Over the last few years, this demo has become a bit of a favourite and it's certainly one that
I can't help but love as well.
All it really involves is some ethanol, which is just ordinary alcohol,
As you can see, it burns with its nice
bluish yellow flame but quite gently. And the reason it burns gently is that it actually has to turn
into a gas before it will burn - it's not the liquid that's burning
so it can only burn as fast as the liquid evaporates.
In the case in the bottle, I slosh the ethanol all around
coating the inside surfaces, allowing it to evaporate
and creating this highly combustible mixture of
ethanol vapour and air which is ready to burn. If we ignite it and slow down the footage,
you can see that at first, it burns with a really lovely clean blue flame
which is a sign of what is called complete combustion.
This is where these is abundant oxygen to react with the fuel so you just get the classic combustion products
of carbon dioxide and water.
As the reaction progresses, the oxygen starts to get consumed and so,
as well as the carbon dioxide and water, you start to produce lots of tiny
carbon particles which glow yellow with the heat of the flame.
There's an interesting thing that you can see the end of the reaction
which is this yellow billowing flame and that's probably because of
fresh oxygen being drawn into the bottle through the normal processes and mixing around
with what's left of the fuel inside
So anyway so far so fun
but besides the fact that this looks really cool, what I think is interesting about this
is that it's basically just a rocket.
A rocket is a device for burning fuel
and creating a force by projecting the combustion products
out the back and that's exactly what's going on here.
So if this is generating some sort to force,
maybe we can get to propel something, such as me.
That was underwhelming.
So clearly the rocket isn't strong enough to have much of an effect on a mass such as mine.
So let's back up a bit and see if we can determine how much thrust
it is generating and we can do that crudely on a balance like this.
So that looked like it produced around 600 grams of thrust.
That's about six Newtons, which is not bad but you can kinda see why
the bottle was a bit out of its depth in trying to push on me. But the bottle on its own
is around 100 times lighter than I am so let's see how that gets on.
You can really see the acceleration as the fuel burns.
The burning gas creates a build up of pressure inside the bottle
that simultaneously forces the exhaust out the back and propels the bottle forward.
What we're seeing here is a really beautiful demonstration of one of the most
fundamental laws in physics - the conservation of momentum.
Where one object, in this case the exhaust gas, moves in the opposite direction to the other object, the bottle,
balancing out the overall momentum and keeping Isaac Newton,
and therefore the universe, happy.
We can also try some more fuels. This is methanol, which is similar to ethanol but has a smaller molecule.
It seems the pack a bit more of a punch.
And this is propanol, which is a larger molecule with more carbon in it.
You can see that it burns with a lot more the yellow flame, which you will remember was a sign of unburnt carbon particles.
Now this all looks very rudimentary but in fact some early rockets
were propelled with ethanol. The infamous V2 rockets
used by the German army in the second world war, combined their ethanol with pure oxygen.
Fully fueled, they weighed over twelve tons, around four which was fuel.
Clearly with the few grams a fuel we're using here
we're not going to be sending anything into space anytime soon.
or are we?
Now that's rocket science.
thanks