# Conservation of mass relation for a closed system

• mech-eng
In summary, the electrical work being done at the boundary of a closed system is related to the presence of a resistor or fan.
mech-eng

Source: Çengel/Cimbala.

Above part is confusing for me from some points. First of it what can a closed system refer to? I just understand a piston/cylinder arrangement with an ideal gas in it. But this concept must be more comprehensive than I have it in my mind.

Second, does "closed system undergoing a change" mean that only it's temperature, pressure, specific volume or density is changing?

Thank you.

mech-eng said:
First of it what can a closed system refer to?
Something that cannot exchange matter with the environment (if this would be physics, we would also have to forbid any exchange of energy).
mech-eng said:
Second, does "closed system undergoing a change" mean that only it's temperature, pressure, specific volume or density is changing?
It does not matter what changes, the mass stays constant in every case.

mech-eng said:
View attachment 197574
Second, does "closed system undergoing a change" mean that only it's temperature, pressure, specific volume or density is changing?
Its average temperature and pressure are changing, although there may be variations of temperature and pressure with spatial location within the system. The average specific volume of a closed system is, of course, constant, but the specific volume may be varying with spatial position within the system. The term "closed system undergoing a change" means that work is being done on the system at its boundary and/or heat is being exchanged with the surroundings at its boundary.

Chestermiller said:
Its average temperature and pressure are changing, although there may be variations of temperature and pressure with spatial location within the system. The average specific volume of a closed system is, of course, constant, but the specific volume may be varying with spatial position within the system. The term "closed system undergoing a change" means that work is being done on the system at its boundary and/or heat is being exchanged with the surroundings at its boundary.

Cannot it be the work not entering or going out its bounderies but be an electrical work inside it?

Thank you.

mech-eng said:
Cannot it be the work not entering or going out its bounderies but be an electrical work inside it?

Thank you.
Can you please provide a more detailed example of this?

Chestermiller said:
Can you please provide a more detailed example of this?

I know the term electrical work but I do not know what exactly it is. I think the situation might be a wire having an electrical current inside the closed system.

What do you think?

Thank you.

mech-eng said:
I know the term electrical work but I do not know what exactly it is. I think the situation might be a wire having an electrical current inside the closed system.

What do you think?

Thank you.
Are you envisioning an electrical resistor inside the system connected by wires to the outside, with electrical current flowing through the wires so that the resistor is giving off heat?

Chestermiller said:
Are you envisioning an electrical resistor inside the system connected by wires to the outside, with electrical current flowing through the wires so that the resistor is giving off heat?

Yes. And I think this would be the only case.

Thank you.

mech-eng said:
Yes. And I think this would be the only case.

Thank you.
You could also have electrical wires running to a fan. Would you include that too?

mech-eng
Chestermiller said:
You could also have electrical wires running to a fan. Would you include that too?

Yes, this is also the form of work not related to boundaries but I didn't remember it.

Thank you.

mech-eng said:
Yes, this is also the form of work not related to boundaries but I didn't remember it.

Thank you.
Who says it's not related to the boundaries? If you include the resistor or the fan as part of your system, then the electrical wires are passing through the exterior boundary of the system, with one wire carrying current into the system at a higher voltage, and the other wire carrying current out of the system at a lower voltage. So the electrical work is being done at the boundary of your system.

If you do not include the fan as part of your system, then the rotating blades of the fan constitute an interior boundary at which work is being done by the blades exerting forces and displacements on the material in your system. So here again, the work is taking place at the boundary.

If you do not include the resistor as part of your system, then the surface of the resistor constitutes an interior boundary at which heat is being transferred to the material in your system. So, in this case, it is heat that is being exchanged at the boundary.

Chestermiller said:
If you include the resistor or the fan as part of your system, then the electrical wires are passing through the exterior boundary of the system, with one wire carrying current into the system at a higher voltage, and the other wire carrying current out of the system at a lower voltage. So the electrical work is being done at the boundary of your system

I am not very familiar with topics of electricity. Aren't there only one wire and it is the wire having current through, glowing and giving heat and there is only one voltage instead of lower and higher?

Thank you.

mech-eng said:
I am not very familiar with topics of electricity. Aren't there only one wire and it is the wire having current through, glowing and giving heat and there is only one voltage instead of lower and higher?

Thank you.
Like you said, you are not very familiar with electricity.

## What is conservation of mass for a closed system?

The conservation of mass for a closed system is a fundamental principle in physics and chemistry that states that the total mass of a closed system remains constant over time. This means that mass cannot be created or destroyed, only transformed or transferred within the system.

## How does the conservation of mass apply to chemical reactions?

In chemical reactions, the total mass of the reactants is equal to the total mass of the products. This is known as the law of conservation of mass, which is a direct consequence of the conservation of mass for a closed system.

## What is the difference between an open and a closed system?

An open system is one that exchanges both matter and energy with its surroundings, while a closed system only exchanges energy but not matter. This means that in a closed system, the total mass remains constant, while in an open system, the mass can change due to matter entering or leaving the system.

## Why is the conservation of mass important in environmental science?

The conservation of mass is important in environmental science because it helps us understand and predict the impact of human activities on the environment. It allows us to track the movement of pollutants and understand how they can affect different ecosystems and the overall balance of the Earth's systems.

## Is the conservation of mass always true?

The conservation of mass is a well-established principle in physics, and it has been confirmed through countless experiments and observations. However, in certain extreme cases, such as nuclear reactions, mass can be converted into energy, violating the conservation of mass. But in everyday situations, the conservation of mass is always true and can be used to make accurate predictions and calculations.

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