I About the equation for work done on gas

1. Nov 6, 2016

curiosity colour

when do we use W=p dV and W= nRT ln Vi/Vf? i want to know because i got 2 different answer when i use them in the same question

2. Nov 6, 2016

Simon Bridge

The work in a process is the area under the P-V diagram.
Draw the PV diagram for the process.

3. Nov 6, 2016

lychette

What was the question you had to solve? Your second equation is a special case of the application of your first equation!!! It relates to isothermal change in P and V

4. Nov 6, 2016

curiosity colour

hmm, my question does involve isothermal process, it ask me to find the work done on gas.
so for isothermal process we have to use the second equation? but why?

5. Nov 6, 2016

lychette

For an isothermal change PxV =constant x T P x V = nRT and the graph of P against V is the Boyles law graph and P = nRT/V
The work involved going from V1 to V2 is given by ∫P dV
so for isothermal change W = ∫nRT/V dV
W= nRT∫dV/V
W= nRT ln(V1/V2)

If in doubt about the mathemeatics you can always draw the curve of P against V and calculate the area between V1 and V2 by 'counting squares' !! (another way to integrate)

6. Nov 6, 2016

Simon Bridge

... then you should not be getting different answers unless there is not just an isothermal process.
Perhaps you have misapplied the first equation.

7. Nov 7, 2016

Staff: Mentor

8. Nov 11, 2016

curiosity colour

i'm sorry for the late reply, was busying on my assignment. thanks for your answer, it have sort thing out for me :)

9. Nov 11, 2016

curiosity colour

i'm sorry for the late reply, was busying on my assignment. i will check on it, thanks for answering me :D