Question

Thermodynamics doubt:

Answer 1

Is \(\large \Delta U=n C_v \Delta T\) true for all thermodynamic processes? ie adiabatic,isothermal,isobaric,isochoric etc

Answer 2

@Vincent-Lyon.Fr

Answer 3

Yes it is.. that is ALWAYS true..

but \[\Delta Q\] is different for different processes..

for example
adiabatic it is zero
isotherm it is equal to work done
isochoric it is the same as delta U
isobaric it is delta U + some work done

Answer 4

It is true for all processes IF THE SYSTEM IS AN IDEAL GAS or if the system follows Joule's first law ONLY.

@Mashy:
I am surprised by : "isotherm it is equal to work done"
Do you use different sign conventions for Q and W ? I mean do you mean as W work done by the gas on the surroundings?

In Europe, we would have written: "isotherm it is OPPOSITE to work done on the system"
First law is written : \(\Delta U=W+Q\)

Answer 5

I guess so..

we use work done BY the system = +ve

so if i heat up a gas isothermally, it would expand
since heat is transferred TO the system Q would be positive
and since the gas is expanding, gas is doing work and hence word done = +ve :D

Answer 6

and quite frankly speaking i love this convention.. because, system (the gas) is one which we are concentrating on, so, if system does positive work, we consider work as positive makes a lot of sense to me :P

Answer 7

It is a question of familiarity: I prefer "everything that enters is +ve everything that goes out is -ve". But once you were taught with a convention, it is written in your genes ;-)

I guess this convention makes it easier for the students to integrate pdV, because we have to integrate -pdV, since dV is change in volume of the system.

Only because OS is international, it makes it more difficult to cope with such discrepancies, and probably most people are not even aware of them.

Answer 8

True.. its good you pointed this out.. if AravindG uses your convention, he would be more confused than he was before he asked the question xD..

and yes. the integral physically makes more sense xD

Answer 9

Thank you both. Actually in my classes the convention used is work done by gas +ve in Physics and work done on the gas as positive in Chemistry.

Answer 10

Lol, total schizophrenia!

Answer 11

@AravindG are you sure?
in CBSE syllabus i think even chemistry adopts the same convention!?

Answer 12

Yes I am sure.