10.2 Processes, The first law of thermodynamics

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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.

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Energy change at constant pressure

The First Law of Thermodynamics



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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).
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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.

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change at constant volume


Work done is zero.

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


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

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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.