Topic 10 Sections 2 and 3

 Statement Number Assessment Statement Deduce an expression for the work involved in a volume change of a gas at constant pressure State the first law of thermodynamics Identify the first law of thermodynamics as a statement of the principle of energy conservation Describe the isochoric (isovolumetric), isobaric, isothermal and adiabatic changes of state of an ideal gas Draw and annotate thermodynamic processes and cycles on P-V diagrams Calculate from a P-V diagram the work done in a thermodynamic cycle Solve problems involving state changes of a gas 10.2 Processes (The First law of Thermodynamics)

  By definition: The study of the conditions under which thermal energy can be transferred through performing mechanical work  Macroscopic Properties: Pressure, Volume and Temperature—all used to determine the amount of work that is/can be done by or to a sample of gas. Thermodynamics

 Internal Energy

  Internal energy of a fixed quantity of a gas (constant number of moles) will only depend on the temperature.  It does NOT depend on volume or pressure  Free-Expansion: when a gas is allowed to expand in a way that is not constricted—both the volume an pressure change in such a way that the temperature will remain constant (in an ideal gas)  Thus—the internal energy is constant for a given temperature of ideal gas. Internal Energy

  The complete set of objects being considered in a particular scenario/problem  Open System  Mass is free to enter and/or leave the system  Closed System  Mass is not free to enter and/or leave the system. The quantity of the gas will remain constant  Isolated System  No energy in any form can enter or leave the system Systems

  The State of a system is known when particular quantifiable characteristics of the system are known, such as the following:  Pressure  Volume  Temperature  Internal Energy  State Function: a characteristic of the system.  If two gases, originally in different (thermodynamic) states, are brought to the same state, the gases will have the same internal energy—no matter how they got there. State of a System

  Thermal Energy and Work  Doing work, or adding or removing thermal energy  Related to a CHANGE in the state, not in the state itself  A gas does not “contain” thermal energy—it can transfer it when it changes state  A gas does not “contain” work—it has work done to it when compressed, or work done by it when expanded Non-state functions

 Work Done by/to a Gas  Imagine a Piston—cross sectional area A  Change the position of the piston by applying a force to expand or compress the gas  Volume changes W = P· Δ V

 PV diagrams

 PV Diagrams  Total work done by the gas as it expands (or to the gas as it’s compressed) = area under the curve  Closed loop? Total (net) work done to/by the system = enclosed area

  Those processes in which the pressure of the system remains constant while the volume and temperature change  Results in a horizontal line on a PV diagram (Isobar) Isobaric Processes

  Those processes in which the volume remains constant while the pressure and temperature change  Results in a vertical line on the PV diagram (an Isochore)  No work is done during an isochoric process Isochoric Processes

  Those processes in which the temperature remains constant (and, as a result, the internal energy)  The pressure and volume will each change Isothermal Process

  Thermodynamic Processes are any processes that will result in the change of the state of a system  Heating a gas  Compressing the gas (doing work TO the gas)  Expansion of the gas (work done BY the gas)