[Music]
thank you one of the things i've noticed
when i give demonstration lectures is
that the most popular demonstrations
seem to be the ones that involve
explosions
so i thought we'd have a lecture
entirely about explosions and explosives
and the science behind them
now just before we begin i
do want to say that you should not try
to replicate any of these experiments at
home and also please don't try to make
explosives at home as you'll see in this
lecture when explosives detonate they do
so extremely quickly much faster than
your human senses can respond to in
other words explosives are very
unforgiving so please don't make
exposures at home
okay so we'll begin the lecture then of
course by creating an explosion what
we're going to do is to recreate on a
smaller scale the the largest explosion
that ever happened on the surface of the
earth in recorded history
and it happened in 1883 and it happened
on the volcanic island of krakatoa
and there was an explosion which was
equivalent to roughly 200 million tons
of tnt
the shock wave traveled around the earth
seven times the bang was heard three
thousand miles away
now there's a theory about how the
explosion occurs it
goes like this
the volcano had been erupting and
throwing out lava and it left behind a
gigantic underground chamber and sea
water rushed in and filled the chamber
now the walls of the chamber were very
hot the sea water turned to steam the
pressure built up and eventually there
was an explosion
so we'll try and create that effect on a
slightly smaller scale here in the
lecture theater
so
this is a a glass tube that contains one
milliliter of water if we just bring the
camera up on the screen you can have a
look at this it's been sealed at both
ends it contains just one milliliter of
water what i'm going to do now is to
heat it up
and raise the temperature
with this bunsen burner
and as the temperature rises the water
starts to evaporate
and the pressure begins to build up and
at some point it will explode it will be
quite loud so i'm going to wear ear
defenders and you may wish to cover your
ears for this one
it's a little bit hard to know exactly
when it's going to go
so this is one milliliter of water when
krakatoa exploded
it's thought that around a cubic
kilometer of water was turned into steam
so the water's turning to steam the
pressure is building up and eventually
it reached the point where the strength
of the container is not sufficient to
withstand that pressure and that's when
we get the explosion
so we call this a physical explosion
explosion caused by a build up and then
a sudden release of pressure
alright
okay so that's an example of a physical
explosion the build up and then the
sudden release of pressure
now if we want to make an explosion for
some practical application this isn't
very convenient we have to have the
flame we have to wait for the pressure
to build up we're not sure exactly when
it's going to go off and so on we need a
different way of creating explosions
we're going to do that using chemistry
and so we're going to have chemicals
that explode we call those explosives
now for around a thousand years there
was only one practical explosive which
was known and that's gun powder and in
the trade we refer to gunpowder as black
powder they mean the same thing
now gunpowder is just a simple mixture
of three ingredients and we've got the
ingredients here we can bring the camera
up this is
the first one this is charcoal familiar
from barbecues and so on it's finely
powded this acts as a fuel
we have
sulfur one of the elements this lovely
yellow powder this also acts as a fuel
and it helps to promote the burning
and the very important ingredient is
this one this is potassium nitrate also
known as saltpeter we can think of this
as a very concentrated source of oxygen
so when this gets hot it gives off
oxygen and the charcoal and the sulfur
can burn in that released oxygen
so the fuels the charcoal and sulfur
don't burn with the oxygen of the air
they burn with oxygen released from this
very concentrated form of oxygen
so let's have a look at some gun powder
in this dish i have one and a half grams
of a commercial gun powder
and i'm going to ignite this
electrically there's a little device in
there called an electric match it looks
just like a an ordinary match but i can
set it off by passing an electrical
signal to it and i'll do that using this
box this is called an exploder by the
way this is not called a detonator we'll
find out what the detonator is later in
the lecture that's something quite
different
so i'll charge this up and then we'll
set off one and a half grams of
gunpowder just in the open on the bench
as you can see we have a lovely smoke
ring
it burns very quickly but you notice we
didn't get a bang we didn't get an
explosion
gunpowder is what we call a low
explosive when we set it off in the open
it burns very rapidly but it doesn't
give us a bang it doesn't explode to get
an explosion we need to do something
else we need to confine the gunpowder
and so that's what we're doing over here
in this
cardboard tube
we have
again one and a half grams of gunpowder
the same quantity of the same type of
gunpowder again there's an electrical
match inside the tube
but this time the tube is sealed it's
bound up with tape and so the gunpowder
is very tightly confined
now what happens when we confine the
gunpowder is that as it burns it
produces energy in the form of heat the
heat can't escape as it can on the bench
instead it's trapped
and so the temperature rises
and when the temperature rises the rate
of chemical reaction the rate of burning
increases and that increases the rate of
heat production and so on we get this
feedback leading to very rapid
combustion we call that an explosion
so again we'll set this off electrically
and we'll use a different exploder this
time we're going to use this
machine this is uh dates from the early
part of the 20th century so dupont
blasting machine it's a genuine antique
blasting machine and remarkably this
still works so we'll use this to set off
the explosion and at this point i wonder
if anybody would like to volunteer to
come and use this
i think you were the first would you
