Question

can anyone explain "work done in an adiabatic expansion"?

Answer 1

@Photon336 @sweetburger @freckles @zepdrix @dan815

Answer 2

i have one doubt..why does the temperature do down?

Answer 3

*go

Answer 4

temperature must increase for the gas to expand.. right?

Answer 5

The heat re-mains constant in this process

Answer 6

ok...so?

Answer 7

do u mean to say temp. won't change..?

Answer 8

Are you familiar with internal energy?

Answer 9

yes

Answer 10

@Photon336 ?

Answer 11

@aaronq @sweetburger

Answer 12

but this is not isothermal process...this is adiabatic process where change in heat is zero..

Answer 13

@ShadowLegendX @Luigi0210

Answer 14

@mayankdevnani @Astrophysics

Answer 15

https://www.youtube.com/watch?v=w04Kd3wIrMI

Answer 16

thanks i saw..the videos..but it doesn't say anything abt "sign"...i wanna know why temp. decreases in an adiabatic "expansion". coz temperature must increase for the gas to expand.. right? @mayankdevnani

Answer 17

@ParthKohli @samigupta8

Answer 18

Bcz ∆U=∆W in adiabatic...hence work done is given as -PextdV ....n volume change wud be positive so work is negative....so change in internal energy is -ve

Answer 19

ya..but i wanna know the physical meaning..

Answer 20

not on formula basis..

Answer 21

Okay so lets first establish: Adiabatic means no transfer of energy through the heating process: \(q_{ad}=0\)

Now lets take a look at what happens to an ideal gas it expands from volume \(V_i\) to \(V_f\):
The gas is performing work on the surroundings \(w_{ad}<0\)
This means the change in internal energy is falling, \(\Delta U<0 \), because the lose of energy is done adiabatic \(\Delta U=w_{ad}\)

The internal energy of an ideal gas depends only on the temperature and not the volume. This means the temperature is falling from \(T_i\) to \(T_f\) when the internal energy is falling. For this reason we can write the internal energy as:
\[\Large w_{ad}=\Delta U=C_V \Delta T\]


The fact the temperature is dropping in an adiabatic expansion can also be understood though statistical mechanics (what I consider the physical meaning if people ask):
Adiabatic literally mean the population of the individual energy states is not altered, but rather the states are moving closer towards each other. In order to forefil this, the temperature within the Boltzmann distribution needs to decrease. (as explained in more detail in my tutorial in thermodynamics and the internal energy).


Within in the macroscopic framework of thermodynamics not all concepts can be explains perfectly. I admit that.