1 / 43

Thermal Physics

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.

imani-huffman
Download Presentation

Thermal Physics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Thermal Physics Thermometric property - a characteristic of an object that varies with temperature

  2. Heat versus Temperature • Temperature: average kinetic energy of each particle in a substance • degree of hotness or coldness • indicates direction of heat flow

  3. 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

  4. 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)

  5. Thermometers

  6. 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

  7. 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

  8. 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

  9. 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)

  10. 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

  11. 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

  12. Heat Transfer - Convection • Occurs in fluids (liquids and gases) • Temperature difference causes mass movement of fluid particles - density differences • Convection cells (convection currents)

  13. Heat Transfer - Radiation • No medium required • Heat travels as electromagnetic waves • Most reflected at atmosphere, some is absorbed

  14. 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

  15. Heat Equation • Heat required to produce a temperature change • Q = mcT • Can heat a substance using electrical energy • Electrical energy = VIt = mcT

  16. 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

  17. Calorimeter

  18. 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.

  19. Solution: • Heat lost by one sample plus heat gained by other sample equals 0

  20. 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.

  21. Solution: • Heat lost by silver plus heat gained by water equals 0

  22. 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

  23. 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

  24. 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

  25. 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:

  26. Example #1 • Find the heat required to melt 10 kg of gold. (latent heat of fusion = 6.3 x 104 JKg-1)

  27. Solution

  28. Example #2 • Find the heat required to vaporize 100 grams of lead (latent heat of vaporization = 2.04 x 104 JKg-1)

  29. Solution

  30. 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

  31. 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

  32. 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

  33. Solution-Part A. 300°C 20°C 1 4 Times and temperatures on graph?

  34. B. C.

  35. Evaporation Evaporation takes place at all temperatures and results in the cooling of a liquid

  36. 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

More Related