Every mathematician knows it is impossible to understand an elementary course in thermodynamics. ~V.I. Arnold

Transcription

1 Every mathematician knows it is impossible to understand an elementary course in thermodynamics. ~V.I. Arnold

2 Radiation Radiation: Heat energy transmitted by electromagnetic waves Q t = εσat 4 emissivity 0 ε 1 Stefan-Boltzmann constant σ = W m 2 K 4 the same for all materials

3 ε 1 good emitter good absorber poor reflector ε 0 poor emitter poor absorber good reflector ε =1 ideal blackbody ε =0 perfect reflector

4 Coffee is too hot to drink, you want to add some creamer anyway, but if you want the coffee to cool quickly, should you (a) add the creamer right away (b) wait to add the creamer (c) It doesn t matter

5 You re a consultant for a cookware manufacturer who wishes to make a pan that will have two features: (1) absorb thermal energy from a flame as quickly as possible, and (2) have an inner surface that remains as hot as possible when cooking. You should recommend a pan with the 1. outer and inner surfaces black. 2. outer and inner surfaces shiny. 3. outer surface shiny and inner surface black. 4. outer surface black and inner surface shiny.

6 States of Matter Solid Liquid Gas Solid X melting sublimation Liquid freezing X evaporation Gas antisublimation deposition condensation X

7

8 Evaporation When evaporation occurs from a liquid, like water, particles of the substance (like the H20 molecules) become part of the gas above the liquid. What happens to the temperature of the liquid, assuming the liquid is otherwise isolated from the environment? Why? It gets cooler. The molecules that escape have a faster speed than average. There is a minimum speed to escape, since there is an attractive force between molecules. When the fastest molecules leave, the average speed drops, so the temperature decreases

9 Condensation When particles of gas, like H2O molecules in air, condense from a gas state into a liquid, like into liquid water, they become part of the liquid. What happens to the temperature of the liquid, assuming the liquid is otherwise isolated from it s environment? Why? It gets warmer. Although the vapor molecules have the same average speed as the ones in the liquid, the attraction between molecules causes vapor molecules to speed up right before they hit the surface. By the time they become part of the liquid, they are moving faster, and the average temperature goes up.

10 Boiling: can think of as internal evaporation, a cooling process. P in P out If P in <P out bubble is crushed

11 Boiling: can think of as internal evaporation, a cooling process. P out If P in <P out bubble is crushed

12 Boiling: can think of as internal evaporation, a cooling process. P in P out Heating liquid P in

13 Boiling: can think of as internal evaporation, a cooling process. P in Pout Heating liquid P in

14 Boiling: can think of as internal evaporation, a cooling process. Boiling is a type of internal evaporation (a cooling process). Within a liquid, a tiny vapor bubble of the substance forms (typically around pieces of dust or other impurities). If the pressure of the surrounding liquid is greater than the internal pressure of the bubble, the bubble is crushed. When the liquid is heated, the internal pressure of the vapor bubble increases allowing it to rise to the surface instead of being crushed. Once a liquid begins to boil, the rate of evaporation is equal to the rate of condensation, so the temperature does not change.

15 Vapor Pressure Water Vapor Air is removed, container then plugged Vapor Pressure is the pressure of the vapor above the liquid Liquid Water

16 Vapor Pressure Water Vapor Air is removed, container then plugged Vapor Pressure is the pressure of the vapor above the liquid Liquid Water Boiling Point: P vapor = P atm

17 Food cooked in boiling water at a mountain top cooks slower than when cooked at sea level. If the temperature under the pot of boiling water is increased, the food will cook (a) faster than before at the mountain top. (b) faster than before at sea level. (c) faster than before at both places. (d) no differently than it did before the increase.