Thermodynamics

Thermodynamic Process in which energy is transferred as heat and work

1 st Law of Thermodynamics Internal energy (U) of a system is increased if heat is added to it or work is done on it  U = Q + W Q – positive if heat added to it W – positive if work done on it Same as the Law of Conservation of Energy

Example An amount of heat equal to 2500 J is added to a system and 1800J of work is done on the system. What is the change in internal energy? What would the change be if 1800J of work was done by the system?

Thermochemical Processes Isothermal Process carried out at constant temp P V A B

If the constant temp is diff, the shape will stay be the same, but it will be shifted up or down B is at lower temp than A Since U depends on the change in temp, if temp is constant  U is 0  U = Q + W 0 = Q + W Q = W True for Isothermal In an isothermal process, the work done by it is equal to the heat put into it

Adiabatic Process in which no heat flows in or out Can only occur if the system is very well insulated or if the process happens so fast that heat has no time to flow  U = Q + W  U = 0 + W  U = +W Work done on a gas increases its internal energy

P V isothermal adiabatic

Other Processes Isobaric – pressure constant Isochoric – volume constant P P VV

Work Work is done when the volume of a gas changes Work is given by the area under the PV curve W = P  V 1 L atm = J

Example What does more work if starting and ending at the same volumes, Isothermal or adiabatic processes?

Example If you stretch a rubber band suddenly the temperature increase. Why? A gas is compressed adiabatically at constant pressure of 2.0 atm from 10L to 2.0L. How much does the internal energy of the system change?

Example Sketch a process of an isothermal expansion and then an isobaric compression. In an isochoric process, the internal energy of the system changes by 100J. What is the work done?