Heat Heat Q is a transfer of energy from one object to another because it’s cooler. It’s not something an object “has” (we will call the energy inside an object its “internal energy”)
Heat Calories and Joules (James Joule 1849) 1 calorie = 4.186 J Food calorie: 1 Cal = 1000 cal Raising your 500 kg car with the energy in a large cream-filled 500 Cal donut: . Compare to Empire State Building: 443 m
Specific heat How much does T rise when heat energy is added? Typical specific heats Water 4186 Ice 2090 Steam 2010 Glass 840 Steel 500
Phase changes
Energy and phase changes
Latent heat during phase Changes Water Latent heats L Ice melting/freezing 3.34 x 105 Water boil/condense 2.26 x 106
Latent heat during phase Changes Which takes the most energy for the same quantity of H20? melting ice at 0 C. raising T from 0 to 100 C boiling it
Calorimetry Conservation of energy: +, – Q’s all sum to zero. or Qgained by cold objects = Qlost by hot objects (on both sides use positive quantities) 50 g of hot steel at 300 C is added to 100 g of water at 20 C. What is the final temperature? csteel = 0.12 cal/g/oC
Blackbody Radiation blackbody: an object that when cool absorbs all light, and when hot, emits light in a spectrum that depends only on T. Emissivity = 1 means perfect blackbody emissivity e Black paper 0.9 White paper 0.7 Anodized aluminum, Black 0.8 Aluminum highly polished 0.05
Blackbody spectrum, color
Radiated and absorbed power Hot vacuum oven with a cold object inside A piece of metal at room temperature (20 C) is put into a vacuum oven at 300C. What determines how fast its temperature rises?
Temperature of the earth:
P2. A rough black metal sphere is heated white-hot to 3000 K, and puts out 1000 W of radiation energy. If it is instead held at 1500 K, it will put out ______ W of radiation energy (ignore the T of the environment). A. 60 B. 250 C. 500 D. 750 E. 1000
P3. If instead the sphere is kept at 3000 K, but the radius of the sphere is doubled, it will put out ______ W of radiation energy. A. 500 B. 1000 C. 2000 D. 4000 E. 8000
Thermal conduction Newton’s law of cooling Thermal conductivity Highest for dense materials, metals
Typical thermal conductivities k [J/(smoC)] Thermal conduction Typical thermal conductivities k [J/(smoC)] Copper 390 Steel (common) 80 Stainless steel 14 Glass 0.84 Wood 0.1 Air 0.023 Styrofoam 0.01 If air is great a insulator, why use fiberglass in houses, feathers in sleeping bags?
Our best insulators simply keep air from circulating!
Thermal convection
Thermal convection
P4. You put the end of a rod in the fire and the other end in a tub of water. The rod that would heat the water fastest will be _______ short and fat long and fat short and thin long and thin