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Isothermal Process
This model illustrates the isothermal process in a gas, i.e. the process of quasi-static expansion or compression of an ideal gas that is in contact with a heat reservoir (T = const).

Modify the temperature under which the experiment takes place, and observe the impact on the gas. The state of the gas is presented on the P-V diagram. Isothermal processes in an ideal gas can be represented on the (P,V)-diagram by a set of hyperbolas P~1/V that relate to different temperatures. For one mole of an ideal gas,
PV = const = RT,
where R = 8.31 J/(mole K) is the universal gas constant.
The internal energy of an ideal gas is the function of temperature T only (in accordance with Joule's law), therefore, the first law of thermodynamics takes the form of
Q = W
The heat that is extracted by the gas from the heat reservoir in isothermal expansion is converted into work. In isothermal compression, the work done on the gas by the external bodies is converted into heat that, in turn, is absorbed by the heat reservoir.
The temperature of the reservoir can be modified. Study an isothermal process' dependency graph p(V). The energy diagram displays the quantity of heat Q that is obtained by the gas, the work W that produced by the gas, and the change in its internal energy ΔU.
Note that in a process of isothermal expansion or compression, the internal energy of an ideal gas does not change, and the absorbed heat is entirely transformed into work.

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