# Thermodynamics piston-cylinder closed system

• Andrew Pierce
In summary: The process will be a vertical line on the P-V diagram, as the pressure is constant. In summary, the problem involves an insulated piston-cylinder device containing 5 L of saturated liquid water at a constant pressure of 175 kPa. A current of 8 A flows through a resistor in the water for 45 minutes, causing half of the liquid to evaporate and resulting in a paddle-wheel work of 400 kJ. Using the saturation tables, the enthalpy at the initial state, h1, can be found to be 487.01 kJ/kg. The enthalpy at the final state, h2, can be found using the lever rule or by adding half of the enthalpy of vaporization to
Andrew Pierce

## 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.

## 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:

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Andrew Pierce said:

## 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.

## 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:

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.

## What is a thermodynamics piston-cylinder closed system?

A thermodynamics piston-cylinder closed system is a type of thermodynamic system that consists of a piston and a cylinder, where the piston is able to move freely within the cylinder and the entire system is insulated and isolated from its surroundings.

## How does a thermodynamics piston-cylinder closed system work?

In a closed system, the mass and energy within the system are constant, meaning that no mass or energy can enter or leave the system. The movement of the piston within the cylinder allows for the volume and pressure of the gas inside the system to change, which in turn affects the temperature and other thermodynamic properties of the system.

## What are the laws of thermodynamics that apply to a piston-cylinder closed system?

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another. The second law of thermodynamics states that the total entropy of a closed system will always increase over time, or remain constant in ideal cases. The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero temperature is zero.

## What is the purpose of a thermodynamics piston-cylinder closed system?

A piston-cylinder closed system is often used as a model for studying various thermodynamic processes and phenomena, such as heat transfer, work, and energy conversion. It is also commonly used in engineering applications, such as in internal combustion engines and refrigeration systems.

## What are some real-life examples of a thermodynamics piston-cylinder closed system?

Some examples of a piston-cylinder closed system in real life include a car engine, where the piston moves within the cylinder to convert thermal energy from burning fuel into mechanical energy to power the car, and a refrigerator, where the piston compresses and expands a refrigerant fluid to remove heat from the inside of the refrigerator.

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