Energy
Design Lots of forms of energy Lots of forms of energy transfer Understand how works Understand how to calculate or estimate
Types of Energy Three main types Kinetic (1/2 mv2) Potential (gmz) Internal Bond vibrations VDW Charges
Types of Energy TRANSFER Two main types Heat Work What’s the difference between heat and energy? State function (temperature, energy) Path function (heat, work)
Heat (Q) It is NOT a property It is a mechanism to transfer energy (thermal) Flow is always in one direction. Adiabatic is a lack of heat flow Insulation Sometimes want to prevent heat flow
Heat Transfer Conduction Convection Radiation Touch the hot stove Open oven and feel rush of heat Radiation Sun “red hot”
Work (W) Electric/Magnetic work Mechanical work (spring work) Electric Engineers and Physics 2 Circuit Mechanical work (spring work) Apply force over a distance Spring Boundary work Boundary changes size Piston Shaft work Rotating shafts Motor Flow Work Pressure change as fluid flows PV work
Flow Work The work it takes to have substances flow Internal energy + Flow work = enthalpy U + PV = H Convenient to combine them Use enthalpy instead of internal energy for “flow processes”
Boundary Work Isochoric – constant V Isobaric – constant P Isothermal – constant T Polytropic – common example PVk = Constant Let’s calculate each for an IG But FIRST, CW questions
Enthalpy/Internal Energy Calculate enthalpy, pressure, volume, and internal energy of saturated steam at 40oC Does H=U+PV?
Work/Heat Changes U/H Some processes are isenthalpic or constant energy However, in the presence of work or heat, U/H of a substance WILL change How do we calculate those changes?
Changes in Internal Energy or Enthalpy for water Look up values at the two conditions, calculate difference
Changes in Internal Energy (IG) Heat capacity (C) For internal energy – Cv “constant volume” How much energy it takes to change temperature ONLY a function of temperature for IG Energy and Cv
Changes in Enthalpy(IG) For enthalpy – Cp “constant pressure” ONLY a function of temperature for IG Enthalpy and Cp
Changes in Internal Energy (IG) For a room 5 m X 4 m x 3 m at 283.15 K and 1.0 bar, how much energy would it take to increase the air to 298 K (room temperature)? For that same change of energy, how much would the quality of water as a saturated liquid (at 283.15 K) change?
Changes for REAL gases Something called departure functions or residual properties (chapter 5)
Other forms of energy change “Latent” heat The energy it takes to change phase Evaporation Heat of vaporization Melting Freezing Cooling or heating process (sign of enthalpy change)
Concept Warehouse But first, what do you think is safer, steam at 150 oC or 105 oC WHY? Enthalpy v energy CW