like to come on down let's have a
handful volunteer please
[Applause]
try to come and stand there just not yet
just turn around just face the audience
what's your name
isaac isaac do you know how to use this
have you seen this on cartoons what do
you think you do okay what you do is
you're going to take hold of the handle
with both hands and not yet but when i
tell you
i'm going to raise it up okay and then
when i tell you you're going to push it
down nice and firmly okay now this is
going to be quite loud so i'm going to
give you a pair of ear defenders pop
those on and again you might want to
just cover your ears for this one
okay so when you're ready off you go
excellent
thank you very much
[Applause]
okay so that's the effect of confinement
on a low explosive like gunpowder it
accelerates the rate of burning we get
that that nice explosion
now
gunpowder is just a mixture of fuels
sulfur and charcoal and this source of
oxygen potassium nitrate we call that an
oxidizer
and people searched for alternatives to
gunpowder other kinds of explosives and
so they started to experiment with
different choices of fuels and different
oxidizers and they invented a whole
range of different explosives and these
are still used today in things like
fireworks and pyrotechnics
i'm going to mention just one of them
it's this one it's called flash powder
and again it's a simple mixture between
a fuel in this case a very finely
powdered metal and an oxidizer not
potassium nitrate but a different
oxidizer but again a very concentrated
source of oxygen
so i have one and a half grams of flash
powder and it's called flash powder
because it produces a very bright flash
it's used in the theater for example
when you know when the genie appears you
want a bright flash and a puff of smoke
so that's where you might use flash
powder so let's set off one and a half
grams of flash powder and again we'll do
this in the open on the bench
we'll bring the lights down for this but
it will be very bright
so i suggest you don't look directly at
this instead you look off to one side
so this is uh one and a half grams of
flash powder now burning in the open so
again it's very bright and i suggest you
look to one side
so here we go
[Applause]
so
again we had a very rapid combustion but
we didn't have a bang we didn't have an
explosion but we know what to do to get
a bang out of our flash powder we need
to confine it so that's what we're going
to do now
so this apparatus consists really just
of a heavy cylinder of steel with a
whole board down the middle and in the
bottom of the hole we have again one and
a half grams of exactly the same type of
flash powder but now the confinement
produced by the metal walls should
accelerate the rate of burning again we
should get an explosion again we've got
the electrical match and we'll use our
oldie blasting machine to set this off
so there's a chance now for another
volunteer to have a go who'd like to
have a go uh yes on the end there in
yellow so the hands-on volunteer
that's come to stand there
what's your name
daniel all right don't forget to turn
around you saw what to do didn't you so
put those on and wait for me to get mine
on this will be i think pretty loud so
again i recommend that you cover your
ears
okay off you go daniel
[Music]
lovely thank you very much
[Applause]
okay so once again then the confinement
of this low explosive leads to an
accelerated rate of burning and we get
an explosion
but people were constantly searching for
ways to improve explosives to make them
burn even faster what can we do to make
our explosives burn even faster
well the exposures we looked at so far
are mixtures they're mixtures between a
fuel and a source of oxygen
and it seems obvious that the better we
can mix them together the faster they
will burn because they burn by the fuel
coming into contact with the oxidizer
so we can test that uh idea out by using
this this is called lycopodium it's a
very fine powder and it's uh flammable
so i'm going to put a little on a mat
and we'll just see if we can set fire to
this
and there is it's just about
could just see a little flame it's just
caught fire so this is flammable what's
happening is that there's a chemical
reaction taking place we call it burning
between the fuel or the lycopodium
powder and the oxygen of the air
but it's a very slow chemical reaction
because it only happens where the fuel
comes into contact with the oxygen it
only happens at the surface of that
little pile
so i'll blow that out
and what we can do now is try something
different we can take some more
lycopodium powder and this time we'll
mix it very thoroughly with the air
first
and then we'll set fire to it so i'm
going to put some more lycopodium powder
into this tube
and i'll use the tube to blow the powder
into the air and that will mix the
powder in the air
very thoroughly there'll be much more
surface area of contact between the
powder and the oxygen of the air and
that should
improve the rate of combustion
so i'll use this bunsen burner
to set fire to the powder we'll bring
the light stand for this
and when the lights are down i'll then
blow the powder through the flame and
we'll see if that increases the rate of
combustion
[Applause]
okay so clearly then mixing the
lycopodium more thoroughly with the air
produced a faster rate of combustion
but using the oxygen from the air isn't
um isn't very good because the air has a
very low density each cubic meter of air
doesn't contain very much oxygen
so we can do better if we use a more
concentrated form of oxygen so i'm going
to use now liquid oxygen minus 183
degrees centigrade so this is just
oxygen that's been cooled down until it
turns into this and beautiful blue
liquid
in the dish i have some ordinary cotton
wool
so i'm going to soak the cotton wool in
the oxygen so the cotton wool
will function as the fuel
but now we have a much more concentrated
source of oxygen
and again we'll bring the lights down
for this
so this is cotton wool burning in liquid
oxygen
[Applause]
so
that certainly burnt pretty quickly with
a little explosion there
but this again is not a very practical
way to make an explosive because liquid
