Triboelectric effect and charge | Physics | Khan Academy


– I’m guessing that you’ve
had the experience of rubbing a balloon against your
hair and then when you take the balloon away from your
hair, your hair sticks up. And if you haven’t had that experience, you might think about trying to lead a more rich and fun life, but I’m guessing most of
you all have done that. And you had a sense that it
had something to do with the balloon or your hair, somehow
exchanging charge or now one is going to be more positive
or negative than the other, and so now they are somehow attracted. And if you were thinking of those things, you are generally right. What you just experienced
after you rubbed the balloon on your head, and then
your hair is now attracted to the balloon, that’s actually called the triboelectric effect,
let me write that down, tribo, triboelectric, electric effect. And human beings have been observing this for a long long time,
and it wasn’t necessarily with balloons at birthday
parties or whatever, it’s with other things, they
rub a silk cloth on a piece of glass and then they’ll
see that there’s some type of attraction, or they
might see that if they do that enough, one of the
objects might discharge when it touches another object. People have observed
things like lightning, where it looks like there’s
some type of a buildup and some type of a potential
and then all of a sudden it discharges and you have
this lightning and then this thunder blast sound that happens too. So this is something
that humans have observed for a long long time, and
scientists or people with a, I guess you could say a
scientific mind have been trying to understand it for a long
long time, and trying to come up with a framework for what
exactly is happening. Well lucky for us, we now
have a framework for it that explains it quite well. And that framework for what is going on with that triboelectric effect, is a framework around charge. Is a framework that we
now have around charge. And this tells us, this way
of looking at the world, says look, there’s some things that just have a property called charge. Some things have a positive charge, Some things have a positive
charge, and it’s somewhat of an arbitrary name, we just
happen to call it positive. And some things have what we
say is an opposite charge, or a negative charge, a negative charge. We could have called
this the magenta charge, and this the green charge,
we could have called this the hippopotamus charge and
this the ostrich charge. And we could have said that
hippopotami, I believe plural for hippopotamus, they’re
always attracted to ostriches, but they always repel other
hippopotami, and likewise. The like charges repel or like hippo… You get the general idea. But I’ll stick to the words
that people are used to using. And so if we say something has a charge, say a positive charge, and something else has a negative charge, then in our framework
that we’re setting up, these two things are going to attract. So opposite charges are going to attract, while like charges are going to repel. So if you have a positive charge, and you have a positive charge, these things are going to accelerate, are going to accelerate
away from each other. And that’s not just true
for positive positive, that’s also true for
negative and negative, these two things are going to repel because they are like charges. Now it’s very interesting
to think about this because we are so used to
thinking in terms of charge, even you know if, especially
in kind of the world of electricity you have the
positive and negative terminal. You think of charging up
your phone or whatever else. That it seems like, we completely,
charge is just something that is fundamental about the universe, and that’s true to some, that’s true, but you’d have to appreciate
that these are arbitrary words and they’re really just
to describe a property that we have observed in the world. And if you go down to the
atomic level, we can get to a fundamental level of where
the charge is happening. But once again, these are really models for our brain to describe,
these are frameworks and models for our brain to be able
to predict and describe what we observe in the world. But if we run with this model, we can imagine at the atomic scale, the nuclei of atoms are composed
of protons and neutrons. So if you have some
protons, and then you have some neutrons, I’ll do two of
each, you have some neutrons, and based on this framework,
protons have a positive charge. Protons have a positive charge. Now once again, this convention
of calling them positive and putting a plus on it,
it’s not like protons have a little plus sign
tattooed onto them somehow. We could have called
those, we could have said they have a red charge, or
we could have even said, we wouldn’t of had to
even use the word charge, this is just a convention
that we have decided to use. And so we say protons have
positive charge and then, kind of buzzing around the
nucleus of an atom, you often, or usually, or often have electrons. Electrons have a lot less mass. Mass is another interesting thing. We associate mass as just,
oh this is just something that we get, we understand
it in our everyday life, but even mass, this is
just a property of objects, it’s just a property of matter, and we feel like we understand
it because on our scales we understand notions of
things like weight and volume, but even mass can get quite exotic. But anyway, the whole
point of this video is not to talk about mass,
it’s to talk about charge. But all of these things that
we talk about in physics, these are just properties
that will help us deal with these notions, these
behaviors in different frameworks. But anyway, let’s get
back to this little atom that I was constructing. So this atom, let’s say
it has two electrons, and obviously this is not drawn to scale, and each of these electrons
