Intermediate Topic 376 of 1102 Thermodynamics & Energetics
Isothermal Process
A change at constant temperature.
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In-depth explanation of Isothermal Process
An isothermal process is one carried out at constant temperature, typically by keeping the system in contact with a large thermal reservoir that absorbs or supplies heat as needed. For an ideal gas the internal energy depends only on temperature, so in an isothermal change delta U equals zero and the heat exchanged equals the negative of the work done. A reversible isothermal expansion of an ideal gas does work equal to minus n R T times the natural logarithm of the ratio of final to initial volume. Isothermal conditions occur in many laboratory experiments and in biological systems held at constant body temperature. They are common reference cases for analyzing gas behavior.
Going deeper: Isothermal Process
Key ideas: For an ideal gas, delta U equals zero and q equals minus w. Reversible work is minus n R T ln(V2/V1), and compressing to a smaller volume needs work input and releases heat. P times V stays constant, which is Boyle's law.
Common mistakes and connections: Common error: applying delta U equals zero to non-ideal gases or to chemical reactions. Isothermal does not mean no heat flow, because heat flows to hold the temperature constant. This process is the standard reference case for gas expansion and for the Carnot cycle's two heat-exchange steps.
Visualization of Isothermal Process
At constant temperature PV stays constant, so pressure falls as volume rises. Internal energy of an ideal gas is unchanged, giving q = -w. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Slow expansion of one mole of ideal gas at 300 K from 1 L to 2 L in a thermostat absorbs about 1729 J of heat.
Frequently asked questions about Isothermal Process
What is Isothermal Process?
A change at constant temperature.
Is Isothermal Process a beginner, intermediate or advanced topic?
Isothermal Process is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Isothermal Process?
Study these earlier topics in Thermodynamics & Energetics first: Coupled Reactions, Reversible Process, Irreversible Process.
What comes after Isothermal Process?
The next topics in Thermodynamics & Energetics are Adiabatic Process and Isobaric Process.
Can I practise Isothermal Process with quizzes?
Yes. The interactive lesson includes a self-check question on Isothermal Process, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Isothermal Process fits in Thermodynamics & Energetics
Isothermal Process is topic 48 of 91 in Thermodynamics & Energetics.