oxygen if we leave it sitting around
we'll warm up to room temperature and
it'll evaporate so it will just sort of
disappear after an hour or two
we need something else something a
little bit more permanent
and the the breakthrough came when
a swiss chemist christian schonbein was
doing some chemistry in his kitchen
of all places one day and he had a
little accident
he'd been working with a mixture of
nitric and sulfuric acid which is a very
corrosive mixture and he spilled some on
the kitchen table and he reached for a
piece of cloth to wipe it up and it was
a piece of cotton cloth
so he wiped up the acids with the cotton
cloth he hung the cloth up to dry over
the stove and when it dried out it
exploded
and he discovered some new chemistry he
discovered something called
nitrocellulose
also known as gun cotton
what was happening there was really
rather special the effect of the nitric
and sulfuric acids was to introduce
oxygen and also nitrogen as well into
the actual molecules of the fuel itself
so the fuel is cellulose these long long
molecules a natural material and the
effect of the acids was introduced
oxygen right into the molecule itself
so this is the best possible mixture we
could have between the fuel and the
oxygen having them built into the same
molecule so let me show you some
nitrocellulose
thanks chris
so this is some that i made earlier this
is just
ordinary um ordinary cotton wool i could
buy from the chemist but it's been
treated with nitric and sulfuric acids
it's been washed and dried and it still
looks and feels pretty much like cotton
wool
right up until the point where i set
fire to it
now
this is actually going to be fairly
bright so again you might just look
slightly to one side so this is nitrous
cellulose
[Applause]
now
nitrocellulose
when it burns produces relatively little
smoke we had quite a large pile of
nitrocellulose so we got hardly any
smoke compared to the tiny amount of
gunpowder for instance the gunpowder
produces a lot of smoke because the
products the output the end result of
the chemical reaction includes materials
like potassium carbonate which are
solids at room temperature so it's those
tiny particles of solid which make up
the smoke the nitrocellulose the
molecules rearrange themselves into
things like water vapor carbon dioxide
nitrogen gas things which are invisible
gases present in the air anyway and so
the nitrocellulose more or less seems to
disappear
so nitrocellulose is used a lot in
pyrotechnics and partly because it
produces very little smoke it's a very
convenient material to use and so what
i'm going to do now is put on a little
pyrotechnic show for you and i hope
you'll enjoy this so we'll bring the
lights down
so this is these are nitrocellulose
pyrotechnics
[Applause]
okay so in all the the explosions and
the pyrotechnics we've seen there is a
sudden release of energy
when the explosion occurs so where does
that energy come from
well we can gain some
insight into this this source of energy
with a little demonstration and for this
i'm going to need
one very brave volunteer so who's
feeling extremely brave
lots of people volunteering their
friends who'd like to come on down and
do this one you seem very keen you're
very brave let's have a hand for a
volunteer
[Applause]
if you'd like to come and stand just
there and pop those on what's your name
lucas right lucas what i want you to do
is hold up your right hand um arms
length that's it i want you to form your
hand into a little cup
well just one hand is fine it's a cup
like that that's right this is just soap
solution just sort of washing up liquid
really and i'm going to put a little
puddle of this in your hand
you just say you're brave didn't you
okay there we go
it's going to create a little explosion
in your hand
now
this is this is a an electrical power
supply and we're passing electricity
through a bottle containing uh water
it's actually sodium carbonate solution
and the effect of the electricity is to
break up the water now water um it's
just
that a bit closer there we go
water of course you all know the formula
for water h2o water is a compound of
hydrogen and oxygen
and the effect of the
electricity is to break up the molecules
to produce hydrogen gas and oxygen gas
so this little pile of bubbles in your
hand contains a two to one mixture of
hydrogen and oxygen
okay let me just have a little bit a
little bit more how brave are you
feeling
quite brave you're feeling a bit nervous
i'm not nervous
all right we'll switch that off now
we're going to pop some ear defenders on
you for this thanks chris
thanks very much i'm going to pop some
ear defenders on
and we'll set fire to this mixture of
hydrogen and oxygen
you ready hold your up arm up straight
here we go
well done
okay thanks very much
okay so that little explosion clearly
released some energy but where did the
energy come from
well hydrogen gas consists of molecules
and each molecule
contains a pair of hydrogen atoms like
this
now
oxygen is similar oxygen consists of
oxygen molecules and each molecule again
has a pair of oxygen atoms
now our volunteer's hand had a had some
bubbles inside the bubbles we had a
mixture of two parts hydrogen and one
part oxygen
but it just sat there it didn't do
anything until we applied a flame when
we apply a flame we're adding a bit of
energy we call that activation energy
it's a very important concept the
activation energy is breaking apart the
molecules it's breaking the bonds that
hold the molecules together we have to
put energy in in order to break those
bonds so the bonds the hydrogen oxygen
get broken
and then new bonds can form the
oxygen and the hydrogen can come
together to form water so as they come
together to form new bonds energy is
released
and a lot more energy is released by the
formation of these new bonds that it
cost us to break the original bonds
and so with
two molecules of hydrogen and one of
oxygen we can make two molecules of
water so this is water h2o
so this idea of activation energy is
very important one i can illustrate it
with this model of a ball on a hill
so the ball is at quite a high point on
the hill so it has gravitational
potential energy compared with the
