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P R O J E CT G U T S
CL I M AT E C H AN G E U NI T
Climate Change Energy
Science behind the NetLogo Climate Change with Cars model
Introduction
Climate Change in a Bottle
This document will provide some background material on the science behind
the Climate Change with Cars model. As we use models, it is inevitable that
questions will arise about whether the mechanisms that make the model work
are really similar to the mechanisms at work in the real world. It is hoped that
students will be motivated to gain a better understanding of real world
phenomena as they evaluate the validity of the models that we are using.
Although not all students will care to pursue these lessons to the foundations in
black body radiation and quantum mechanics, it is valuable to provide a path
for these potential investigations. One of the benefits of teaching with models
and simulation is that models provide a “low threshold” for engagement but a
“high ceiling” for serious research. Teachers are not assumed to have mastered
the rather deep science behind these models but perhaps this guide will offer
teachers and motivated students a starting point for future independent
exploration.
Our NetLogo climate model shows a reservoir of
heat (the red particles) stored in the earth. These
particles are shown to be emitting radiation (red
dashed lines). The particles in the earth are called
“heat”.
It may be important to remind students that heat is
best thought of not as a material object--but rather
as “energy” that can take several forms. William
Thompson (Lord Kelvin) 1851, in his "On the
Dynamical Theory of Heat", puts is this way: “heat is
not a substance, but a dynamical form of
mechanical effect”. The kinetic theory of heat is a
much celebrate early victory of the scientific
approach, on par with Newton’s laws. The kinetic
theory paved the way for the branch of science
that became thermodynamics.
Figure 1 Lord Kelvin, William Thompson
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CLIMATE CHANGE UNIT
Understanding Heat as Kinetic Energy
The heat energy in the earth (red dots) is stored in the form of kinetic (moving)
energy. The fact that the particles are moving shows that heat is stored. A more
accurate model would show, not more particles, but, rather, particles moving faster
as temperature increased. I believe that this is an important point for students to
understand since it relates to fundamental understandings of matter and energy.
Crosscutting concepts have
value because they provide
students with connections and
intellectual tools that are
related across the differing
areas of disciplinary content
and can enrich their
application of practices and
their understanding of core
ideas. -- Next Generation
Science Standards Framework
p. 233
Maxwell’s unification of
electricity, magnetism and optics
under the concept of electromagnetic radiation was almost as
profound as Newton’s insight that
the concept of gravity could
make sense of things as
apparently unconnected as the
motion of the moon and an
apple falling from a tree. There
may be value in pointing out to
students that one of the goals of
scientific understanding is to
discover simple rules that underlie
disparate phenomena, or events
that do not appear to be
connected. This feature of
scientific explanations is a good
example of what is called a cross
cutting concept in the Next
Generation Science Standards.
Figure 2 When Joule first determined the precise connection
between kinetic energy and heat, he used a “mixer” device like
the one above.
Understanding Radiation
It is somewhat mysterious that moving particles
should be able to “generate” energy in the form
of electromagnetic radiation (radio waves, light).
These are the red dashed lines that we see
emerging from the earth in the climate change
model. Understanding how materials absorbed
and emitted energy was important scientific work
in the mid to late 1800’s (see Balfour Stewart,
Gustav Kirchhoff).
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CLIMATE CHANGE UNIT
Absorption of Infra-red Radiation
Understanding Radiation (cont’d)
The basic idea works like this: James Clerk Maxwell, in 1865, published a set of
equations that describe electricity, magnetism, and optics as manifestations of
one thing, the electromagnetic field.
Maxwell observed that moving charged particles back and forth created
electro-magnetic fields. These (initially mysterious) forces could be sensed at a
distance from the source. Maxwell’s ideas prepared the way for antennas, metal
conductors that allowed for the rapid oscillation (back and forth movement) of
charged particles. Broadcast antennas generate radio waves--energy that
radiates out in all directions at the speed of light. One of Maxwell’s greatest
contributions was the discovery that light and infrared radiation were all part of
the continuum of energy that could be created by vibrating particles.
Figure 3 Rattlesnakes can sense
infrared radiation with special sensors
integrated with their vision system.
In our model the heat in the earth is stored in
the form of molecules that are charged
particles. As these particles oscillate, they
generate electromagnetic radiation mostly in
the form of infrared radiation. Infrared radiation
is a kind of light that is just outside of the kind of
light that we can see with our eyes
(rattlesnakes, vampire bats, bed bugs and some
other animals can see it!) but we can sense it as
heat.
An analogy that might help students to understand this situation is to consider
the vibrating particles in the earth as a sort of giant cell phone making a call to
the universe. The energy emitting from the “earth phone” is constantly draining
the (heat) battery of the earth. The sun is constantly charging this “battery”.
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CLIMATE CHANGE UNIT
Most of the energy that the earth emits in the infrared range is lost to outer
space. Certain molecules, however, are by chance “tuned” to receive the
infrared frequency that the earth is broadcasting. Methane and carbon dioxide,
the “greenhouse gases”, are the most important of these molecules. When these
molecules absorb the infrared electro-magnetic energy, the energy turns back
into kinetic energy--that is, the gas molecule’s temperature (velocity) goes up.
In the model, the carbon dioxide is shown radiating infrared back to the earth.
This is not quite right but it is a convenient way to add the heat energy back into
the reservoir that we are measuring. It is important to know when and where we
are taking “shortcuts of convenience” in our models.
To make the students’ understanding of infrared radiation a bit more personal
consider pointing out that the greater part of the calories that they require each
day are lost as a infrared radiation. Calculations on thermal radiation of a
human at equilibrium with the environment reveal that humans are radiating
infrared energy at a rate of about 100 watts. The total energy radiated in one
day is about 9 MJ (mega joules), or 2000 kcal (food calories). So that’s where all
those calories go!
Finally, it is worth pointing out that consideration of the ways in which objects
absorb energy -- especially the observation that energy seems to be absorbed
in discrete “chunks” or “quanta” rather than continuously -- led directly to the
development of quantum mechanics -- or the current best theory of how matter
works at the smallest scales.
More reading:
How Heating with Friction Works
An accessible article about heating.
http://www.askamathematician.com/2009/12/q-could-a-simple-cup-of-coffeebe-heated-by-a-hand-held-device-designed-to-not-only-mix-but-heat-thewater-through-friction-and-is-that-more-efficient-than-heating-on-a-stove-andthen-mixing/
CLIMATE CHANGE UNIT
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More reading:
Topic: Infrared detection in animals
http://www.mapoflife.org/topics/topic_311_Infrared-detection-in-animals/
Black body radiation
Although not an easy read, these articles explains how objects absorb and emit
radiation.
http://en.wikipedia.org/wiki/Black-body_radiation
http://simple.wikipedia.org/wiki/Black-body_radiation
Radiation to Quantum Mechanics
Article explaining how problems in nineteenth century investigation of black
body radiation led to the development of quantum mechanics.
http://voyager.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html