10.2 Processes, The first law of thermodynamics

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

Energy change at constant pressure

The First Law of Thermodynamics

State the first law of thermodynamics.
FIRST LAW OF THERMODYNAMICS: The heat supplied to a mass of gas is equal to the increase in its internal energy plus the work done by the gas (expansion is positive work).
First law of thermodynamics

Students should be familiar with the terms system and surroundings. They should also appreciate that if a system and its surroundings are at different temperatures and the system undergoes a process, the energy transferred by non-mechanical means to or from the system is referred to as thermal energy (heat).
Identify the first law of thermodynamics as a statement of the principle of energy conservation.
PRINCIPLE OF THE CONSERVATION OF ENERGY: Energy may be transformed from one form to another, but it cannot be created or destroyed ie the total energy of a system and its surroundings is constant.

Describe the isochoric (isovolumetric), isobaric, isothermal and adiabatic changes of state of an ideal gas.
ISOCHORIC CHANGE: A change in a gas where the volume remains constant.

change at constant volume

Work done is zero.

ISOBARIC CHANGE: A change in a gas where the pressure remains constant.
change at constant pressure

Some energy raises the temperature and some does work to expand the gas.

ISOTHERMAL CHANGE: A change in a gas where the temperature remains constant.
without change in temperature

ADIABATIC CHANGE: A change in a gas where no energy enters or leaves the system.

no heat enters or leaves the gas

In each process, the energy transferred, the work done and the internal energy change should be addressed. Students should realize that a rapid compression or expansion of a gas is approximately adiabatic.
APPLET OF A CARNOT CYCLE - a piston performing isothermal and adiabatic changes

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.