bottom of the hill
energy would be released if the ball
could roll from the top to the bottom of
the hill but it can't because it's stuck
in this little uh well this little
valley
if i add a little bit of energy to this
nothing much happens
but if i had enough energy to get it
over the hump
we call that the activation energy
then the rest of the energy can escape i
get the activation energy back again
plus all this extra energy
so when we set fire to the hydrogen
oxygen we are we are adding
the activation energy
and then we get the activation of your
back again plus a lot of extra energy
so that's the idea of activation energy
i'm going to illustrate activation
energy with a different kind of
experiment and for this we're going to
bring the lights down because we're
going to use not heat as the source of
activation energy but rather light so
we're going to do an experiment now
which involves a glass tube a heavy
glass tube filled with a mixture of
hydrogen gas and chlorine gas
now again the two gases are sitting
there mixed together nothing very much
is happening
but if we apply some activation energy
we can break apart the bonds of the
molecules and they can react to form
hydrogen chloride and at the same time
release some energy
so we're going to use light to provide
the source of activation energy
now light consists of particles called
photons
and the energy of each photon depends
upon its color in other words it depends
upon its wavelength or its frequency
so long wavelength light
such as this red light the photons have
a relatively low energy
so what's happening now is the red light
from the projector is passing through
this tube which contains hydrogen and
chlorine but no reaction is taking place
because the photons don't have
sufficient energy
at this point i'm going to put some ear
defenders on and then we'll explore some
different wavelengths of light
so let's
go to a higher frequency or shorter
wavelength of light
we'll try some
yellow blight
higher energy but still not enough to
get the ball over the hill not enough to
reach the activation energy
there's some green light
high energy still but not still quite
enough energy to get over the hill but
let's try the shortest wavelength of all
some blue light with the photons have
the the highest energy
[Applause]
so that's an example of activation
energy we add a little bit of energy to
get the reaction going and then it
releases a lot more energy
now different reactions will have
different levels of activation energy
sometimes the that hill at the top will
be quite high that's a high activation
energy we need to put a lot of energy in
to get the reaction going sometimes the
activation energy will be very small
i'm going to show you a reaction now
where the activation energy is so low
that the reaction happens spontaneously
what i'm going to do is to add this gray
powder it's called magnesium silicide to
some hydrochloric acid
now the reaction that takes place
produces a gas called silene now you've
all heard of methane gas i'm sure
methane is natural gas it's the gas that
you use for cooking and and so on
and a molecule of methane has a carbon
atom and four hydrogen atoms
well cylinder is very similar except
that it has a silicon atom and four
hydrogen atoms and like methane it will
burn in the air but the activation
energy is so low that this will ignite
as soon as it's formed
so we'll bring the light stand for this
i'll add the magnesium silicide to the
hydrochloric acid and as soon as the
silane gas is formed it will ignite in
the air
[Applause]
okay so that's a system with a very low
activation energy what i'm going to show
you now is an explosive that has
a very low activation energy not so low
that it goes off spontaneously but
almost
this is an explosive called
nitrogen triiodide
and the molecules consist of nitrogen
bonded to iodine
but the bonds are very easily disturbed
and when that happens
an explosion takes place now this will
be quite a loud explosion
so again i'm going to wear ear defenders
and you may wish to cover your ears
the
the nitrogen tri iodide
is this black solid sitting here on this
filter paper all i'm going to do is give
a gentle tap with this stick
and that should be sufficient to set it
off
so here we go
[Applause]
now
i'm sure being all being observant
scientists you'll have noticed that
cloud of purple vapor that's produced
that's iodine
so the molecule consists of nitrogen
combined with iodine it breaks apart to
release the iodine and nitrogen
now nitrogen plays a very important role
in explosives and i'll explain why that
is with the help of this model
this is a model of a molecule of
nitrogen and it consists of two atoms of
nitrogen joined together by this very
strong triple bond now this is one of
the strongest chemical bonds in the
whole of chemistry
it means that nitrogen is a very
unreactive material
so nitrogen as you know is about
four-fifths of the air it's very hard to
get the air the nitrogen in the air to
react with anything that's because the
bond is very strong
so to get the nitrogen to react we'd
have to pull the atoms apart and break
this very strong bond that would require
a great deal of energy
now energy of course is conserved so if
we took two atoms of nitrogen and
brought them together so that this bond
is formed that same amount of energy
must be released
so when nitrogen gas is formed a lot of
energy is released that was what was
happening with that nitrogen tri iodide
a lot of that energy was coming from the
formation of this nitrogen nitrogen bond
so nitrogen plays an important role in
explosives and there's a very similar
explosive a little bit less sensitive
than nitrogen tried which plays a very
important role because it saves many
lives every year it's used in car
airbags now a car airbag as you know is
a safety device where a bag is inflated
uh moments before uh well
as a collision is occurring your car
collides with something the airbag is
set off
and the airbag will protect you
hopefully from injury
now
i
imagine that most of you have never
actually seen i hope most of you have
never seen an airbag going off the reel
so what we'll do today is actually set
one off for you so we've got
