- #1

EastWindBreaks

- 128

- 3

## Homework Statement

## Homework Equations

unreliable source of solution:

## The Attempt at a Solution

[/B]

I came across this problem while practicing for the final exam in thermodynamic,my attempt: i recall this type of problems are not steady-flow system. Since the change in kinetic and potential energy is neglected, and the system does not produce work, therefore, the energy balance equation should becomes:

dE/dt= Q_dot+ m_dot(h_inlet)

h=u+Pv

since the question asks for Q instead of Q_dot: ( m_dot can not be found from given information alone either)

ΔE=ΔU=m(U2-U1)

therefore, m(U2-U1) = Q+ m* (H_inlet)

at this point, I see I can find final mass from ideal gas equation PV=mRT,

i can also find H and U2 from the given temperature and pressure from ideal gas property table of air. (T1=T2, P1=P2)

and U1 of the evacuated bottle is just 0, Q can be finally solved.

what I do not understand is that the solution assuming it's a steady-flow system, even if it is, dE/dt should be 0, so i guess they made a typo there. also, steady or not, it does not seem to alter the solution because dE/dt is 0 under steady-flow, but ΔE is not 0, correct?

#### Attachments

Last edited: