VIEWS: 3 PAGES: 43 POSTED ON: 10/21/2011 Public Domain
Thermal Physics Thermometric property - a characteristic of an object that varies with temperature Heat versus Temperature Temperature: average kinetic energy of each particle in a substance degree of hotness or coldness indicates direction of heat flow Heat versus Temperature Heat - total thermal energy (internal energy) absorbed or transferred Internal energy - KE and PE PE due to bond energy (chemical energy stored in bonds) and electromagnetic forces between particles Example: bucket vs thimble of water at 100°C Measurement of Temperature Alcohol vs. mercury thermometers Alcohol - used at lower temperatures Alcohol freezes at -114°C and boils at 78.5°C Mercury - used at higher temperatures Mercury freezes at -39°C and boils at 357°C(675F) Thermometers QuickTime™ and a decompressor are need ed to see this picture. QuickTime™ a nd a decompressor are need ed to see this picture. Constructing Thermometers Bulb - thin glass for quick heat transfer Clinical thermometer - restriction prevents backflow Vacuum above liquid Calibrated using two fixed points (0C and 100C), mark scale evenly Thermometers-constructed using thermometric properties Expansion of liquid in capillary tube Resistance in wire (thermistor) Different rates of expansion of metals (bimetallic strips) Volume of gas at constant pressure (gas expansion rate is linear) Color change of solid when heated(pyrometers Conversions K = °C + 273 °F = 9/5 °C + 32 Absolute zero - no motion of particles in substance at 0 K Water freezing point 32°F, 0°C, 273 K Water boiling point 212°F, 100°C, 373 K Room temperature 20°C Relating temperature to velocity of particles Kinetic energy = 1/2 mv2 Kinetic energy = 3/2 kT k = Boltzmann constant 1.38 x 10-23 J/K T = Temperature in Kelvin m = mass in kilograms v = root mean square velocity (rms) rms velocity Number of particles at different speeds is not a normal distribution - some particles move VERY fast Peak velocity is most probable velocity Vrms = (vav2)1/2 QuickTime™ and a decompressor are neede d to see this picture. Heat Transfer - Conduction Occurs in solids, liquids, gases Temperature difference causes transfer of thermal energy from hot to cold by particle collision without net movement of substance Heat Transfer - Convection Occurs in fluids (liquids and gases) Temperature difference causes mass movement of fluid particles - density differences Convection cells (convection currents) QuickTime™ and a decompressor are need ed to see this picture. Heat Transfer - Radiation No medium required Heat travels as electromagnetic waves Most reflected at atmosphere, some is absorbed Heat Capacity Objects with a high heat capacity take in heat at a slower rate - heat slowly, and also cool slowly Heat capacity Q/T units are JK-1 Specific heat capacity - heat capacity per unit mass - heat required to raise the temperature of 1 kg by one Kelvin Variable for specific heat….c Heat Equation Heat required to produce a temperature change Q = mcT Can heat a substance using electrical energy Electrical energy = VIt = mcT Mixtures Heat lost by one substance equals heat gained by the other substance - total heat gained or lost by the system is 0 Calorimeter - allows minimal energy loss to surroundings Calorimeter QuickTime™ and a decompressor are neede d to see this picture. Example #1 Mix 100 grams of water at 20°C and 200 grams of water at 40°C. Find the final temperature of the water. Solution: Heat lost by one sample plus heat gained by other sample equals 0 mcT mcT 0 1 1 0.1kg(4180 kJkg K )(Tf 20) 1 1 0.2kg(4180 kJkg K )(Tf 40) 0 418Tf 8360 836Tf 33440 0 Tf 33C Example #2 A 100 gram block of Ag at 100°C is placed in 100 grams of water at room temperature. Find the final temperature. Solution: Heat lost by silver plus heat gained by water equals 0 mcT mcT 0 1 1 0.1kg(235 kJkg K )(Tf 100 ) 1 1 0.2kg(4180 kJkg K )(Tf 20) 0 23.5Tf 2350 418Tf 8360 0 Tf 24C Kinetic Theory All matter is composed of extremely small particles All particles are in constant motion If particles collide with other particles, KE is conserved A mutually attractive force exists between particles Matter Matter - has mass and occupies space Four phases - solid, liquid, gas, plasma Plasma - made by heating gas atoms until they ionize - separate into positively and negatively charged particles - sun, other stars composed of plasma QuickTime™ and a decompressor are neede d to see this picture. Characteristic Solid Liquid Gas Shape definite variable variable Volume definite definite variable Compressibility almost slightly highly incompressible compressible compressible Diffusion small slow fast Density highest high low Bonds strong relatively weak strong Particle closely larger very large spacing packed spaces spaces PE high higher highest Thermal least middle most energy Latent Heat Latent heat of transformation: heat required to change 1kg of a substance from one phase to another Equation: Q = mL No temperature change during a phase change - heat is used to change PE - heat needed to break bonds, heat released when bonds are formed Latent Heat Latent heat of fusion: solid to liquid (heat is absorbed) or liquid to solid (heat is released) Latent heat of vaporization: liquid to gas (heat is absorbed) or gas to liquid (heat is released) Heating curves: QuickTime™ and a decompressor are need ed to see this picture. Example #1 Find the heat required to melt 10 kg of gold. (latent heat of fusion = 6.3 x 104 JKg-1) Solution Q mH f 1 10kg(6.3x10 Jkg ) 4 6.3x10 J 5 Example #2 Find the heat required to vaporize 100 grams of lead (latent heat of vaporization = 2.04 x 104 JKg-1) Solution Q mHv 1 0.1kg(2.04x10 Jkg ) 4 2.04x10 J 3 Example #3 Find the heat required to change 2 kg of ice at -10°C to steam at 120°C Specific heat of ice 2060 Jkg-1K-1 Latent heat of fusion 3.34 x 105 Jkg-1 Specific heat of water 4180 Jkg-1K-1 Latent heat of vaporization 2.26 x 106 Jkg-1 Specific heat of steam 2020 Jkg-1K-1 Solution: Heat ice from -10°C to 0°C Melt ice Heat water from 0°C to 100°C Evaporation of water Heat vapor from 100°C to 120°C Q mcT mH f mcT mHv mcT 1 1 2 kg(2060Jkg K )(10C) 1 2kg(3.34x10 Jkg ) 5 1 1 2 kg(4180Jkg K )(100C) 1 2kg(2.26x10 Jkg ) 6 1 1 2 kg(2020Jkg K )(20C) 6 5.5x10 J Example #4 Heat is added to a mass of 5 kg at room temperature (20°C) at a rate of 500 watts for 1 minute until the substance begins to melt at 300°C. The substance takes 3 minutes to melt. A. Sketch a graph of temperature vs. time B. Find the specific heat of the solid C. Find the latent heat of fusion Solution-Part A. QuickTime™ and a decompressor are neede d to se e this picture. 300°C 20°C 1 4 Times and temperatures on graph? B. Q mcT 500watts x 60s 5kg(c)(280C) 1 1 c 21.4Jkg K C. Q mH f 500watts(180sec) 5kg(H f ) 1 Hf 1.8x10 Jkg 4 Evaporation Evaporation takes place at all temperatures and results in the cooling of a liquid Evaporation Change from liquid state to gaseous state, occurs at a temperature below boiling point Particles near surface have enough KE to overcome attractive forces of nearby particles, lowers KE of substance