- When you press down on the trigger of a BBQ lighter
you'll notice it's quite hard work.
Like it's not like pressing the button on a remote control,
you actually have to put some effort into it.
(pressing lighter)
And there's a reason for that.
When you press down on the trigger
you're working against quite a stiff spring
so that all that work you do is stored
as potential energy in the compressed spring.
And in front of that spring is a tiny hammer
so that when the mechanism finally gives way
and the spring is released all that potential energy stored
in the spring is turned into kinetic energy in the hammer.
The hammer then strikes a special kind
of crystal called a Piezoelectric crystal.
And when you hit one of these things
it generates a voltage across the crystal.
This is the inside of a BBQ lighter.
This is the mechanism that houses the spring
and the hammer and the crystal.
These two wires are coming off the crystal
and they're drawing that voltage
up to the top of the lighter here.
There's still a gap between the two wires at the top here
but the voltage that you produce when you press down
on the piezoelectric crystal is so high
that electricity can actually jump across that gap,
producing a spark.
If you send a flammable gas up to the top as well
then the spark will ignite the gas.
For a while now I've wanted to understand
how the piezoelectric effect works on the molecular level.
And I finally do
and I've also built something to make it easy
for anyone to understand how it works.
So that's what this video is about.
There's a few different types
of piezoelectric crystal that you can buy,
but I'm gonna look at quartz in particular
which is the first piezoelectric crystal
to ever be discovered.
This is what a giant quartz crystal looks like.
It's also a healing crystal,
according to the website that I bought it from
it would align your chakras.
I don't know if you take it orally or...
If you were to take a slice of quartz
away from this crystal and compress it
you'll be able to measure a voltage across the slice
but you'd have to slice it at just the right angle.
Like, I don't know if this is the right angle
but it'd be something like that
but it's got to be just right.
And to see why you've got to get the angle just right,
we need to look at the lattice structure of quartz.
So quartz is made of silicon dioxide,
so it's silicon and oxygen.
And this is what it looks like.
At first it looks like quite a complicated structure
but as you rotate the crystal
you find these symmetries like this one,
and here's another
but there's one particular angle that I'm interested in
which is this one.
Notice this hexagonal shape here.
It's actually a spiral going into the screen
but for simplicity, we're gonna think of it as a ring.
So these three crunchy peanut butter lids
represent silicon atoms
and these three smooth peanut butter lids represent oxygen.
So when you compress a crystal, of course,
you're squishing these hexagons.
But crucially the bond between oxygen and silicon,
isn't quite even
like the oxygen is a bit more aggressive
in the way it holds onto its electrons.
So the oxygens are a little bit negatively charged
and the silicons are a little bit positively charged.
So think about where the average
of all the positive charge is in this diagram.
It's a bit like the center of mass
but it's the center of charge.
So it's there in the middle
between these three silicon atoms.
But watch what happens when you compress the crystal
the two sides silicons move outwards
but they don't move vertically.
And the bottom silicon moves upwards.
So the average position
of these three positive charges moves upward slightly.
And similarly,
think about the average position
of the three negative charges.
Look, the two side negative charges, move outwards,
but that top negative charge moves downwards.
So the average
of the three negative charges moves downwards.
So when you compress a crystal, of course,
in this exact orientation,
you're slightly shifting
all the negative charges in one direction
and all the positive charges in the other direction.
Suppose you've got a square slice of quartz crystal.
Let's represent all the positive charge as gold
and all the negative charge as red
and in an uncompressed crystal
those negative charges and positive charges overlap exactly.
And so you've got this neutral greenish color
but when you compress the crystal
the negative charges shift in one direction
and the positive charges shift in the other direction.
In the bulk of the crystal
it's still neutral overall,
but at the faces you've got this buildup
of positive and negative charge.
If you then wire up these faces,
well the positively charged face
will be trying to pull electrons
towards it from inside the wire
and the negatively charged face will be repelling electrons.
And if you bring the ends
of those wires close enough together
the electricity will be able to jump the gap
producing a spark.
So that's how quartz is piezoelectric.
But in general,
any crystal can be piezoelectric
so long as it fulfills a couple of criteria.
The first one is
that the lattice needs to contain some polar bonds.
That just means that some of the atoms end up
with a slight positive charge
and some of the atoms end up with a slight negative charge
like silicon and oxygen in quartz.
That's why diamond isn't piezoelectric.
You can squeeze a diamond as much as you like
but all the carbon atoms inside are neutral.
So there's no charge for you to move around.
The second criteria is more subtle is to do with symmetry.
The crystal needs to have a certain type of symmetry
or more accurately, a lack of a certain type of symmetry.
Take a look again at this hexagonal arrangement
of atoms from the quartz crystal.
Look at this silicon atom
and then draw a line through the central point
to the opposite side and see what's there.
It's not another silicon atom, it's an oxygen atom.
So there's something different on the opposite side
when you go through the central point.
That means it doesn't have point symmetry.
To see why a lack of point symmetry is important,
look at this arrangement
of atoms that does have point symmetry.
See how silicon atoms that are opposite.
Silicon atoms, oxygen atoms are opposite oxygen atoms.
When you compress the crystal
you move the charges symmetrically.
So the average position of those charges doesn't change.
It stays in the middle.
The piezoelectric effect has a lot
of uses besides just a high voltage power source
like in a BBQ lighter.
It can also be used as a sensor.
This is a piezoelectric disc.
It's probably not quartz, so there'll be something else.
And if I press down on the disc
I can get a bit of a voltage across it.
It's sensitive enough to be used as a pickup
for sound so I can actually use it as a microphone.
It's not a great microphone but it is a microphone.
Interestingly, the piezoelectric effect is reversible.
Actually in a previous video,
I mentioned how speakers can be used as microphones
and microphones can be used as speakers.
And it's true of this piezoelectric microphone.
It can be used as a speaker.
It's not a great speaker, but it is a speaker.
When you apply a voltage
across a piezoelectric crystal, it will deform.
It's the opposite of the piezoelectric effect.
My absolute favorite example of the use
of the piezoelectric effect is in the quartz watch.
Inside one of these watches,
there's a tiny crystal of quartz
that vibrates exactly 32,768 times per second.
If you know your powers of two
then that might sound familiar, but why?
Well, this video is already getting too long
so my next video is gonna be
about the amazing mechanism inside a quartz watch.
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