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## Electric potential & potential difference

Current time:0:00Total duration:13:08

# Intro to potential difference (& voltage)

## Video transcript

- [Instructor] If we have a bulb connected to a bunch of wires, the question that we're
gonna answer in this video is how do we setup a current in these wires? And we'll also learn what a battery does and what does voltage really mean? And to do all of this, let's start by thinking of an analogy. And the analogy is this
picture of happy kids playing on a slide. If, as the kids slide down, someone could pick them up and put them back at the top continuously, then I'm pretty sure you can guess, there'll be a continuous
motion of these kids. And so, there'll be a
continuous current of kids. And so we'll keep this picture in our mind as we try and understand how to produce a continuous current of charges. So let's start looking
at this a little deeper. If you look at the kids
who are on the slide, what's making them move from here to here? I'm pretty sure you know the
answer to this, it's gravity. Gravity's the one that's
pushing them down the slide. In a similar manner,
there are charges present everywhere in the wire already. We've already seen that
these wires are all metals and metals have loads of electrons, which are negatively charged particles, free to move inside of them. The only problem is they're
not moving, they're at rest. So to make them move,
we need to push them. Just like how gravity pushes these kids, we need an electric force
to push these electrons. And that electric push is
what is given by a battery. So of course, there are charges
inside a battery as well. But what's important is
that one side of the battery is positive, the other side is negative. That starts pushing these electrons. Now, for simplicity, what we will do is we'll assume that these
are not negative electrons, but we'll assume them to be
some kind of positive charge. The only reason being, the
direction of the current is the direction in which
positive charges move. That's how we chose the
convention, isn't it? So thinking in terms
of electrons would be, might be slightly annoying. So we can focus more on the
concept over here easily, if we just assume that these are positive charges, all right? So, if we think of them
as positive charges, we can now see the positive
terminal of the battery pushes these charges away from them and the negative terminal of the battery pulls these charges towards it. And as a result, we now
have an electric current motion of charges in this direction. And just like over here, for
continuous motion of the kids, someone needs to continuously
keep picking these kids up and putting them back over here, pushing them against gravity. Over here, someone needs to
keep picking up these charges and push them back up continuously against the electric force. And that job is done by the
chemicals inside the battery. There are some chemical
reactions that go on and these chemical reactions
start pushing these charges up against the electric
force, and as a result, we have a continuous motion of charges creating a continuous
supply of electric current. And if you could see those charges moving, it might look somewhat like this. That's it, a very brief animation. Now, to understand voltages,
let's come back over here. As these kids move down from
more height to less height, they gain energy. If this was a very smooth slide, they might have gained kinetic
energy, they might speed up. But if this was a rough slide let's say, then because of the
friction between their body and the slide, then there
is a lot of heat generated. So as the kid keeps moving down, there is a lot of heat
generated in the slide. And since heat is a form
of energy, we could ask, where did that energy come from? And you may already
know the answer to this. We say that energy was
already stored up in this kid. As the kid moved down,
slowly the stored up energy got converted to heat,
and this stored up energy is what we call potential energy. And so, every kid at this
point, top most point of the slide has very
high potential energy. And so we could say this
point is high potential point. High potential. And as they move down
they lose potential energy and convert it into heat. And by the time they come over here, they would have lost all
that potential energy. So we can say that this
is low potential point. Low potential. And as the kids move back up,
they gain potential energy. And who gives them that potential energy? Well again, you might
guess, it's this person who's pushing them up, he's
the one who's transferring the potential energy back into the kids. And again, the kids lose
that potential energy and so the cycle repeats. In a similar manner, as the
charges move through the bulb, there is heat generated in the bulb. Again we could ask, where
did this heat come from? In a similar manner we could say, the charges over here
might have already had, might already have stored energy. And as they move through the bulb, they lose that stored energy. And so we can say that
the charges over here have high potential energy,
and the charges over here, as they move down, they lose so they have low potential energy. The only difference
between these two cases, one is that this is due to gravity, this is due to electricity. But another major difference you can see is that over here, as
kids are moving down, they're continuously
losing potential energy and producing heat. But over here, we're going to assume that not much heat is
created in the wires. I mean, you know that in
reality, the wires do get hot, but they don't get as hot
as the bulb, isn't it? So to keep things
simple, we like to assume in all the circuits that we deal with, that there is absolutely no
heat generated in the wire. And so that means that
the charges don't lose any potential energy up til this point. Then they lose all the potential energy gets converted to heat,
and then they come back. And so in this case, we can
now say that all the charges over here in this section of the wire have high potential energy, so
this is high potential point. And all the charges over
here, they have lost their potential energy, and again, they have the same energy
because they don't lose energy to the wire, in reality they do, but we're assuming they won't. And so we can say all
these charges over here are at low potential. So every time charge
moves through the bulb, they lose potential energy. And every time the
chemicals in the battery push them back up, they
gain potential energy. And that energy comes from
the chemicals of the battery. Just like how over here, the
energy comes from this guy. And voltage is simply the difference in the potential energy of the charges. And that's why voltage is
more technically called just potential difference. It just tells us what's the difference in the potential energy of charges. So for example, we'll take an example and understand voltage. For example, over here, if we
said the potential difference between these two points is 1.5 volts. Than it means there is
a difference in energy, potential energy, of
1.5 joules per coulomb. The per coulomb part says,
that every coulomb as it moves from here to here, it gains
1.5 joules of potential energy. And similarly, when a coulomb
moves from here to here, it loses 1.5 joules of potential energy and gets converted to heat. So voltage just indicates
how much potential energy a coulomb gains or loses. If this was a nine volt battery, then every coulomb gains nine joules of potential energy as it comes here. And per coulomb, it would lose nine joules of potential energy as it
goes from here to here. So there will be more heat
generated per coulomb. That's what a nine volt battery would do. And this number also
means that if there were two coulombs that go from here to here, then they would lose a
total of three joules of potential energy,
two times this number. If there are 10 coulombs
that move from here to here, they would lose a total
of 10 times this number, 15 joules of potential energy, and so on. So knowing the voltage helps us calculate how much potential energy
would be lost or gained when charges move from
one point to another. Also, if we were to take these two points in the circuit, then
the potential difference between them is zero. Can you understand why? Pause the video and think about it. Well, that's because we
assume there is no loss in potential energy as charges
move through this wire. And therefore, the potential difference between these two points is zero. Similarly I hope you
agree, potential difference between these two points is also zero. There's only a potential
difference between these two points and since
this is a high potential, we tend to put a plus
sign for high potential and negative sign for low potential. And similarly, there's
a potential difference between these two points. And therefore, we always say, we can get a continuous supply of
current if we maintain a potential difference
between ends of the wire. All right, one last thing is in some textbooks they define
voltage in terms of work done. It's pretty much the same thing. So let's just look at what that is. You see over here, when
the battery is transferring the charges from low
potential to high potential, we saw it's the battery
who's transferring energy into the charges, just like how this guy transfers energy into the kid. In physics, whenever you transfer energy, we say you are doing work, that's it. So over here, we can
say the battery is doing 1.5 joules of work per coulomb
that it transfers, right? And so from this, we can now
say voltage also tells us how much work is being done. Work is being done in transferring charges from one point to another per coulomb, per coulomb, that's it. So to summarize what we
learned, we saw that in order to maintain an electric
current in a circuit, we need to maintain a
potential difference. The potential difference or
voltage, is simply an indicator of how much potential energy
is gained or lost per coulomb, when it moves from one point to another. It can also be thought of as
how much work needs to be done to transfer the potential energy per coulomb from one point to another.