Objectives Describe thermodynamic processes on P-V diagrams. State and apply second law principles. Apply the laws of thermodynamics to the Otto cycle.

Today’s Plan Finish efficiency discussion. Discuss Thermodynamics. Discuss the Otto cycle. Homework problems due Monday! Quiz Friday.

Do Now (11/5/13): Rank the following diagrams in order from highest to lowest in terms of the work done. Justify your response. A.B.C.

Thermodynamics Study of processes in which energy is transferred as heat and as work. System vs. environment Closed system: no mass enters Open system: mass may enter or leave Isolated system: no energy passes across boundaries

Thermal Energy Internal energy—sum of all energies in a system. U=KE + PE (molecular level) U=(3/2)NkT for an ideal monatomic gas where k=1.38 x J/K U=(3/2)nRT, where R=8.315 J/mol-K

First Law Energy is neither created nor destroyed; changes from one form to another. Transferred by work or heat. U can change only if system is not isolated. (Energy moves between system and surroundings)

Work Energy transfer between system and surroundings due to organized motion in the surroundings. (rubbing a block of wood vigorously, stir a glass of water, allow a gas to expand against an external pressure.)

Work on/by an Enclosed Gas Ideal gas in moveable piston. Expands slowly so it remains near equilibrium throughout. Isobaric process. Pressure inside equals pressure exerted by piston (ie from outside) W by gas = - W on gas by the piston

Pressure-Volume Work P= F/A F=P*A W by = F*d = P  V Graphical representation. Area under curve = work done by gas

Heat Energy transfer between system and surroundings as a result of random motion in the surroundings. Flows spontaneously from high temp to low temp. Work can be used to make heat flow opposite natural flow direction.

First Law  U=W on + Q into Where U=internal energy, Q=net heat added to system, W=net work done on the system. Conventionally, heat added is +, lost is negative Work done on system is +, done by system is negative Statement of energy conservation

Thermodynamic Systems Isothermal—work done by the gas equals the heat added to the gas.  U=0 Adiabatic—no heat is allowed to flow into or out of the system. Q=0 therefore,  U=W on. Isobaric—pressure is constant Isochoric—volume is constant

Isothermal Constant temperature PV=constant Graphical representation and isotherms As heat is added slowly, gas expands at constant temperature. Work is done by the gas. U=0, so W by = Q in.

Adiabatic No heat is exchanged between the system and surroundings. Q = 0. U = W on Internal energy decreases as gas expands. (U=3/2 Nk  T, so temperature will decrease.)

Isobaric Pressure of system remains constant. W= P  V

Isochoric Volume remains constant W = 0.

Second Law Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.

Heat Engine

Carnot Cycle

Refrigerator/Heat Pump

Refrigeration Schematic

Refrigeration Cycle

Entropy Disorder  S=Q/T where T=absolute temperature The entropy of an isolated system never decreases. It only stays the same or increases. If not isolated, the change in entropy of the system plus in the environment is greater than 0.

Second Law Restatement Natural processes tend to move toward a state of greater disorder.

Nikolaus Otto The four stroke engine was first demonstrated by Nikolaus Otto in 1876, hence it is also known as the Otto cycle. The technically correct term is actually four stroke cycle. The four stroke engine is probably the most common engine type nowadays. It powers almost all cars and trucks.

starting point Position 1 being the beginning of the intake stroke of the engine. The pressure is near atmospheric pressure and the gas volume is at a minimum.

Intake Cycle Position 1  2 Copyright 2000, Matt Keveney. All rights reserved. Between Stage 1 and Stage 2 the piston is pulled out of the cylinder with the intake valve open. The pressure remains constant, and the gas volume increases as fuel/air mixture is drawn into the cylinder through the intake valve.

Compression Cycle Position 2  3 Copyright 2000, Matt Keveney. All rights reserved. Stage 2 begins the compression stroke of the engine with the closing of the intake valve. Between Stage 2 and Stage 3, the piston moves back into the cylinder, the gas volume decreases, and the pressure increases because work is done on the gas by the piston.

Stage 3 Stage 3 is the beginning of the combustion of the fuel/air mixture. The combustion occurs very quickly and the volume remains constant. Heat is released during combustion which increases both the temperature and the pressure, according to the equation of state.

Power Cycle Position 4  5 Copyright 2000, Matt Keveney. All rights reserved. Stage 4 begins the power stroke of the engine. Between Stage 4 and Stage 5, the piston is driven towards the crankshaft, the volume in increased, and the pressure falls as work is done by the gas on the piston.

Stage 5 At Stage 5 the exhaust valve is opened and the residual heat in the gas is exchanged with the surroundings. The volume remains constant and the pressure adjusts back to atmospheric conditions.

Exhaust Cycle Position 6  1 Copyright 2000, Matt Keveney. All rights reserved. Stage 6 begins the exhaust stroke of the engine during which the piston moves back into the cylinder, the volume decreases and the pressure remains constant. At the end of the exhaust stroke, conditions have returned to Stage 1 and the process repeats itself.

Practice: Work with your group to complete the conceptual questions. One paper per group will be collected Each person should use a different color writing utensil Each person should have written roughly the same amount of work