have a negative charge, and they’re kind of jumping around here, buzzing around this nucleus of this atom. And the reason why, this
model, even going down to the atomic scale and thinking
in protons and electrons is interesting, is that it
allows us to start explaining what is happening in the
triboelectric effect. What is happening in the
triboelectric effect is when you rub that balloon on your
hair, because of the property of the balloon, the
material of the balloon, and the materials of your hair, when they come in contact and they rub, the balloon is grabbing
electrons from your hair. So the balloon is grabbing
electrons from your hair, and so it is getting
more negatively charged, it is getting more negatively charged, and your hair is getting
more positively charged, or essentially it’s lost these electrons. And so when you put the
balloon now close to your hair, remember like charges repel each other, so the electrons in your
hair try to move away from these other electrons,
the negative charge tries to move away from the negative charge, and I guess you could say that the tips of your hair will
then become more positive. Are more positive and
they will be attracted, and they will be attracted to the balloon. So we can think about
what’s happening in terms of transfer of electrons,
that’s exactly what’s happening. And so when you think
that way, it’s like ok, we are scientists, this is a nice model, we can start to think about
what’s happening here. This model actually explains
a whole ton of behavior that we’ve observed in the
universe, including things like, lightning and whatever else,
you know the static shock that you get when you
might touch a doorknob after rubbing your shoes along the carpet. But we like to start, we
like to quantify things, so we can start seeing how much they repel or how much they attract each other. And so the fundamental unit of charge, or one of the fundamental units of charge, or I guess you could say the
elementary unit of charge is defined in terms of the charge of a proton or an electron. So the fundamental, or I
guess you could say the elementary unit of charge
is denoted by the letter e, and this is the charge of a proton, this is e for elementary,
charge of proton. And the charge of an electron,
even though an electron has a much, much, much, much
smaller mass than a proton, most of the mass of an atom is from the protons and the neutrons. So an electron has a
much, much smaller mass than the protons and the neutrons, but it has the same but
opposite charge as a proton. So sometimes the convention
is to write negative e, or maybe even negative one
e, sometimes depending on whether you view this as a
kind of the actual charge or whether you view this as a unit, but here I’ll view this
as the actual charge. You could view negative e as the charge, as the charge of an electron. And something that has no charge, like a neutron, we say they’re neutral, and actually that is why
they are called neutrons, because they are neutral,
they don’t have charge. So that right over there, that
over there is, is a neutron. Now when we start to get
on kind of a larger scale, not on a sub-atomic scale anymore, talking about electrons and
protons, the unit of charge, in general the unit of
charge that we typically use is the coulomb, is the coulomb. Coulomb, it’s named for
Charles Augustin de Coulomb, so if we’re talking
about the guy, and he was an 18th Century French physicist,
we would use capital C, but if we’re talking about the units, we would use lowercase c,
the coulomb, the coulomb. And the coulomb is
defined, so one coulomb, let me write it right
over here, one coulomb and it uses the abbreviation
uppercase C, is equal, or I’ll say approximately equal to, we’re going to round here,
it’s approximately equal to 6.24, 6.24 times 10 to the
eighteenth e, or you could say, in magnitude wise, it’s
equal to the charge of 6.24 times 10 to
the eighteenth protons, or magnitude wise, it would be the opposite if
you’re talking about electrons, it would be 6.24 times 10
to the eighteenth electrons. Now if you want to go
the other way around, what is the charge of, the
magnitude of the charge of say a proton in terms of coulombs, well you would just take
the inverse of this. So you could say that e
is approximately equal to the inverse of this which is 1.60, I guess you could say
the reciprocal of this, 1.60 times 10 to the negative 19, times 10 to the negative 19 coulombs. So hopefully this gives
you an appreciation for, I guess at a base level, what charge is. And in some ways it’s like
it’s this everyday thing, you’re used to it, we’re used
to dealing with electricity and we’ll talk much more
about that in depth. But at some levels it is this thing, one of the mysteries of the universe, how did these two particles
know to attract each other, you know it looks like
they’re at a distance, how do they immediately
exert a force on each other. how do these particles know
immediately to repel each other, it’s not like they have
a wire connecting them that they’re communicating somehow, or I guess once you get
to quantum mechanical, an argument can be made that
they are communicating somehow. But in our everyday,
kind of logical sense, it’s like well at a distance, how do these things actually
know to repel or attract, and what is this charge anyway? You know we’ve put all
these names around it but to kind of help us think
about it and have a framework and predict what will happen. But do we really know
what this charge thing is. So on one level it’s kind
of plain and mundane, and it deals with balloons and hair, but on another level it’s this deep thing about this universe, it’s
a deep property of matter that we can manipulate and we
can predict, but it is still this very fundamental and
somewhat mysterious thing.

