Thermodynamic Processes Chapter 10
Work at Constant Pressure Constant Pressure Work The definition of work becomes W=Fd for a constant force F along a distance d. Using the definition of pressure, can we deduce a formulae for the work done by the piston below?
The First law of Thermodynamics
In the data booklet as…
Why the changes in signs?! It’s important to get the positive and negatives right here… If Q is positive – HEAT ENERGY is being added TO the system If Q is negative – HEAT ENERGY is being LOST by the system
If work is negative – WORK is being done ON the system (the gas is being compressed) If work is positive – WORK is being done BY the system (the gas is expanding) If U is positive – GAIN in internal energy of the gas If U is negative – LOSS in internal energy of the gas
Definitions Isochoric – (Isovolumetric) A change that takes place at constant volume Isobaric – A change that takes place at constant pressure Isothermal – A change that takes place at constant temperature
Which graph shows which change?! In which process would the work done be zero? How would you work out the work done during the process?
One more… Adiabatic- A change in the gas where no energy enters or leaves the system Q= 0
Adiabatic Process Adiabatic process must take place rapidly and the system is not in thermal equilibrium with its surroundings Any RAPID expansion or compression CAN be assumed to be ADIABATIC (Q=0)
First Law of Thermodynamics Is just the Conservation of Energy law in a different format!
Heat engines! http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/carnot.htm http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heaeng.html#c1
Heat Engines Usually depicted like the diagram below
Carnot Cycle- most efficient cycle (imaginary)
Otto Cycle
Heat Pump (Refrigerator)
Second Law of Thermodynamics Many ways of saying this!! Thermal energy may be completely converted to work in a single process, but that continuous conversion of this energy into work requires a cyclical process and the transfer of some energy from the system. Thermal energy can not be spontaneous transferred from a region of cold temperature to a region of hot temperature
Entropy and the Second Law The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibrium—the state of maximum entropy.
Entropy Is a measure of the degree of disorder of a system. Although local entropy may decrease (heat pumps), any process will increase the total entropy of the system and surroundings, that is, the universe. The overall entropy of the universe is increasing. All natural processes increase the entropy of the universe.
Entropy The change in Entropy when a quantity of heat energy flows into a body is equal to… ΔS = Q/T Units?!
Entropy http://www.saburchill.com/physics/chapters/0123.html