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En provenance du cours de The University of Chicago

Global Warming I: The Science and Modeling of Climate Change

157 notes

The University of Chicago

Global Warming I: The Science and Modeling of Climate Change

157 notes

This class describes the science of global warming and the forecast for humans’ impact on Earth’s climate. Intended for an audience without much scientific background but a healthy sense of curiosity, the class brings together insights and perspectives from physics, chemistry, biology, earth and atmospheric sciences, and even some economics—all based on a foundation of simple mathematics (algebra).

À partir de la leçon

Heat, Light, and Energy

A primer on how to use units to describe numbers when describing temperature, energy, and light. Even if you don't plan on doing calculations yourself, understanding how units work will help to follow the rest of the lectures in the class. If you are interested in practicing your analysis skills, using units to guide calculations, there are some exercises in the Part II of this class.

Using Units3:43

Units of Energy5:12

Rencontrer les enseignants

David Archer
Professor
Geophysical Sciences

[MUSIC]

So global warming is ultimately about heat.

But have you ever thought about what heat actually is?

Heat is an expression of the amount of kinetic

energy that the atoms in a substance has.

So when atoms are moving around very quickly,

that feels hot to us and if they are moving very slowly, that feels cold.

Which is amazing that you can feel that at all, because atoms are such tiny things.

You can't feel individual atoms.

You can feel huge collections of atoms in a solid surface like this.

But an individual atom you can't really feel.

And yet, how fast that atom is jiggling, you can feel that instantly,

because if you touch it, the fast jiggling of the atoms will bump up

against the atoms of your skin and make those jiggle more quickly too.

And if they jiggle too quickly, they can actually rip apart the chemical bonds

of your skin and that's how you burn yourself, and that's why

we have evolved to be able to sense how fast the atoms are wiggling around.

So it's like we have little atomic speedometers in our finger tips.

It's really cool.

So it turns out that each mode of motion of the atoms

contains on average one-half KT of energy.

So this is actually how the temperature is defined.

This T is the temperature.

K is the Boltzmann constant.

It's just a number you can look up in a book that doesn't change.

So if you have a gas that consists of single atoms,

not in molecules like argon is single atoms in the air,

that thing is flying around in space, and

because there are three dimensions of physical

space it turns out that a single atom gas like this will

tend to have three-halves KT of energy, one for

each of the three dimensions of physical space.

And then, if you have a molecule like oxygen,

one of the main gasses in our atmosphere, O2 has two atoms and

these two atoms can vibrate relative to each other.

And so, that vibration sets up another mode of

motion that the molecule can carry heat.

And the fact that it's now sort of a line instead of a point

means that it can rotate, as well.

And there are two different dimensions of rotation of this,

so you get two rotational modes from this atom, as well.

So if you think about thermopane windows where the whole point

is to stop heat from being transferred from the outside to the inside.

The very best thermopane window that you could get would have two panes of

glass and then it would have a vacuum in between.

Because then there would be nothing.

No way that the jiggling atoms of this pane of glass could touch the jiggling

atoms of that one, and transfer the heat by conduction.

Unfortunately, they can't make windows with a vacuum inside there,

because of all the pressure from the atmosphere pushing in.

The panes of glass would get broken.

So what they do instead is for the best, for

the good windows they put argon in-between the two panes, because the atom of argon,

so it goes to the warm side and picks up some extra kinetic energy, and

then it goes to the cold side and gives it off, and that's heat loss from the window.

But it's, argon can only carry a relatively small amount of

energy compared to if you put oxygen into this pane of glass.

When it comes to the warm side, it is like a bigger battery.

It can carry more energy.

It goes to the warm side, to the cold side, and it would

carry twice as much energy per molecule, because it has more modes of motion.

[MUSIC]

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