not a real car but nevertheless a real
steering wheel with a genuine airbag
inside
and we have a battery at the front of
the car's little switch in a moment i'm
going to drive the or push the car into
the wall it will close the switch and
set off the airbag this is actually
quite loud
obviously if you're involved in the
collision the last thing you're worried
about is a is a loud bang but if i can
find my ear defenders i'm going to pop
these on
and then we'll have a look at the car
airbag
so here we go
[Applause]
okay so that's a car airbag that
contains an explosive called sodium
azide it's a compound of sodium and
nitrogen when it decomposes when it
explodes it produces sodium metal and
nitrogen gas and the formation of those
nitrogen bonds that produces a lot of
the energy of that explosion now people
searching for new explosives realize
that schernbein's discovery of nitrous
cellulose or gum cotton was an extremely
important one because he had worked out
how to introduce oxygen and nitrogen
directly into molecules in order to turn
them into explosives and his first
discovery was was nitrocellulose or gum
cotton but people took the same idea and
began to search for other explosives in
which nitric and sulfuric acids were
used to introduce the oxygen and the
nitrogen directly into the molecule
so people searched uh lots of different
uh
possible compounds but the real
discovery was made by an italian chemist
ascanio sobrero in 1847 when he tried
nitrating that is treating with nitric
acid not cotton but another substance
called glycerin now glycerine is a
product a byproduct of the manufacture
of soap and if we nitrate glycerin we
get a substance called nitroglycerin
so this is some nitroglycerine
this is a small quantity which i made
this morning here in the chemistry
department
and you can see
it's
a clear liquid it looks a bit like water
slightly viscous
you'll see in a moment it behaves rather
differently from water
so what i'm going to do first of all is
to place a few drops
of the nitroglycerine onto a heatproof
mat
and we'll set fire to it and we'll see
what happens
i'll put the rest of it away
and let's see what happens if we just
heat this with a blow torch
and try and set fire to it
so i hope you saw there it just burns it
burns with a lovely green flame
there's no explosion
just a very small green flame it behaved
a bit like the nitrocellulose it burned
very rapidly
the nitroglycerin can do something very
different and i can illustrate that in
the following way i'm going to take a
filter paper and place it on this
steel anvil
i'm now going to get the same quantity
of nitroglycerine
and place them on the filter paper
there we go
and this time i'm going to hit the
nitroglycerin with a hammer
and the effect should be rather
different from before
so this
is likely to be very loud so i'm going
to put ear defenders on
i'm also going to put on a face shield
and i'm going to wear a glove
this is definitely one you shouldn't try
at home
okay so this is just five drops of
nitroglycerine
[Applause]
so what happened there was very
different in the first case it just
burned but here something very different
happened we call this
detonation and detonation is
a very specific physical effect and
we'll have a look at that again in a
moment
now nitroglycerin has quite a reputation
for being a dangerous material and
that's because in the early days there
were a lot of accidents when it went off
by mistake when it was being made or
when it was being transported
and although nitroglycerin is an
extremely powerful explosive it wasn't a
very practical explosive it was just too
dangerous to use
and
this was very disappointing and so
people tried to find a solution and the
person who really solved the problem was
a swedish chemist called alfred nobel
and he tried combining the nitroglycerin
with other materials in order to make it
less sensitive
and one day he hit on this material so
this is a clay-like material it's called
keselger
and hazelguard is very absorbent so it
can absorb many times its own weight of
nitroglycerin
and when it does so it remains a very
powerful explosive but it's much less
sensitive and he called this material
dynamite and dynamite's usually made up
into sticks and the sticks can be pushed
into boreholes in rock if we're doing
quarrying or mining or tunneling
it's very safe explosive because it's
much less sensitive than pure
nitroglycerin and so it's safe to store
and transport
so nobel had solved one of the great
problems with nitroglycerin he'd worked
out how to prevent it going off
accidentally
but there's another problem with
nitroglycerin is it doesn't always go
off when you want it to it's a kind of
unreliable explosive
and when it's made into dynamite
because it's very insensitive it's very
hard to get it to go off setting
dynamite off is actually very difficult
so that led to nobel's other great
invention and this is called a
destinator
and this is a modern electric detonator
it just consists of a small metal tube
and inside is a very small quantity of
an explosive that's quite sensitive call
it a primary explosive
so the explosive in here is quite easy
to set off
this one is actually a modern destinator
which is set off electrically and
nobel's original detonator would have
been set off by a slow burning fuse
this was a brilliant invention because
the detonator
although it contains a sensitive
explosive it only contains a tiny
quantity of that explosive so although
detonators are quite dangerous things
there's a limit to how much damage a
single detonator can do
so the detonator is kept well apart from
the the main explosive the dynamite
right up until the moment that we want
to use the explosive and then the
detonator is placed in contact with the
explosive
the effect of the detonator when it goes
off is to create a small explosion that
provides a very sharp shock it's rather
like the hammer blow and that hammer
blow is enough to set off the main
explosive so the dynamite is safe
because it's very insensitive the
detonator is relatively safe because it
doesn't contain very much explosive
and those two inventions of nobel really
opened up the world of high explosives
