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Thermal Physics

Thermal Physics. Thermal Physics is the study of temperature and heat and how they effect matter. Heat leads to change in internal energy which shows as a change of temperature and is evident with the expansion or contraction of matter. Temperature.

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Thermal Physics

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  1. Thermal Physics • Thermal Physics is the study of temperature and heat and how they effect matter. • Heat leads to change in internal energy which shows as a change of temperature and is evident with the expansion or contraction of matter

  2. Temperature • Temperature is the hotness or coldness of matter • Heat energy travels from a hot object to a cold object • If two objects are in contact thermal contact energy can be exchanged between them • The exchange of energy is called heat

  3. Thermal Equilibrium • Two objects are in thermal equilibrium if they are in contact and no exchange of energy takes place • Zeroth Law of Thermodynamics states that if object A and B are in thermal equilibrium with object C then A and B are in thermal equilibrium with each other. • Two objects in thermal equilibrium have the same temperature.

  4. Thermometers • A thermometer is a calibrated device to measure temperature. They are much smaller than the system so they can reach equilibrium without great loss of energy from the system.

  5. Types of Thermometers • Change in volume of liquid (Mercury) • Length change of a solid • Change of pressure of gas with constant volume. (change of v with constant p) • Electric resistance of a conductor • Change of color of a hot object

  6. Temperature Scales • Kelvin calibrated using a gas thermometer Absolute zero = 0 Kelvin = - 273.15c Triple point of water is where ice, water and water vapor coexist. At 0.01oc and 4.58 mm Hg is used to establish Kelvin scale. • Celcius scale TC = TK- 273.15 • Fahrenheit scale TF = 9/5TC + 32

  7. Thermal expansion of solids and liquids • As the temperature of a substance increases the volume increases. Thermal expansion occurs due to a change in the average separation of the constituent atoms or molecules. • Atoms in a solid a separated by an average of 10-10m and vibrate. As temperature increases so does the separation.

  8. Linear Expansion • Let Lo be the original length  be the coefficient of linear expansion ΔT be the change in temperature Then ΔL =  LoΔT • Coefficient are published values particular to the type of material

  9. Area Expansion • Let the lengths of the sides be = L then A = L2 let Ao = original area ΔA = Ao ΔT •  is the coefficient of area expansion

  10. Volume Expansion • Similar to both length and area expansion volume expansion can be shown as Δv = vo ΔT  is the coefficient of volume expansion • Note that  = 2  and  = 3  • Liquids generally have volume coefficients ten times greater than solids

  11. Ideal Gas • An ideal gas is one that has atoms or molecules that move randomly and have no long range forces on each other. Each particle is like a point. • 1 Mole of gas has 6.02*1023 particles • 1 mole of gas occupies 22.4 liters

  12. Ideal Gas Equation • Pv = nRT • R is the ideal gas constant R= 8.31 when using Pa and m3 R= 0.0821 when using atmospheres and liters

  13. Kinetic Theory of Gases • The number of atoms/ molecules in a gas are large and the average separation is great compared to their size • particles obey Newton’s laws of motion and move randomly • Particles interact only through short range forces having elastic collision, including walls • All molecules in a gas are identical

  14. Boltzmann’s Constant • From Pv = nRT you get Pv = kBRT where kB = n/NA NA = Avogadro’s number = 6.02 * 1023

  15. Force on Container Walls • F = N/3(mv2/d) • where N = number of particles m = mass of one particle v = the average speed of the particles d = the length of the edge of the container • Total pressure on the walls of the container • P = 2/3(N/vc)(1/2mv2) vc= container volume

  16. Molecular Interpretation of Temperature • Temperature of a gas is a direct measure of the average molecular kinetic energy of the gas particles. 1/2mv2 = 3/2kBT • Total translational kinetic energy of N particles KEtotal = N(1/2mv2) = 3/2NkBT • For monatomic gases translational KE is the only type of energy the particles have. • Where U = 3/2nRT

  17. Root-Mean-Square • Diatomic and polyatomic gases have additional energies due to vibration and rotation. Their average velocity is calculated from • vrms = • m = molar mass in kg per mole

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