Thermal Physics

TEMPERATURE measures the tendency for energy to leave an object spontaneously a measure of the average kinetic energy of the molecules in the object Will both fingers feel the same temperature when they're put in the warm water?

Thermal Equilibrium Two objects in contact are said to be in equilibrium if their temperatures are the same Does the thermometer measure the temperature the water had before the thermometer was put in? Zeroth law of equilibrium If body A is in thermal equilibrium with body B and body B is in thermal equilibrium with body C, then body A is in thermal equilibrium with body C.

Kelvin and Celcius Temperature Scales Temperature is a direct measure the average translational KE of the molecules or atoms in the material High temperature  high average KE The Kelvin scale directly measures the average KE of the molecules Kelvin is identical to Celcius except the zero point is set at o CCC

Internal Energy, U The sum total of all the energy associated with molecular motions and positions

Work & Heat Work has been done..if the energy exchange is associated with organized motion of molecules in the surroundings

Work & Heat Heat.. the transfer of energy is a result of random motions in the surroundings Heat can be transferred only between systems that are at different temperatures

First Law of Thermodynamics

Heat Transfer Mechanisms

CONVECTION Convection is the process in which heat is carried from one place to another by the bulk movement of a fluid convection currents

CONDUCTION Conduction is the process whereby heat is transferred directly through a material One mechanism for conduction occurs when the atoms or molecules in a hotter part of the material vibrate or move with greater energy than those in a cooler part. By means of collisions, the more energetic molecules pass on some of their energy to their less energetic neighbors.

CONDUCTION OF HEAT THROUGH A MATERIAL The heat Q conducted during a time t through a bar of length L and cross-sectional area A is SI Units of Thermal Conductivity: J/(s·m·C o ) thermal conductivity

THERMAL RADIATION Radiation is the process in which energy is transferred by means of electromagnetic waves It doesn’t need a material medium between the two systems exchanging heat.

IDEAL GAS ASSUMPTIONS ABOUT THE GAS 1. On average, the space between molecules is much larger than the actual size of the molecules 2. Molecules collide elastically with the container walls and each other 3. While molecules exert forces on each other during a collision, there are no long-range forces between them, so there is no internal potential energy

Boyle’s Law - Temperature and moles of gas are held constant pV = p’V’ The relationship between volume and pressure in a contained gas is an indirect proportion

Kinetic Molecular Theory A molecule of mass m collides elastically with one wall ∆p = -mv – mv = -2mv a = L ∆t = 2L / v

f av = ∆p / ∆t = -2mv/ (2L/ v) = mv 2 / L P = f av / area = f av / L 2 Assume 1/3 of all molecules are moving in this back-and -forth manner… P = 1/3 N (mv 2 / L 3 ) = 1/3 N (mv 2 / V) PV = 1/3 N (mv 2 ) = 2/3 N (1/2mv 2 ) = 2/3N(KE av ) PV = nRT = NkT k = R/6.02 x = 1.38 x J/K (k = Botlzmann’s constant) KE = 3/2 kT U = 3/2NkT = 3/2nRT