Science Notes on Kinetic Theory
Kinetic Theory






Theory – an explanation based on facts and observations. Theories are used to help us understand
our world. They sometimes used models to assist understanding
Kinetic Theory –states that all matter is made of atoms that are constantly moving
- as energy is added, the atoms move faster
- as energy is removed, the atoms move slower
- if I keep adding energy, the atoms will continue to pick up speed, moving faster and faster
- if I keep removing energy, the atoms will continue to slow down, moving slower and slower
Temperature – temperature changes change density





Hot objects are less dense than cold objects
Temperature is the average speed of atoms in a substance
Thermometer – used to measure temperature
When a thermometer is put in contact with a surface, the atoms of the substance and the atoms of the
thermometer begin colliding into each other; the atoms in the thermometer and substance begin to
travel at the same speed
Thermal expansion – how much an object expands or contracts when temperature changes; different
materials will expand and contract in different amounts
How does Energy get transformed? (Why does density change when temperature changes???)


Energy always moves from hotter temps to lower temps
When objects with 2 different temperatures are mixed, the faster atoms slow down and the slower
atoms speed up. Where they touch, the energy gets transferred.
History of the Thermometer




One of the first known thermometers was invented by Galileo (1593)
Galileo found that as things get cold, they contract and when the get warm/hot they expand
Galileo put a glass bulb at the end of a long tube. When the bulb was heated or cooled, the liquid
moved up and down the tube (Thermometer #1)
Galileo had no numbers on his thermometer – was inconsistent, inaccurate and bulky
How does Galileo’s thermometer work?






As atoms gain energy, they go faster
As they go faster the atoms collide into each other more vigorously
As they begin to hit each other harder they begin to bounce further apart and the object expands
NOTE Atoms do NOT change size. They move farther apart when heated and closer together when
cooled.
Different metals will expand and contract at different rates
Galileo’s thermometers were not very accurate and they were bulky to use in experiments


Later – Galileo improved his first attempt with a second thermometer based on density
Galileo did not have the technology available later on
Thermal Expansion – how much an object expands or contracts with temperature changes


Different materials will expand and contract by different amounts
Brass (high thermal expansion) will change a lot, but wood (has low thermal expansion and will not
expand or contract much
Example: Water (at room temperature) has a density of 1g/mL…BUT…when you heat it, the atoms
move faster and farther apart. If I cool the water, the atoms move slower and closer together
Density and Temperature



Density is how tightly packed together atoms are
If I heat something up, the atoms bounce around and spread out – that something becomes less dense
When objects are colder, the atoms move slower and closer together
Daniel Fahrenheit (1724) - First Accurate Thermometer/starts at 32 degrees F










Fahrenheit decided to make a better thermometer than Galileo’s so he could sell them and make a
profit
Fahrenheit had 100 years more of technology behind his thermometer than Galileo
Fahrenheit made his thermometer with the numbers on it and then began measuring the temperature
of things
People in Fahrenheit’s time knew how to make thinner glass and hotter fires
He used a very small and skinny version of a test tube that was both portable and accurate
He made a mixture of ammonium chloride, ice and salt (the coldest liquid he could come up with)
and put a thermometer in it
Stuck a thermometer in the mixture and marked the lowest point and called it “0”
He then stuck that thermometer into ice and found that water freezes at 32 degrees Fahrenheit
He then stuck the thermometer into boiling water and found that water boils at 212 degrees
Fahrenheit
Fahrenheit found 3 Fixed Points
1. Water freezes and melts at 32 degrees
2. Water boils at 212 degrees
3. Most people have a normal temperature of 98.6 degrees
Next came – Anders Celsius – 1742

Anders didn’t like Fahrenheit’s thermometer…thought it was great…BUT…
1. It was difficult to reproduce
2. Math calculations were difficult
3. There was no pattern





Wanted a metric thermometer based on water
Celsius measured things to make his thermometer
He took a blank thermometer and stuck it in freezing water
He watched the mercury drop to the lowest point and marked it “0” – the point where water freezes
He then stuck that thermometer into boiling water and watched the mercury rise to a point he labeled
100 degrees – and called it the boiling point
So his thermometer had a bottom of 0 degrees (freezing) and a top of 100 degrees (boiling)
It thus had 100 divisions and he called his thermometer a centigrade thermometer
He based his thermometer on the metric system and a number system based on 10 – easy to make
calculations
It was called a centigrade or Celsius thermometer – both the same thing
Today, this is the only thermometer used by scientists worldwide
Body temperature on a Celsius thermometer is 37 degrees
(Mercury is more sensitive to temperature so it makes the best thermometer, but is not used much
anymore because it is toxic – can be absorbed through the skin or breathed in







Next is Lord Kelvin – 1848









(William Thomson – knighted Lord Kelvin by the queen)
Used Celsius’ thermometer, but started at the coldest temperature possible…..Absolute Zero
ABSOLUTE ZERO =0 DEGREES KELVIN (which in Celsius degrees would be -273)
Starting at Absolute zero, he made his freezing point 273 degrees Kelvin and his boiling point 373
degrees Kelvin
Found that as temperature decreases (and things get colder) pressure goes down
Found that as temperature increases (and things get hotter) pressure goes up
Found that removing heat (energy) would result in the atoms slowing down
Asked if you keep removing energy and the atoms keep slowing down, at what temperature would
the atoms stop moving? This would be ABSOLUTE ZERO – 0 DEGREES KELVIN
We have never reached absolute zero but did come close
September of 2003, scientists at MIT chilled a cloud of sodium atoms to a record-breaking 0.45
nanokelvin – that is .00000000045 K
RE-CAP

4 thermometers beginning with …
1. Galileo’s thermometer –a glass bulb at the end of a long tube…bulky and not very accurate
2. Fahrenheit – first accurate thermometer; based on the properties of water; has many
numbers on it and goes from 0 degrees to well over 100 degrees; water freezes at 32 degrees
and boils at 212 degrees on the Fahrenheit thermometer
3. Celsius – based on water also; begins at 0 degrees (the freezing point of water) and ends at
100 degrees (the boiling point of water)
4. Kelvin – also based on the properties of water; starts at absolute zero and goes from 273
degrees (freezing point of water) to 373 degrees (boiling point of water)
CONVERTING CELSIUS TO KELVIN and KELVIN TO CELSIUS
K = C + 273
C = K -273
PLANK TEMPERATURE – just as Kelvin theorized about absolute zero (the coldest temperature
possible), there is also a theory of the hottest temperature possible and it is called Plank Temperature



This temperature is 1.4 x 10 to the 32nd power K
AT THIS TEMERATURE ALL MATTER IS DESTROYED INTO A MIXED MUSH OF INDISTINGUISHABEL HOT
ATOMIC SLUDGE. ATOMS WOULD BE MOVING SO FAST THAT THEY WOULD LOSE THEIR IDENTITY
SCIENTISTS HAVE CREATED THE HOTTEST TEMPERATURE EVER RECORDED INSIDE A REACTOR AT 4
TRILLION DEGREES – 250,000 TIMES HOTTER THAT THE CORE OF OUR SUN.
THREE FIXED POINTS IN FARENHEIT – CELSIUS – KELVIN
F
C
K
BODY TEMPERATURE
98
37
310K
WATER FREEZING
32
0
-273K
WATER BOILING
212
100
373K