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### question on U=q+w equation

Posted: **Sun Jan 29, 2017 5:39 pm**

by **Julianne Seog 3K**

in the course reader, it says that if the change in pressure is 0( meaning constant pressure),then the change in internal energy is equal to q+w. How would you calculate internal energy if the pressure wasnt constant while the system is getting compressed?

### Re: question on U=q+w equation

Posted: **Sun Jan 29, 2017 6:08 pm**

by **Audrey Magsig 1E**

If the pressure isn't constant, the reaction is most likely reversible. Therefore, the internal energy of the system would be 0 as all energy would be conserved.

### Re: question on U=q+w equation

Posted: **Fri Feb 03, 2017 2:45 pm**

by **Janice Kim 3I**

If the pressure is not constant, you are calculating the work for a reversible pathway. If you are in isothermal conditions (temp is constant) and you have a system that is being compressed with changing volume, the work expression would be -nrtln(V2/V1), where V2 is the final volume and V1 is the initial value. Use this value of w to use in the deltaU = q+w expression to calculate the internal energy.