First Law of Thermodynamics Physics 102 Professor Lee Carkner Lecture 5 (Session: )

PAL #5 Phase Change  Final temperature of melted Frosty  Four heats:  Warm up Frosty to 0 C: m ice c ice (0-(-5))  Melt Frosty: m ice L ice  Warm up melted Frosty: m water c water (T f -0)  Cool down air: m air c air (T f -20)  (100)(2100)(5)+(100)(333000)+(100)(4186)(T f )+(9700) (837)(T f -20) = 0  1.05X X X10 5 T f +8.12X10 6 T f -1.62X10 8 = 0  8.54X10 6 T f = 1.28X10 8  T f = 15 C

Energy  We know that in mechanics energy is conserved   In what ways can energy be expressed?   Heat can flow in or out   Related to expansion or compression  The internal energy might change  Related to temperature

 Consider a piston of gas with weight on the top and a thermal reservoir at the bottom   Weight can be added or subtracted so that the system does work on the weight or the weight does work on the system   If we add weight and do 6 J of work we either increase the internal energy by 6 J or produce 6 J of heat or some combination that adds up to 6

The First Law of Thermodynamics  This conservation of energy is called the First Law of Thermodynamics  U = Q - W   If work is done by the system W is positive, if work is done on the system W is negative  Positive work is the useful work we get out   Heat flow in is +, heat flow out is -

PV Diagram  How much work is done if a gas expands and raises a piston?   Depends on:  Pressure   Change in volume   The relationship between P and  V can be complicated, but  Work equals area under curve in PV diagram

The P-V Curve   If the volume decreases, work is done on the system and the work is negative   If the process is cyclical and returns to the same point by two different paths the area between the paths is equal to the work (and also equal to the heat)

P-V Diagrams

Internal Energy and Temperature  Internal energy (U) is directly related to temperature   High T, large U  Low T, small U   KE = (3/2)kT for one molecule  U = (3/2)nRT for n moles  Note that everything has some internal energy, we want to know about the change in internal energy (  U)

Types of Processes  We want to understand 5 basic types of thermodynamic processes  For each you should know:      PV diagram

Isobaric  In an isobaric process the pressure does not change   Since the area under the PV curve is a rectangle:  W=P  V

Isochoric   e.g. a sealed hollow cylinder  W = 0 so  U = Q  If any heat is applied to the system it goes directly into internal energy

Isothermal  An isothermal process happens at constant temperature   Since  T = 0:  U = 0 so Q=W   We can use calculus to find the area under the curve  W = nRTln(V f /V i )

Adiabatic  Adiabatic processes are ones in which no heat is transferred   Since there is no heat: Q=0 so  U = -W

P-V Diagram P V Isobaric (P=const.) Isochoric (V=const) Isothermal (T=const) Adiabatic (Q=0)

Cyclical Process  A cyclical process returns to its initial state   U = 0 so Q=W  There are many different ways to produce a cyclical process 

Next Time  Read:  Homework : Ch 15: P 7, 10, 17, 29  Test 1 next Friday  About 10 multiple choice (~25%)  About 4 problems (~75%)  Equation and constant sheet given  I have posted equation sheet and practice problems

As a pot of water boils, the temperature of the water, A)Increases B)Decreases C)Stays the same D)Fluctuates unpredictably E)It depends on the temperature of the stove

Water condenses out of the air onto a cold piece of metal. Due to this condensation, the temperature of the air around the metal, A)Increases B)Decreases C)Stays the same D)Fluctuates unpredictably E)It depends on the temperature of the metal

The temperature of a solid is held constant and the pressure is lowered. When the pressure gets very close to zero the solid will, A)Become a gas B)Become a liquid C)Stay a solid D)Be at the triple point E)Be at the critical point