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An isothermal process is when there is a change in the system but the temperature remains constant: ΔT = 0. This occurs when a system is in contact with an outside thermal reservoir and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange.  Although ΔT = 0, Q ≠ 0 in an isothermal process.

For an ideal gas that is operating at an isothermal process

p = constant/V

To calculate work for an isothermal process

W = nRTln(Vb/VA)

As stated by the first law of thermodynamics

ΔU = Q + W

Therefore, for an isothermal process

–Q = W

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