and as a result he became extremely
wealthy in fact he became one of the
richest people in the world at the time
and when he died in his will he gave his
vast fortune to the creation of the
nobel prizes which are today the most
prestigious prizes in science
and there's even one of the elements
named after him it's called nobelian
now dynamite was sort of the beginning
really there were lots of other
explosives invented i'll mention one of
them if you
take a solvent called toluene and treat
that with nitric and sulfuric acid you
get
trinitrotoluene and the abbreviation for
that is tnt again an explosive that many
people have heard of
so don't confuse tnt with dynamite
they're quite different dynamite
consists of nitroglycerin absorbed into
kieselger or some other material whereas
tnt is a completely different chemical
compound
and people continue to research new
kinds of explosives this is a very new
one this is the latest british explosive
this has been available for a couple of
months now this is a plastic explosive
it's called pe-7 and a plastic explosive
an explosive that's squidgy and moldable
so this is like sort of plasticine or
play-doh
this is a very powerful explosive but i
should just say that none of these
explosives are live this is actually a
training
block of material it's identical to
real p7 in every respect except that the
explosive component has been replaced by
an inner material it's also been colored
blue so that you can tell that it's
inert the real explosive is white
so when we
looked at the combustion of gunpowder we
saw that gunpowder in the open just
birds it burns very rapidly this is a
burning process there's no explosion
when we hit the nitroglycerin with a
hammer we get something very different
we get this loud explosion we call that
a detonation
the the burning of gunpowder and other
explosives of that kind we give that a
fancy word we call it deflagration it
just means burning
so low explosives burn whereas
nitroglycerin is something very
different we call it a high explosive
and high explosives detonate i want to
explain the difference between a low
explosive and high explosive and explain
what's meant by detonation
so first of all going to demonstrate
this idea of deflagration or burning
so this is a metal track and it contains
a powder the powder is actually finely
powdered nitrocellulose it's the kind of
powder that's used to reload the the
bullets for guns
and what i'm going to do is to set fire
to this trail of powder at one end
and you can see what's happening
the
trail of powder is burning along
what's going on here is that a a piece
of the pile of the powder is burning and
releasing energy in the form of heat
that heat energy is raising the
temperature of the next piece of powder
providing the activation energy so that
it can burn it then releases heat and in
turn ignites the piece of powder to the
next
next door to it
so what's happening then is the flame is
progressing by the transfer of heat so
that's what we call deflagration that's
a low explosive
detonation is something completely
different and to explain this i'm going
to need
about six volunteers something like that
who'd like to come on out
any
guys in the front shall we have yes go
on you guys come out
go back from you
to there eight of you come on then let's
have a handful of volunteers
if you'd like to form a row
oh gosh we could do you want to come to
this end
maybe if you come to this if you come
and stand just here just trying to get
to a roughly height order okay that'll
do
right what i want you to do is to face
this wall what's your name
tom all right you just stand upright
like that now what's your name
bridget okay if you just stand back a
little bit face uh tom's back put your
hands on his shoulder shoulders grip
them very tightly if you stand back a
little bit have your arms very rigid
locked rigid that's good so if i move
you he'll move okay so keep your arms
locked and rigid okay you do the same
thing what's your name harry harry okay
put your arms on her shoulder stand back
a bit keep your arms nice and locked so
that you move together okay same thing
what's your name thank you okay
you are
joe
mark mark okay put your hands on his
shoulders arms nice and rigid so you're
locked together that's good oh right
okay can you reach
put your arms on your shoulders what's
your name
kim and you are abby abby all right arms
on her shoulders good
now what i'm going to try to do keep
your arms nice and rigid and locked
i'm going to give you a shove and we'll
see what happens
okay i'm going to do that one more time
so when i give you a push
good the push sort of works its way down
now what i want to imagine is each of
our volunteers is like a molecule in a
block of explosive and their arms are
like the bonds between the molecules
if one of the molecules gets a push or a
knock with a hammer then the wave
travels through the block now if there's
a block of regular material that would
travel at a particular speed we call
that the speed of sound this compression
wave traveling through a material we
call a sound wave it's how you're
hearing my voice right now through
compression waves traveling through the
air those compression waves those sound
waves can travel through solid materials
as well okay let's have a big hand for
our volunteers
i want you to imagine that this little
dot is a source of sound waves so those
circles spreading out of sound waves the
light circles are regions of high
pressure and in between we have regions
of low pressure so that's sound wave
spreading out from an object
now let's imagine that the object starts
to move
well you can see what happens the sound
waves ahead of the object pile up and
get closer together and the ones behind
it are sort of trailed out and they get
further apart
now we call this the doppler effect it's
something that's very familiar if you
hear a car go past it makes a particular
noise it goes
okay the note the pitch drops as it goes
past you and you can see why ahead of
the object the sound waves are bunched
up the wavelength is shorter the
frequency or the pitch is higher
likewise at the back the waves are
spread out so the frequency or the pitch
is lower