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20 thoughts on “Triboelectric effect and charge | Physics | Khan Academy”

  1. Derbis the Eternal says:


  2. Mack Ben says:

    Take a break my friend for you have done good 🙂

  3. JonDecagon says:


  4. Benjamin Agaoglu says:

    I hope this isn't a ridiculous question but how did he get from 6.24X10^18 to 1.6X10^-19? He said by taking the inverse?

  5. Anoir Trabelsi says:

    Please continue !

  6. akash parvatikar says:

    I think there is a reason why electrons have a ''negative'' charge…

  7. B Cuz says:

    why's this guy such a fuckin scatterbrain? it's like he's trying to distract you on purpose

  8. Cosmo John says:

    Dear Khan Academy
    Why is it that Static cling Vinyl decals never lose its static effect?

  9. Lakshmi Narasimha Murthy Vishnubhotla says:

    That's a great picture, Sal! And as usual, thank you Khan academy.

  10. ørchid. says:

    Did Khan inhale some helium?

  11. FEDOR SYKORA says:

    7:05 so like guys and girls?

  12. Cosmin Popa says:

    Great, now I want a hippopotamus for Christmas 😐

  13. 4pharaoh says:

    Tell me more, tell me more… Why are so many of the insulators (Teflon, silicon, plastics etc.) so far up in the Tribo series when they are suppose to hold on to their electrons? Why are the voltages due to ‘rubbing’ so high? Why does the balloon so easily grab an electron from the wool? What is the best guess as to what is happening at the atomic level? So many questions, so  few answered. Thanks.

  14. Eri says:

    lost it at hippopotami XD

  15. Jam Silva says:

    Thanks a lot for this video sir… This really helps me in my teaching (Physics subjects although i am a Math Major )here at Philippines. Thanks you very much and God bless !

  16. Levon Tabirian says:


  17. Michael Pierce says:

    I assume if you are grounded and lose electrons due to Triboelectric effect that you are essentially drawing a current of electrons from ground?

  18. 신성윤 says:

    Awesome lecture! It is really nice to note that assigning "+" for a positively charged particle and "-" for a negatively charged one is just a convention. That was what used to drive me crazy.

  19. Nathaniel Clarke says:

    if the if both the balloon and the hair roots have a negative charge, why is the hair pulled up wards in stead of down, or simply cancelling out?

  20. Strychninesonics says:

    It's hippopotamuses.

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