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NOTHING IN THE WORLD IS MORE DANGEROUS THAN
SINCERE IGNORANCE AND CONSCIENTIOUS
STUPIDITY.
- Martin Luther King, Jr. -
WE SPEND MUCH OF OUR LIVES THINKING
ABOUT TEMPERATURE – HOT AND COLD.
WE CHECK OUT WEATHER REPORTS.
MANY OF OUR CARS HAVE OUTSIDE
THERMOMETERS.
OUR CARS AND OUR HOUSES HAVE WAYS OF
CONTROLLING THE TEMPERATURE – AC AND
HEATERS OR HEAT PUMPS.
THIS IS ALL BECAUSE OUR BODIES HAVE
EVOLVED TO OPERATE AT CERTAIN
TEMPERATURES.
IN THIS UNIT, WE ARE GOING TO BE
CONCERNED WITH THERMAL ENERGY AND
THE RELATIONSHIP BETWEEN HEAT AND
TEMPERATURE.
IF WE TRIED TO DEFINE TEMPERATURE,
WE COULD COME UP WITH SEVERAL
DIFFERENT WAYS TO DESCRIBE IT.
SOME OF THESE ARE GIVEN ON THE NEXT
SLIDE.
•The degree of hotness or coldness of a body or
environment.
•A measure of the warmth or coldness of an object
or substance with reference to some standard value.
•A measure of the average kinetic energy of the
particles in a sample of matter, expressed in terms
of units or degrees designated on a standard scale.
•A measure of the ability of a substance, or more
generally of any physical system, to transfer heat
energy to another physical system.
•Any of various standardized numerical measures of
this ability, such as the Kelvin, Fahrenheit, and
Celsius scale (the reading on a thermometer).
WHEN WE TAKE A THERMOMETER READING,
WE ARE USING THE THERMAL EXPANSION
PROPERTIES OF A LIQUID.
MOST SUBSTANCES EXPAND WHEN HEATED.
SO, WE PUT A LIQUID IN A GLASS TUBE,
MARK THE HEIGHT OF THE LIQUID IN
REFERENCE TO CERTAIN KNOWN
PHENOMENA, AND MARK OFF A SCALE.
THE TEMPERATURE WE READ WILL THEN BE
PROPORTIONAL TO THE AMOUNT OF HEAT
OR THERMAL ENERGY OF THE SUBSTANCE.
SO ON A MARCOSCOPIC SCALE (LARGE)
TEMPERATURE WOULD BE A READING ON A
THERMOMETER THAT WE’VE CALIBRATED
RELATIVE TO CHANGES IN THE STATE OF
MATTER.
ON A MICROSCOPIC SCALE (VERY SMALL),
TEMPERATURE WOULD BE A MEASURE OF THE
AVERAGE KINETIC ENERGY OF THE PARTICLES
(ATOMS OR MOLECULES) MAKING UP THE
SUBSTANCE.
KINETIC ENERGY IS GIVEN AS
KE = ½ m x v2
where m = mass and v = velocity
WHEN WE THINK ABOUT KINETIC ENERGY,
WE THINK ABOUT SOMETHING IN MOTION.
WHEN WE THINK ABOUT SOMETHING LIKE
SODIUM CHLORIDE, NaCl, WE DON’T THINK
ABOUT THE PARTICLES (SODIUM IONS, Na+,
AND CHLORIDE IONS, Cl-) MOVING
AROUND.
THEY CAN VIBRATE AROUND FIXED POINTS
IN THE CRYSTAL.
PARTICLES IN A GAS OR A LIQUID CAN
ALSO MOVE RELATIVE TO EACH OTHER.
THE AVERAGE KINETIC ENERGY WILL BE A
FUNCTION OF THE TEMPERATURE. AS THE
TEMPERATURE INCREASES, THERE WILL BE A
GREATER PERCENTAGE OF PARTICLES MOVING
AT HIGHER SPEEDS.
QUESTION
IF YOU HAVE TWO SAMPLES OF A GAS – A
CONTAINER OF HELIUM, He, AND A
CONTAINER OF OXYGEN, O2, WILL THE
PARTICLES IN EACH GAS HAVE THE SAME
AVERAGE SPEED?
THINK ABOUT TWO SCENARIOS:
1) YOU ARE SITTING AT BREAKFAST WITH
A CUP OF HOT COFFEE. YOU GET A
TEXT MESSAGE FROM A FRIEND. BY
THE TIME YOU ANSWER IT (10
MINUTES LATER), YOUR COFFEE IS
LUKE WARM.
2) YOU ARE SITTING IN YOUR BACK YARD
WITH A GLASS OF ICE TEA WHEN
GEORGE COMES OVER. YOU SHOOT A
FEW HOOPS, AND WHEN YOU SIT BACK
DOWN, ALL THE ICE IN YOUR TEA HAS
MELTED.
WHAT REALLY HAPPENED IN THOSE TWO
CASES?
WE SAY THAT HEAT FLOWS FROM AN
OBJECT AT A HIGHER TEMPERATURE TO
AN OBJECT AT A LOWER TEMPERATURE.
IN FACT, WE USE THAT AS OUR
DEFINITION OF HEAT.
HEAT IS THE TRANSFER OF ENERGY FROM
THE HIGHER TEMPERATURE OBJECT TO
THE LOWER TEMPERATURE OBJECT.
SO, ANOTHER WAY WE CAN THINK ABOUT
TEMPERATURE IS THAT IT IS A MEASURE OF
THE ABILITY OF A SUBSTANCE TO TRANSFER
HEAT ENERGY TO ANOTHER SUBSTANCE.
WHAT WOULD HAPPEN IF YOU PLACE A SMALL
METAL CUP OF COLD WATER (5o C) INSIDE A
LARGER STYROFOAM CUP OF HOT WATER (70o
C)?
THEY WOULD EVENTUALLY COME TO THE SAME
TEMPERATURE. THIS IS CALLED THERMAL
EQUILIBRIUM.