Irreversible isothermal Process Work External pressure not provided

AI Thread Summary
The discussion centers on the challenges of calculating work in an irreversible isothermal process without provided external pressure. The formula for work, W = -P_ext(ΔV), is highlighted, but the absence of P_ext complicates the calculations. Participants express confusion over how external pressures of 2 bar and 1 bar are derived and question the validity of changing P_ext in irreversible processes. The conversation also touches on the importance of sketching a PV diagram to visualize the isothermal behavior of an ideal gas. Overall, the participants seek clarity on the relationship between external pressure and work in this thermodynamic context.
Aurelius120
Messages
269
Reaction score
24
Homework Statement
An ideal gas is irreversibly isothermally expanded from ##(8bar ,4L)##to##(2bar,16L)##to##(1bar,32L) ##Find heat.
Relevant Equations
NA
Screenshot_20231231_031131_Chrome.jpg

It is clear that the process is isothermal else it is not possible to find heat absorbed.
$$W=-P_{ext}(\Delta V)$$

However ##P_{ext}## is not given. How do I proceed?
I tried taking ##W=-(P_2V_2-P_1V_1+P_3V_3-P_2V_2)=\Delta(PV)## but it is wrong for obvious reasons.
 
Physics news on Phys.org
The one solution I found uses
##W_1=-(2)(16-4)## and ##W_2=-(1)(32-16)##
##W=(W_1+W_2) bar.Litre=-4000J##
And ##Q=4000J##

How external pressure becomes ##2\ bar## and ##1\ bar## is beyond me. It also seems wrong that ##P_{ext}## should change in an irreversible process? That is like the only thing that is good about irreversible calculations.
 
Sketch a PV diagram of the process. Do you know what an isotherm looks like for an ideal gas?
 
Mister T said:
Sketch a PV diagram of the process. Do you know what an isotherm looks like for an ideal gas?
images.jpeg

Something like this correct?
 
Looks like you've found the way!
Aurelius120 said:
How external pressure becomes ##2\ bar## and ##1\ bar## is beyond me.
A slow compression expansion.

Aurelius120 said:
It also seems wrong that ##P_{ext}## should change in an irreversible process?
A dramatic example would be an explosion.
 
Last edited:
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
View full post »

Thread 'Struggle to understand the concept of the question (ice cube in water optics question)'

TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
View full post »

Similar threads

Work done in adiabatic process vs work done in isothermal
Replies
2
Views
2K
Is enthalpy just the sum of internal energy and work against external pressure?
Replies
5
Views
984
Deriving an expression for change in internal energy
Replies
5
Views
1K
Thermodynamics Problem: Reversible and Irreversible Processes
Replies
1
Views
2K
Could someone help me study this cyclic process of an ideal gas?
Replies
35
Views
1K
Closed cycle of an ideal gas
Replies
3
Views
1K
Isothermal, isobaric and adiabatic
Replies
6
Views
2K
Work done in reversible and irreversible processes
Replies
4
Views
3K
Chemistry A few questions on an irreversible adiabatic expansion of an ideal gas
Replies
1
Views
2K
Reversible Isothermal Expansion Steam
Replies
2
Views
3K

Hot Threads

Recent Insights

Back
Top