Thermodynamics – March 19, 2013
By: Sung An
Thermodynamics
A section of science that deals with temperature, energy, work, and the transferring of energy
Brownian Motion – Tells us that particles (atoms/molecules) are in a constant state of random motion
Kinetic Molecular Theory (Refined version of Brownian Motion for Physics)
“Stuff” is made up of smaller, randomly moving particles
At the molecular level, electrostatic forces keep particles at the right distances from each other
How far apart and how quickly the atoms and molecules are moving determines the state of matter
Solid
ex: a nail
Liquid
ex: blood (O negative)
Gases
ex: HCN(g)
Low Energy -----------------------------------------------------------------------> High Energy
The amount of energy possessed by particles is called their “Thermal Energy”
Thermal Energy
-Made up of Kinetic Energy and Electrostatic Potential Energy
-Electrostatic Potential Energy is hard to measure, so average Kinetic Energy is usually measured instead
-Average Kinetic Energy can be measured through Temperature
Thus: Low energy = Low temperature and High Energy = High temperature
*We use thermometers to measure temperature, and they work through expansion
Temperature
Giving a measure to the particle’s kinetic energy
K=Kelvin (which is the SI unit of Temperature)
o
C=Celsius
o
F=Fahrenheit
Kelvin scale starts at Absolute Zero, 0K, which is impossible to reach (thus far)
-Particles still have a little energy at absolute zero, they can move a tiny bit
0K=-273oC so 0oC=273K
o
F=9/5 x oC =32
Example: Convert 22oC to Kelvin and Fahrenheit
K->
0oC=273K
22oC=295K
o
F->
o
F=9/5 x 22 +32
= 39.6 +32
=71.6oF
Scenario: In a forensics lab, the corpse’s body is at the same temperature as the fabric it lies upon.
Yet the body feels colder. Why is this?
To answer this, one looks at heat. Heat is defined as the transfer of thermal energy from one source to
another.
Firstly, heat can only be “given” to the cold, as the cold is simply an absence of heat and so cannot be
given away.
Secondly, when two objects of different temperatures touch, they attempt to equalize their
temperatures.
In the case of the forensics lab, the scientist’s hands are warmer than the room temperature and the
objects within the room. So the temperature of the fabric and corpse are the same. But the corpse is a
better conductor than the fabric, which is an insulator, so heat transfers more easily to the corpse than
it does to the fabric when touched by the hands. (assuming that this procedure was done barehanded)
Conductors include most metals, whereas insulators are objects like fabric, hair, glass, etc.
How to Transfer Heat Energy
1) Convection
T
In the image above, the fire is the source of heat and the air molecules around the fire are very
hot and move rapidly. They transfer this kinetic energy to the molecules above them, and so on,
but lose less average kinetic energy as the transference progresses. Once they reach near the
top of the room, the molecules have become colder and thus move less. This means that the
particles are closer together, and more dense than the heated particles. Because of this
difference in density, the cold air sinks and gets reheated and rises up once more. This cycle
continues, and is called Convection, and happens with fluids. Fire also causes carbon monoxide
(lighter than air by a little), so stay close to the ground to avoid inhaling it. But because of
convection, it does circulate the room, so as a precaution do not stay in a room full of fire.
2) Conduction – The direct transference of kinetic energy between solids when they come in
contact. For an example, see the dead body (Scenario in Temperature section)
3) Radiation
Radiation comes in the form of Electromagnetic Waves. These waves travel at C, which is
3.0*10^8 m/s. So the amount of time it takes for the waves to travel from the Sun to Earth is
about 8 minutes.
Radiation, Black and White
Black fabric gets warmer faster than white fabric because white reflects all colours but black absorbs
them. Black absorbs radiant energy better, but people in deserts still wear black robes. This is because
black robes cause a convectional current inside them which cool the people down using the moving air
around them.
ENTROPY – Everything eventually becomes or wants to go to a state of Randomness (Chaos)
Proven scientifically and philosophically.
HOMEWORK: Package called “Warmth and Coldness”
Extra Resources:
a) How thermometers work http://home.howstuffworks.com/therm.htm
b) Heat transfer Interactive http://www.wisc-online.com/Objects/ViewObject.aspx?ID=sce304
c) Absolute Zero (Particle Motion) http://www.sciencedaily.com/articles/a/absolute_zero.htm
EXTRA: An example of philosophical entropy (a movie based on survival on a deserted island)
http://www.youtube.com/watch?v=X7ffCWSTNYM