### Internal Energy

Posted:

**Sat Mar 14, 2020 6:21 pm**Can anyone please explain to me why when it is an isothermal reversible expansion, we can assume that internal energy is 0 and q = -w? Thank you

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=130&t=62526

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Posted: **Sat Mar 14, 2020 6:21 pm**

Can anyone please explain to me why when it is an isothermal reversible expansion, we can assume that internal energy is 0 and q = -w? Thank you

Posted: **Sat Mar 14, 2020 6:27 pm**

The internal energy is 0 because isothermal means the temperature is constant

Posted: **Sat Mar 14, 2020 6:31 pm**

If the temperature is constant, there is no heat change and thus the change in internal energy is zero

Posted: **Sat Mar 14, 2020 6:32 pm**

The internal energy of an ideal gas is proportional to its mass and temperature (deltaU = 1.5*n*R*deltaT). Both mass and temperature are constant in an isothermal expansion, so the internal energy is constant (deltaU = 0).

Internal energy, heat, and work are related by deltaU = q + w. deltaU = 0, so q = -w

Internal energy, heat, and work are related by deltaU = q + w. deltaU = 0, so q = -w

Posted: **Sat Mar 14, 2020 6:33 pm**

isothermal means that temperature is constant therefore internal energy would be 0

Posted: **Sat Mar 14, 2020 6:36 pm**

Michael Du 1E wrote:Can anyone please explain to me why when it is an isothermal reversible expansion, we can assume that internal energy is 0 and q = -w? Thank you

Isothermal means the temperature is constant and so the internal energy doesn't change so the change is 0. If change in internal energy is 0 then 0 = q + w and so q = -w

Posted: **Sat Mar 14, 2020 8:18 pm**

Temperature and internal energy are directly proportional. In a sense internal energy is a measure of temperature. If temperature is not changing, then internal energy is 0. If you are expanding then you must be inputting heat. If you are compressing then you must be releasing heat.