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 v 2 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, O 2, 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 (5 o C) INSIDE A LARGER STYROFOAM CUP OF HOT WATER (70 o C)?

THEY WOULD EVENTUALLY COME TO THE SAME TEMPERATURE. THIS IS CALLED THERMAL EQUILIBRIUM.