Chapter 12

Our Study of Nature
What is a Model?
Part I of Course
A. Why do things move the way they do?
1. Newtonian View
- the laws of force and motion
2. Conservation laws
3. Symmetry Principles
4. Relativistic view
Simplified picture of reality to
help us predict how things will
work in some circumstances.
Part II of Course
B. What are things made of?
Different models are used to
describe the same thing at
different levels of detail.
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The First Model of Matter
(Aristotle)
Everything is made of a
combination of four elements
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A Working Model of Matter
Continuous Model (most simple picture)
- consider matter as infinitely divisible
- ignore discrete nature of atoms and molecules
Earth
Water
Air
Fire
• Examples
buildings, boats, pencils, shoes, airplanes
Wrong, but it gets us started in
the right direction: Classification
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Continuous Model of Matter
Allows us to classify according to observable
properties:
1. The four states of matter
2. Changes of state according to temperature
3. Density
4. Color
5. Response to forces that deform
6. Electrical conductivity
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Solid State
Solid: resists changes in size, shape, or volume.
We will discuss each of these today. To classify is only
the first step in understanding
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Liquid
Gas
Liquid: flows to assume the shape of its
container but resists changes in volume
Gas: expands to fill shape and size of container
Examples: water vapor, natural gas, nitrogen
(air)
Note: liquids and gases are both fluids
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Plasmas
Examples of Pasmas
Plasma: ionized gas; electrons become detached from atoms
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Modern
Solid
Liquid
Gas
Plasma
Fire
Lightning
Northern Lights
Fluorescent Lights
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Some materials do not fit neatly
into a state classification
An Interesting Parallel
Four states of matter
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Ancient
silly
putty
Four elements of matter
Earth
Water
Air
Fire
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Over 99% of the visible
universe is in the plasma state
What is the most common state of
matter in the universe?
Solid
Liquid
Gas
Plasma
Earth
Coronal Loops on the Sun (~250,000 miles long)
Eagle Nebula (Fingers are ~1 Light Year)
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Changes of State
Melting and boiling
State is a function of temperature (and pressure).
Different materials change state at different temperatures.
* exist in plasma state at high temp
P1: An unknown solid melts at 200 degrees. Which
a)
b)
c)
d)
*
*
*
of the following would most likely be its boiling
point?
-200 degrees
0 degrees
200 degrees
400 degrees
The temperatures at which substance transform from
solid to liquid and liquid to gas give information about the
strength of the bonds holding the material together.
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Demo: Liquid Nitrogen
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Density
density =
steel
Boiling Point: -196° Celsius (-321° Fahrenheit)
P2:
What happens to the air in the balloon as it is
put into the liquid nitrogen? Why?
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mass
volume
Styrofoam
Objects normally contract when cooled.
What happens to the density? Why?
Demo: bimetallic strip, ball and ring
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Exception: Water
Density
Ice is less dense than water.
¾ Ice floats because it is less dense
than water.
¾ GOOD THING!!!!
¾ If ice sank, lakes would freeze solid from the bottom up
and aquatic life would die.
¾ P3: Ice cubes floating in a drink, filled to the brim. The
ice now melts. Will the water spill over, will the water
level go down, or stay the same?
Density depends on the material. Mass or amount
depends upon the object.
Which is most dense, an iron shaving or all the
water in the ocean?
P4: Which is most dense, a ball bearing or a huge
Cracks in mountain rock and sidewalks, fish in winter. Si, Ge and a few others are like
water.
Video: VE 15-15
cannon ball made of the same material? Which
is most massive?
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The color of an object gives
information about its composition
Color of an object
Sunlight contains all colors of light and has a
continuous spectrum -- all colors
An object will
– Absorb light
– Reflect light
– Emit light (often in the infrared) when heated
When white light falls on an object some colors are absorbed and
some reflected.
Other sources of light may have a
discrete spectrum -- only a few specific colors
Color of an object: characteristic of its material
– clover reflects green colors, absorbs others
Color and properties of emitted light are also characteristic of the material.
discrete absorption spectrum – all colors but
a few lines
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Spectra
Why is the sky blue?
Blue light is more easily
scattered by air molecules
than other colors. You are
seeing that scattered light.
cont.
P5: Why is the sun red in
the evening.
Demo: light scattering carousel
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Clues to internal structure, nature of the
atom. Use a grating to separate the
light (Demo:)
discrete
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Response to forces that deform
Compression
Tension
Shear
Deformation
Elastic deformation: object returns to its original
shape after the deforming force is removed.
elastic constant = force / deformation
Plastic deformation: object retains its new shape
when the force is removed.
Elastic limit: transition point between elastic and
plastic deformation for a material.
Which forces do
Solids resist?
Liquids resist?
Gases resist?
.
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Elastic properties (and
temperature)
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Electrical Conductivity
P6: Which has the larger elastic constant,
rubber band or table top?
Demos – marshmallows, balloon, clay, bouncing balls,
rubber bands, Shape Memory Alloy
Elastic properties can be changed as
temperature is varied.
a. conductor - electric current flows easily
b. nonconductor - resists flow of current
- sometimes called insulator
c. semiconductors - allow current, but not as good
as a condutor
d. ionic materials – nonconductors, but become
conductors when they are
liquid or dissolved in water
e. nonionic materials - nonconductor when liquid
and when dissolved in water
Metals: silver, copper, gold. Semiconductors: germanium, silicon.
Insulators: glass, mica, polyethylene.
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Let’s see it!
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Summary
steel
sugar
salt
Water
We classify matter according to
P7: Which of solutions below will conduct electricity?
distilled water
sugar water
salt water
State (solid, liquid, gas, plasma)
Density
Color
Response to force (deformation)
Conductivity
Over the next several lectures we will discuss
various models that have been used to
explain the origin of these properties.
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P8:
Take home a slide
Which statements are true?
a. The sun emits a continuous spectrum of light.
b. Energized neon gas emits a discrete spectrum
of light
c. The density of a material is defined as its mass
per unit volume
d. Salt is an ionic material since it becomes a
conductor when dissolved in water.
e. The solar wind is a plasma since it is made up
of charged particles that move independently
of each other.
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Take a slide only if
you will really use it!
Don’t look at the sun!
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