• Support PF! Buy your school textbooks, materials and every day products Here!

Thermodynamics piston-cylinder closed system

  • #1

Homework Statement


An insulated piston-cylinder device contains 5 L of saturated liquid water at a constant pressure of 175 kPa. Water is stirred by a paddle wheel while a current of 8 A flows for 45 min through a resistor placed in the water, If one-half of the liquid is evaporated during this constant pressure process and the paddle -wheel work amounts to 400 kJ, determine the voltage of the source. Also, show the process on a P-v diagram with respect to saturation lines.

ElBL8rY.jpg


Homework Equations


ΔE = Ein - Eout
Welectric = VIΔt
if ΔP = 0, ΔH = m(h2 - h1)
ΔH = ΔU + Wboundary


The Attempt at a Solution


V = 5 L
m = 5 kg (by conversion)
P = 175 kPa
I = 8 A
Δt = 45*60 sec = 2700

Assuming that KE, and PE = 0,
ΔE = ΔU

Also since it is insulated we are assuming that the heat lost is 0

We + Wsh - Wboundary = ΔU
We + Wsh = ΔU + Wboundary
We + Wsh = ΔH
We + Wsh = m(h2 - h1)

Here is where I get lost. I know that we can find h1 from the saturated water tables
h1 = 487.01 kj/kg (at Psat = 175 kPa)
I'm confused as to where the second enthalpy value comes from since our state has constant P.

Also the P-v should look something like this I believe:

Bg1l7tO.jpg
 

Attachments

Answers and Replies

  • #2
20,205
4,248

Homework Statement


An insulated piston-cylinder device contains 5 L of saturated liquid water at a constant pressure of 175 kPa. Water is stirred by a paddle wheel while a current of 8 A flows for 45 min through a resistor placed in the water, If one-half of the liquid is evaporated during this constant pressure process and the paddle -wheel work amounts to 400 kJ, determine the voltage of the source. Also, show the process on a P-v diagram with respect to saturation lines.

ElBL8rY.jpg


Homework Equations


ΔE = Ein - Eout
Welectric = VIΔt
if ΔP = 0, ΔH = m(h2 - h1)
ΔH = ΔU + Wboundary


The Attempt at a Solution


V = 5 L
m = 5 kg (by conversion)
P = 175 kPa
I = 8 A
Δt = 45*60 sec = 2700

Assuming that KE, and PE = 0,
ΔE = ΔU

Also since it is insulated we are assuming that the heat lost is 0

We + Wsh - Wboundary = ΔU
We + Wsh = ΔU + Wboundary
We + Wsh = ΔH
We + Wsh = m(h2 - h1)

Here is where I get lost. I know that we can find h1 from the saturated water tables
h1 = 487.01 kj/kg (at Psat = 175 kPa)
I'm confused as to where the second enthalpy value comes from since our state has constant P.

Also the P-v should look something like this I believe:

Bg1l7tO.jpg
If you are using the saturation tables, you can get ##h_2## using the lever rule, or equivalently, as ##h_2=h_1+\Delta h_{vap}/2##.

Regarding the P-V diagram, your result is correct.
 

Related Threads on Thermodynamics piston-cylinder closed system

Replies
4
Views
4K
  • Last Post
Replies
8
Views
4K
Replies
0
Views
6K
Replies
8
Views
5K
  • Last Post
Replies
1
Views
2K
Replies
12
Views
10K
Replies
3
Views
3K
Replies
0
Views
9K
  • Last Post
Replies
3
Views
1K
Replies
1
Views
3K
Top