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### Course: High school physics > Unit 12

Lesson 1: Electric current, resistivity, and Ohm's law# Introduction to circuits and Ohm's law

Ohm’s Law is V = IR, where V = voltage, I = current, and R = resistance. Ohm’s Law allows you to determine characteristics of a circuit, such as how much current is flowing through it, if you know the voltage of the battery in the circuit and how much resistance is in the circuit. Created by Sal Khan.

## Want to join the conversation?

- Can electrons move at the speed of light ?(75 votes)
- No, the faster you push the electron the more energy i takes to push it. the max speed we have sent an electron at is 99.9999992% of the speed of light and takes 4 billion eV to do so. the problem is that the faster you push the electron it acts as though it is getting heavier and therefore takes more energy to push.(126 votes)

- I've been taught that current flows from positive to negative, and you are saying that electrons flow from negative to positive. Is this just like a two lane highway with stuff flowing in each direction? Also, what about LEDS? In order for them to light up, the current has to flow from positive to negative.

Is this just semantics?(18 votes)- It's just a notation thing that's stuck around. The idea of current was discovered before electrons were, and now everything is backwards.(49 votes)

- what happen when there is no resistor in a circuit ?(20 votes)
- It is impossible to have a zero resistance wire because there are factors of the wire to provide resistance. If the resistance is very low, however, then it would probably trip a circuit breaker or blown a fuse. If you didn't have a circuit breaker, which is not likely, then the wire would melt and you would probably start a fire.(24 votes)

- when we have resistors in series, the current through all the resistors is same and the voltage drop (or simply voltage) at each resistor is different.

Question 1: it is fine that voltage drop (potential drop) is different because each resistor offers different resistance (suppose). but how is the current through each resistor same? if we have resistors of different resistance, shouldn't the current be different through each resistor?

similarly, when we have resistors in parallel, the current through each resistor is different but the voltage drop at each resistor is same.

Question 2: current is through each resistor is different because resistance of each resistor is different (suppose). but how is the voltage drop at each resistor same here? shouldn't the voltage drop at each resistor be different because each resistor offers different resistor?(20 votes)- ok lets take resistor as blockages in pipe....so won't the water flow be slowed down by the blockages in pipe? similarly, won't the electron flow be slowed down by the resistor. slowing down electron flow mean less Q/t and this means less a current. doesn't it?(6 votes)

- Around5:40, Sal says "the more things there are for the electrons to bump into, or the less space there is for the electrons to move, the more they slow down'. So shouldn't this mean that the best conductors would be gases (because there are really few atoms or molecules floating around) and not metals like silver or copper which are densely packed with atoms?(4 votes)
- not quite, because you need "free" electrons for good conductivity.

The best conductors are superconductors. There the electrons are in a quantummechanical state that somehow prevent them of bumping into atoms around them.(15 votes)

- Isn't the source of voltage shown in this video a cell, not a battery?(7 votes)
- Hello Mihir,

Maybe. Some sources would identify the schematic symbol as a cell. In practice it makes little difference and the two are used interchangeably.

The important part is to look for the voltage as it changes based on the battery a.k.a. cell chemistry.

Regards,

APD(6 votes)

- Sal says that no matter how many charges flow from one side of the battery to the other, the potential difference remains the same. Why is it so? I thought the potential difference decreases, making the battery/ cell dead.(3 votes)
- When we draw a symbol for a battery in a circuit, we are indicating an imaginary battery whose voltage never changes. A real battery will maintain fairly steady voltage until near the end of its life.(7 votes)

- At9:05,V=I*R, so as voltage across the circuit increases, current in the circuit increases.

But in "High tension wires" high voltage is kept so that there is less flow of current. In this case current is inversely proportional to the voltage.Why so?(5 votes)- V is directly proportional to I

to remove the above sign we introduce a constant of proportionality R(resistance)

which gives us,

V=IR WHERE

v(2 votes)

- I have heard that electrons can travel through a vacuum

(hoping this is not a misconception made by me),

consider an insulating hollow tube where the 'hollowness' is 'filled'(not the appropriate word ;)) with vacuum. Is there any resistance ?(2 votes)- Yes, electrons can travel through vacuum. The electrons won't have any resistance from direct interactions with matter but being a charged particle there can be interactions with electric and/or magnetic fields.(8 votes)

- I think that Kirchhoff's junction rule is a bit unclear about the fact that while the charge is flowing through the resistor, the current
*is*less than before, but the current after the resistor is equal to before.

I was just confused for a while, because they seemed to be contradicting themselves using the formula I = V/R to get less current but then said it was the same. Maybe it should be more clear.

I don't really have a question I guess, but I don't now where else to put this.(2 votes)- Kirchhoff's junction rule is really just an extension of conservation of charge. If you have a junction (a place where 3 or more wires meet), the current flowing into the junction must equal to the current flowing out of the junction. This law does not involve resistors.

As for current flowing, the current is the same throughout ALL points in the circuit. The idea that current slows down in the resistor and than goes back to normal when it exits is a misconception. The addition of the resistor simply restricts the amount of current that flows through the whole circuit. Hope this helps!(4 votes)

## Video transcript

- [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.