Chapter 10 - Thermodynamics ‘heat’-‘movement’

Internal Energy Two main processes can change Internal Energy (U) of a given system Heat (Q) can flow in or out of system Work (W) can be done by the system*

Life is complicated System – Set of objects that we define as a distinct group Environment – everything outside of system Heat flows from one to other, work is done by one on the other

Work by a system Gases do work by increasing in volume – (internal combustion engines) W=Fd, ΔV=Ad -> algebra -> Work = Pressure*ΔVolume W = PΔV

Laws of Thermodynamics 1st Law – Total increase of thermal energy of a system is the sum of the heat added and work done to the system ΔEnergyinternal = Heat – Work ΔU = Q – W SIGNS!!!

Special cases Isovolumetric – volume doesn’t change – no work done by or on Isothermal – no temperature change – internal energy doesn’t change Adiabatic – no heat flow in or out

Laws of Thermodynamics ΔU = Q – W Revisit Isovolumetric, Isothermal, Adiabatic, Isolated systems Chart Page 344

Applications of 1st law Cyclic Processes – back where you started at the end Heat engines – turn thermal energy into mechanical energy (do work with heat) W = Qh-Qc Must have a high temperature source and a ‘cold’ heat sink

Evaporation Evaporation is Phase change from liquid to gas Takes place at the surface of the liquid (hot particles leave; cool ones are left behind) A cooling process How do you cool your body? How do you stay cool at Six Flags? How do you cool a hot cup of coffee? Energy money game, all start with 4, then play rock, paper, scissors with each other. Winner takes a energy unit. When you reach 6 – go to evaporation land. What is left? THE HOT ONES LEAVE

Condensation Condensation is Phase change from gas to liquid loss of energy due to collision loss of energy due to attractive forces A warming process Why do you feel extra warm on a muggy day?

Evaporation>Condensation-cooling The Going Rate Evaporation and Condensation take place at the same time Evaporation>Condensation-cooling Evaporation<Condensation-warming Evaporation=Condensation-equilibrium

The Going Rate What can you conclude about a dish of water on a table for several days that has no change in the water level? Why do you feel chilly after emerging from a shower? Why do you not feel as chilly if you stay in the shower to dry off? Dogs, pigs, etc

There’s Something in the Air Saturation of air must occur for condensation to be present When slow moving molecules collide, they stick together (at higher speeds, they can bounce apart) Cloud formation attributed to condensation warm air rises = expands = cools = molecules stick together = condensation upon molecules/ions Fog - a cloud that forms near the ground

Evaporation/Condensation Human, dog cooling – AZ vs. St. L. Shower examples Cloud formation Fog Adiabatic example – cold cans & blowing air

HSW - Refrigerator Changing pressure to move energy with vapor points Compressor Inside coils Outside coils Expansion valve Step by step through the loop with number examples 500 calories of heat absorbed from food to vaporize cool liquid Compress the gas to raise vaporization temperature (do 100 calories of work) Compressed gas condenses – releasing 600 calories of energy Liquid’s pressure is reduced by expansion valve – start over 500 calories out of the fridge – 600 calories released outside Page 350-351

How Refrigerators work

Efficiency of engines Efficiency – useful work out of total energy given Eff. = Wnet/Qh Eff. = (Qh – Qc)/Qh Eff. = 1 – Qc/Qh Chart on page 353

Laws of Thermodynamics Entropy – disorder of system hotter – faster moving molecules; less entropy (ability to do more work) cooler – slower moving molecules; more entropy (ability to do less work) 2nd Law – natural processes go in a direction that increases the total entropy of the universe

2nd Law Examples Diffusion of liquids/gases Heat always flows from hot to cold Universe ends up cold and dark (all microwaves) Evolution?

THE BIG QUESTION! Which freezes faster – hot water or cold water? ??