now what would happen if the object
moved at the speed of sound
well here's the object now moving at the
same speed as sound waves and now the
sound waves all pile up at the front of
the object they all catch up with each
other and so we get this region of very
intense pressure
and let's just make it move a little bit
faster than sound
and you can see there's these this sort
of bow wave we call that a shock wave
that's where the sound waves are all
catching up with each other
so that shock wave
is is something that would be produced
for example by an aircraft when it's
traveling faster than the speed of sand
so supersonic aircraft would create the
shock wave we can hear the effect of
that shock wave on the ground in the
form of a sonic boom
so a shock wave is the mechanism by
which a high explosive detonates
so in a high explosive the
uh the it's not heat
being uh propagated from one region to
another like in our trail of powder it's
not just heat energy
raising the temperature of the next
piece of explosive it's rather the
passage of a very fast shock wave
traveling much much faster than the
speed of sound
okay thank you for the computer screen
so how does a shock wave set off the
explosive
well i could demonstrate that with with
this apparatus this is a a plastic a
clear plastic tube and it has a piston a
plunger which is uh sealed
and i'm going to take some cotton wool
this is just ordinary cotton wool it's
not gun cotton or anything just ordinary
cotton wool take a little piece of this
and place it in the end of the tube
and then i'm going to seal the tube
so what i'm going to do now is to
compress the air in this tube and i'm
going to do so very quickly now if
you've ever pumped up your bike tires
quickly you know if you're in a hurry
and you pumped up your tires really
quickly you'd notice that the tires and
maybe the pump as well get hot so when
we compress a gas it gets hot
so
i'm going to compress this gas very
rapidly and if we bring the lights down
we'll see what effect this has remember
there's just a little bit of cotton wool
right there in the in the end of the
tube so here we go
so that got so hot
[Applause]
that became so hot that the cotton will
actually ignite it and there's nothing
left in there now just the it looks like
an empty tube when a shock wave passes
through a high explosive that shock wave
is a region of very intense uh pressure
and that pressure heats up the explosive
and causes it to detonate and that
detonation releases energy which is what
drives the shock wave onwards and that
shock wave travels much faster than the
speed of sound
now what i'd like to do now is to try to
demonstrate for you a detonation
shockwave here in the lecture theater
now it's quite difficult because if i
couldn't if i just set off a block of
plastic explosive that would be the end
of the lecture i can't do that obviously
so i'm going to find a way of
demonstrating a destination
and
to do that i'm going to use some of this
this is called shock tubing it's
actually a commercial product it's used
on a
enormous scale by the explosives
industry and it consists of a plastic
tube a hollow plastic tube and the
inside of the tube is coated with a very
fine powder of a high explosive called
hmx and this explosive is about 70
percent more powerful than tnt but
there's just a very light dusting of it
so what i want to do now if we can bring
the
the camera on screen
is to
just step back a moment
what i'm going to do is to blow through
the tube and
watch to see if you can see some of this
hmx powder coming out of the tube
okay so it's just held on by static
electricity so i can blow it out of the
tube
now if i
uh just took a piece of shock tubing and
set fire to it'd be rather like the
nitroglycerin it would just burn
but if instead we can
provide a very sharp shock to the tube
we can initiate a detonation that will
pass down the tube at very high speed
so
let's have a look at that
and to
illustrate shock tubing and show you how
it works i've got a little demonstration
in which i've used shock tubing
to spell out the the name of the
explosive that it contains so this says
hmx
and hopefully this will help you to
remember the name of the explosive
so
i'm going to
initiate this shock tubing with
obviously with a shock and that's going
to be provided by this device which
produces a voltage of two and a half
thousand volts
and then discharges it to make a little
lightning
strike if you like so i'm just going to
point this at you right safe and fire it
you should just be able to see a little
spark
okay over here
i'll try over there so it's not
producing very much energy but it's
producing that energy in a very short
interval of time in the form of a shock
and that can initiate a destination wave
traveling down the shock tube
so
these letters consist of just a single
length of shop tube wound in and out of
the board
and so we'll fire the shock tube
and hopefully you'll be able to
see the the detonation of the of the
tubing
now this uh
this will produce quite a bright flash
so if we bring the lights down you
should be able to see the shock tubing
firing quite clearly
it makes a little bit of a noise you may
or may not wish to cover your ears
okay so here we go this is shock tubing
here we go
[Applause]
okay so that was a destination wave
passing through a few meters of shock
tubing but it's extremely quick so i
doubt that you could see the actual
detonation wave pass along the tubing so
what i'm going to do now is to
set off a longer section of shock tubing
and see if we can actually see the the
detonation wave passing along the tube
and for this i'm going to need four
volunteers who'd like to come and hold
the shock tubing while we set it off
some of the hands are going down i
noticed have you on the end there
and who else should we have we'll have
you
and yes you can come on down and let's
have you as well let's have a handful of
volunteers please
okay can you stay there you stay there
if you come with me
you come with me to this end
maybe there we go
um if you'd like to come and stand about
here
now we're going to give you
some eye protection and some hearing
protection will give you a glove to hold
as well
