Thermodynamics

Topic 10Sections 2 and 3

Statement

Number

Assessment Statement

10.2.1 Deduce an expression for the work involved in a volume change of a gas at constant pressure

10.2.2 State the first law of thermodynamics

10.2.3 Identify the first law of thermodynamics as a statement of the principle of energy conservation

10.2.4 Describe the isochoric (isovolumetric), isobaric, isothermal and adiabatic changes of state of an ideal gas

10.2.5 Draw and annotate thermodynamic processes and cycles on P-V diagrams

10.2.6 Calculate from a P-V diagram the work done in a thermodynamic cycle

10.2.7 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:

The sum of the total kinetic energy of the molecules in a sample of a gas and the potential energy associated with the intermolecular forces with that gas.

Ideal Gases: assume that the intermolecular forces are non-existent, so potential energy = 0

Therefore the internal energy is solely related to the kinetic energy (which is random…each molecule is likely different)

Average Kinetic Energy:

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·ΔV

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)