Let's take a look at something that's, I think,
taken for granted sometimes and perhaps not well understood
or I think can lead to confusion.
And that's the idea of what a scientific model is.
In fact, even a model anywhere in life, really.
What's a model?
You want to buy some clothing or something online.
You might go to the Abercrombie website or something,
and you see a picture of someone wearing their clothing.
And that gives you a prediction for what that piece of clothing
might look like if you were wearing it.
So a model is a way of predicting behaviour, right?
It predicts some kind of a result.
Actually, let's call it that.
We'll call it a results.
It projects a result.
And a child might play with a model car.
And for the child's purposes, it's sufficient.
It predicts the way something rolls on four wheels,
and it moves around, and it's fun in the same way
the car could be fun, right?
So we have these many, many models.
Fashion model, right?
There's a fashion model.
We have a model car.
And they predict behaviour or predict results.
But, of course, they also have also have limitations,
don't they?
They're limited.
But that's OK.
And I think that we would understand that instinctively
with a fashion model or a model car.
The same thing is true of scientific models.
For example, we've introduced in this course the idea
that we can model an atom.
Remove this base unit of matter.
We can model as a sphere, and we treated it, and face-centered
cubic by slicing it up.
But we were able to get some useful information by treating
an atom as a sphere.
Now is that a correct depiction of an atom?
No, it's not.
There's certain finer levels to the structure
of an atom than just what we get from a sphere.
Here's my best attempt at a sphere.
That's the catch light there.
Shade in the back.
Shadows back here.
Look at that.
That's gorgeous.
Gorgeous.
So that's supposed to be a sphere popping right out
of the page there for you.
But it was good.
And actually, in fact, what else we could
we do this model of the atom as a sphere?
We could take two of them and add one extra level.
Make a slightly new model.
That's a spring between them.
And what did we get with that model?
Well, that model took us to Young's modulus.
It was a good enough model for that particular discussion.
You could even discuss the inter-atomic force separation
curve.
And that's how we arrived at Young's modulus
from the slope of the force separation curve.
These atoms model as little hard spheres and we're
at their equilibrium space.
And so that took us there.
This hard sphere model took us to the idea
of a unit cell, which enabled us to go and calculate
density, a physical property.
So it was a good model.
It allowed us to explain a lot.
In fact, we even took the model of an atom as a sphere,
and we built on it with this unit cell,
and then that introduced for us these crystalline
imperfections.
Didn't it?
Crystal imperfections.
We took all these little spheres,
and we realise that there's going to be places where that
regular arrangement of spheres in space breaks down.
And we saw, for example, the edge dislocation.
And what did that do for us?
Well, the edge dislocation allowed us
to explain plastic deformation.
Allowed us to explain plastic deformation.
That step-by-step breaking and reforming of bonds.
Plastic deformation.
And that understanding led to strengthening of metals.
All from this simple model of the atom as a sphere.
But there's limitations to it.
We can't explain just with this model
alone electrical properties, conductivity.
We need to get into what the electrons are doing.
Optical properties-- well, you can't
get that close explaining it with just that hard sphere
model.
You need a more detailed model.
So there are limitations to models.
And with polymers, what we'll do is
we'll treat polymers-- these great, long molecules--
as if they're strings.
So we'll say, polymers can be modelled as strings.
And that is actually a fairly good description-- or model,
I mean-- for explaining many properties.
Mechanical properties, some thermal properties,
dissolution of polymers.
But there's going to be limitations as well,
particularly when you try to get into conductivity and optical
properties.
So I think it's important to realise that there's models.
And a model only needs to be as useful or as valid
as it needs to be for what you're explaining.
So we'll also get into these models of the atom,
and we'll have certain models of the atom
where we have this little nucleus with-- maybe
we have some electrons.
Nucleus with some electron orbiting it.
And you'll see this type of the description
here is going to be useful, but there's
going to be limitations.
And as the learner, I don't want you to get hung up on the fact
that there are limitations.
In fact, as you learn, what you should to keep in mind
is that it's fine for there to be limitations.
But also for you to be aware of that,
so that you know when certain models are valid,
and when they're not.
And that's absolutely fine.
I remember when I was a student, I got hung up on, well,
why are we going to treat the electron as a particle
in any discussion, when we learn later that we
got to treat it as a wave?
And it was frustrating, because I
struggled to think, well, why don't we just
have one model that does everything?
And unfortunately, we don't.
We don't really understand everything that level.
And in fact, sometimes a model is too complicated
for your particular need.
We can model casting of steel with water.
And why is that?
Well, because molten steel has a similar viscosity to water.
And so you can make a physical model, a pilot or a test
facility, and you can pour water through parts of that process
and get a fairly good prediction for what the flow of the molten
steel will be like.
Does it predict solidification of the steel?
Well, no, of course not.
But that's fine, because that's not what you're using it for.
So there's many models and there are limitations to each.
In fact, a mathematical model, where we would model even
individual atoms and their behaviour,
has places where it's applicable and places where it's limited.
So it's important for you to realise that,
and I certainly hope that you can be comfortable with that
as we dive through different models and different ways
of looking at things.