Discovery of Electron Video Lecture | Physics for Airmen Group X - Airforce X Y / Indian Navy SSR

in this video we're going to talk about
how JJ Thompson discovered the electron
in 1897 using the cathode ray tube
experiment so these days we think about
atoms kind of like this they're made up
of a variety of smaller subatomic
particles but in the late 1800s it was
all about John Dalton's model of the
atom in the early 1800s John Dalton had
proposed that all matter all stuff was
made up of tiny indivisible particles
called atoms at first not many people
believed him but throughout the course
of the eighteen hundreds more and more
scientists got on board with his idea
but it remained an open question whether
the atoms were really indivisible were
they like tiny little billiard balls you
know hard and solid or were they made of
smaller things no one really knew the
answer that but JJ Thompson was able to
answer part of that question when he
discovered the electron he discovered
that it was much smaller than atoms he
discovered that Adams had electrons and
so he was able to say for the first time
that atoms weren't totally solid and
indivisible but they were made up of
smaller subatomic particles let's look
at how he discovered the electron so
Thompson used what's called a cathode
ray tube it's a big tube of glass that's
kind of like a bottle you can imagine
it's sort of like this soda bottle it's
sealed all over and then you pump all
the air out of it
okay and it has these two pieces of
metal at this end so Thompson connected
these two pieces of metal to a power
source and here's what happened our ray
shot from this piece of metal across the
tube and created a glowing spot over
here when it hit a special coating on
the inside of the glass now Thompson
didn't know what was going on but
really was going on is that electrons
from this piece of metal
we're shooting out they were attracted
to this piece of metal which has an
opposite charge but they were moving so
fast they shot all the way across the
length of the tube anyway
Thompson sees this ray and he gets
curious as to what it's made of
and so he asks the question the stuff
that the cathode ray is made of does it
have a electrical charge and here's how
he attempts to answer that question he
takes two metal plates and puts them on
either side of the cathode ray tube and
then he turns on electricity which makes
the top plate positively charged and the
bottom plate negatively charged and
check out what happens to the cathode
ray when he turns on the power he see
that he sees it now instead of going
straight through the cathode ray bends
up it bends up towards the positively
charged plate so based on this he
reasons that the cathode ray must be
made of stuff that's negatively charged
since it's attracted to the positively
charged piece of metal and he knows that
opposite charges attract
but just because he wants to be very
careful he's a very careful guy he tries
to check this another way so he uses a
magnet if something has an electrical
charge and it's moving a magnet will
push it in particular directions so he
takes a cathode ray tube if you want to
think that this marker is kind of a
cathode ray tube and he surrounds it
with a magnet just like this we can kind
of show what goes on by slipping the
magnet under like this so anyway he puts
the magnet here and he sees now that the
cathode ray bends in the other direction
it now moves downward now if the cathode
ray is indeed
negatively-charged this is exactly how
he would expect it to move in the
presence of a magnet like this so this
is just further confirmation that
whatever makes up the cathode ray
probably is negatively charged so he
takes the the data that he gets from
these experiments and he's able to draw
a few important conclusions about
cathode rays and what makes up atoms
okay so based on the outcomes of the
experiments with the charge plates and
the magnet Thompson is able to conclude
that cathode rays must be made of stuff
that is negatively charged okay he
combines the information that he got
from the experiments that I just told
you about with some other data that he
collected and he's able to come to the
conclusion that the particles that make
up cathode rays are a thousand times
smaller than a hydrogen atom hydrogen
atoms are the smallest atoms so this
thing that's in the cathode rays must be
really really tiny compared to an atom
and finally Thompson swaps out the type
of metal that's in the cathode tube he
uses a variety of different types of
metal but he finds that all of the
different metals he use he uses give off
the same cathode rays and regardless of
what metal he uses via the size of the
things that are in the cathode rays is
exactly the same so based on these three
conclusions he's able to come up with
the idea that atoms have tiny negatively
charged particles inside them and these
things that we're talking about here are
electrons now if you're like me when I
first learn this you might be confused
how these conclusions definitely tell
him that there are electrons inside
atoms I remember when I first learned
this stuff I was thinking like wait
doesn't this just show
electricity or cathode rays have
negatively charged stuff how does this
tell us anything about the atoms okay
here's why at the time that Thompson
does his experiment everyone for the
most part agrees that all things are
made up of atoms that atoms are the
smallest things that make everything up
there like the smallest things in the
universe
so if Thompson is able to show that
there are even tinier things well where
are those tiny things coming from they
have to be coming from atoms because
everything's made of atoms right that's
the only place they can be coming from
and even if electricity even if you want
to say that you know oh well he's just
showing that electricity is made of
electrons
well then still where are the electrons
coming from that make the electricity
the electrons have to be coming from
other atoms and since he can move and
remove these pieces of metal and swap
other metals in and he still gets
electrons it again shows that all
different types of atoms are able to
release or give off electrons so they
must be in there to begin with anyway
these conclusions that Thompson comes to
have dramatic effects on how scientists
think about what the atom actually looks
like now at the beginning of the video
we said that in Dalton's atomic model
atoms are indivisible that they're not
made of anything smaller
but when Thompson discovered that atoms
have tiny negatively charged electrons
inside them he essentially disproved
this model at least the part about the
indivisible atoms so here is the model
of the atom that Thompson comes up with
he realizes first of all that electrons
are negative we already talked about
that but then he notices that on the
whole atoms in general are usually
electrically neutral
so even though they have these
negatively charged electrons inside them
the atoms as a whole don't have a charge
so Thompson realizes that in order for
the atoms not to have a charge there
must be some positive charge in the atom
that can balance out the negative charge
of the electrons and then the positive
and the negative balance out and the
atom that as a whole doesn't have a
charge so he comes up with a model that
he calls the plum pudding at it now plum
pudding is kind of a British thing so if
you're American it might be better for
you to think about this as like the
blueberry muffin model and here's how it
works
Thompson imagines that the atom is like
a blueberry muffin and that electrons
are stuck in this atom the way
blueberries are stuck in a blueberry
muffin so you have these little
electrons that are stuck in the atom but
then they're surrounded sort of by a
dough and the dough like in a blueberry
muffin
um keeps these electrons or blueberries
held in place but the dough itself has a
positive charge and it's that positively
charged dough that is what balances out
the negative charge of the electrons and
so together they don't have any total
net charge so the atom is electrically
neutral so that's how JJ Thompson's
discovery of the electrons leads to this
plum pudding model yeah but now JJ
Thompson didn't have it quite right
because it turns out that the positive
charge in the atom isn't he isn't sort
of spread out in this dough or pudding
but instead it's concentrated right in
the center of the atom where the nucleus
is so that's gonna be the next step in
discovering what the atom really looks
like if you're interested in how
scientists disproved JJ Thompson's plum
pudding model check out Rutherford's
gold foil experiment where he discovers
that atoms have a very small very dense
positively charged
nucleus