and this is the shock tubing
so if you if you come with me stand
about there what i want you to do
what's your name albert albert all right
albert i want you to hold that end of
the tubing just hold that for a moment
we'll wind this out across the full
width of the lecture theatre okay hold
that just hold it in your gloved hand
what's your name adam adam
okay
now if you'd like to come with me a
little bit
if you'd like to come with me
hopefully there's enough to go all the
way across if you stand about there hold
that and if you come with me what's your
name
bradley bradley and
you're going to be down this end
pretty good you hold that there what's
your name adam adam yeah okay what i
want you to do hold it near at the end
now it's very important i want you to
point that away from your for your face
okay because there'll be a little split
of flame come out of the end hold it up
nice and tight we're going to try and
stretch this out so if you step forward
a little bit
hold it up nice and high
so everybody can see that's good if you
just step forward a little bit that's
good
and excellent
good okay so what we're going to do now
is to uh start a shock wave a detonation
wave at this end of the tubing and we'll
see if we can see it passing all the way
down to the other end of the tube
so again we'll bring the lights down for
this
and here we go three two
one
[Applause]
okay just stay where you are for a
moment if you just stay where you are if
you'd like to head back to your seat so
the two volunteers at the end can go
back to their seats let's just thank
them once again
and stay with just stay where you are
okay so you know you may or may not
think you saw that shock wave traveling
down the tube but i can tell you now
that it was traveling at 2
100 meters per second that's nearly
seven times the speed of sound so you'll
be quite lucky if you did actually see
it
but i really wanted to show you a
detonation wave i wanted you to see a
destination wave passing at seven times
the speed of sound so what we've done
and i think this may be the first time
this has ever been done is we've wound
nearly a kilometer there's something
just over 800 meters of shock tubing
wound around the walls of the lecture
theatre so we're going to bring the two
ends out now to the middle of the
lecture theater
thank you chris
all right you come with me you're going
to hold this end
okay that's it you stand there
and you come with me we may overlap you
a little bit that's fine keep going
you've got a we've got a few knots in
that shouldn't matter
okay so i think this is a first
we tried a smaller scale demonstration
earlier in the week it was it was very
interesting so this is the first time
you've tried it with such an enormous
length of this shot tubing so this
should take about
point four of a second just under half a
second for the shock wave to travel
along this tube eight times around the
lecture theatre and then back along this
tube
if it all works
i hope you're happy so
you are completely surrounded by an
explosive that's 70 more powerful than
tnt
okay so here we go then we'll bring the
lights down
let's hold up nice and high
and we're firing in three
two
one
[Applause]
so when an explosive detonates of course
it produces gas so we've now got 800
meters of tubing full of high pressure
gas
if we bring that
you hear that gas escaping
extraordinary let's have a big hand for
our two volunteers
thank you
okay so um that pretty much brings us to
to the end of the lecture obviously it's
a lecture about explosions so of course
we have to end with a big explosion
um
before we do though
i just want to say a big thank you first
of all to the department of chemistry
for hosting this i'd like to thank our
sponsors of course microsoft research
event horizon and lemaitre for their
very generous support i'd also like to
ask you to join me in thanking somebody
who's put a lot of effort into helping
me prepare and deliver this lecture and
that's chris braxton
[Applause]
okay so we need a good explosion to
finish
and we're going to
create another physical explosion if you
remember at the beginning of the lecture
we had that little tube of glass water
which we heated up the pressure built up
and it burst that was a physical
explosion so we're going to do something
similar but we're going to scale it up a
little bit
and instead of using
water this time we're going to use
liquid nitrogen
so this is
liquid nitrogen this is this is at
-196 degrees centigrade
so nitrogen remember that's about
four-fifths of the air if we cool it
down to minus 196 degrees centigrade
extremely cold it becomes a liquid so we
have some liquid nitrogen here
and
if we were to add liquid nitrogen to
warm water
it would raise the temperature of the
liquid nitrogen it would evaporate and
turn into a gas and in the process it
would expand rather like the water
boiling and turning to steam
so first of all demonstrate that by
adding some liquid nitrogen
into some hot water
[Applause]
okay so when we add the liquid nitrogen
to the hot water the liquid nitrogen
turns from a liquid into a gas and it
expands
and that's fine because it can expand
and escape into the room but what would
happen if we did the same thing that we
used a closed container a sealed
container
well
the container that we're going to use is
this
it's a a half liter um fizzy drinks
bottle so i'm gonna put some liquid
nitrogen into here screw the lid on so
it becomes a sealed container
and of course when you're taught how to
use liquid nitrogen you're told under no
circumstances keep your liquid nitrogen
in a sealed container you'll find out
why in a moment
and then going to drop it into here we
have a bucket inside the bucket contains
some warm water so the nitrogen will
evaporate and turn into a gas would like
to expand but it'll be trapped inside
the bottle the pressure will build up
and and you can guess the rest
so
let's get some liquid nitrogen
i'm going to fill this about one third
full
of liquid nitrogen
there we go
so this is the the final demonstration
of the lecture i hope you've enjoyed
yourselves and i hope you enjoy
the uh the final demo here we go
[Applause]
thank you very much
[Applause]
you