this site, would all are similar. But there is a quirk. Pause this video and thinkabout what do you think would be the direction for the present? Well, if you knew about electrons and “whats going on”, you would say, well, theelectrons are flowing in this direction.And so for this electric current, I would say that it was flowing in, I would express thecurrent going like that. Well, it turns out thatthe convention we use is the opposite of that. And that’s really a historical caprice. When Benjamin Franklin wasfirst studying circuits, he did not know about electrons. They would be discoveredroughly 150 years later. He only well known what hewas labeling as indictment, and he arbitrarily labeledpositive and negative, he just knew they only inverses, he knew something like charge was flowing. And so, in his studies of electricity, he expressed current as going from the positive tothe negative terminal. And so we still use that convention today, even though that is theopposite of future directions 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 .. 