Conservation of mass - equation understanding

  • #1
Ketler
3
0
Homework Statement
Understanding conservation of mass equation
Relevant Equations
$$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt}$$
Hello All,

I have a problem to understand this equation:

$$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt} $$

It supposed to describe change in the mass of the control volume during a process.

Two terms on the left are the total mass flow rates in and out of the system. I struggle to understand RHS.

What $$ \frac{dm_{CV}}{dt} $$ means and why it is not equal to $$\dot{dm_{CV}}$$?

Many thanks for all your help.

Lukas
 
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  • #2
The rhs is the rate of change of mass within the control volume. It's like a bank account. (Rate of money in) minus (rate of money out) equal (rate of accumulation of money within the account).
 
  • #3
Thanks for your answer. So if it is a rate of change, why is it not written as:
$$ \dot{dm_{CV}} $$

My assumption is, LHS can be also written as:
$$ \frac{{dm_{in}}}{dt} - \frac{{dm_{out}}}{dt} = ... $$
 
  • #4
Ketler said:
Homework Statement: Understanding conservation of mass equation
Relevant Equations: $$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt}$$

Hello All,

I have a problem to understand this equation:

$$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt} $$

It supposed to describe change in the mass of the control volume during a process.

Two terms on the left are the total mass flow rates in and out of the system. I struggle to understand RHS.

What $$ \frac{dm_{CV}}{dt} $$ means and why it is not equal to $$\dot{dm_{CV}}$$?

Many thanks for all your help.

Lukas
Can you define your variables, please?
 
  • #5
Ketler said:
Thanks for your answer. So if it is a rate of change, why is it not written as:
$$ \dot{dm_{CV}} $$

My assumption is, LHS can be also written as:
$$ \frac{{dm_{in}}}{dt} - \frac{{dm_{out}}}{dt} = ... $$
No. It's a notational thing. The over-dot does not mean a time derivative. ##\dot{m}_{in}## the rate of flow in: $$\dot{m}_{in}=\rho_{in}v_{in}A_{in}$$where ##\rho_{in}## is the density of the inlet stream, ##v_{in}## is the velocity of the inlet stream (at the inlet to the control volume), and ##A_{in}## is the cross sectional area of the inlet flow conduit.
 
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Likes Ketler
  • #6
Chestermiller said:
No. It's a notational thing. The over-dot does not mean a time derivative. ##\dot{m}_{in}## the rate of flow in: $$\dot{m}_{in}=\rho_{in}v_{in}A_{in}$$where ##\rho_{in}## is the density of the inlet stream, ##v_{in}## is the velocity of the inlet stream (at the inlet to the control volume), and ##A_{in}## is the cross sectional area of the inlet flow conduit.
what does CV mean here?
 
  • #7
pines-demon said:
what does CV mean here?
Control volume
 

FAQ: Conservation of mass - equation understanding

What is the law of conservation of mass?

The law of conservation of mass states that in a closed system, the mass of the system must remain constant over time, regardless of the processes acting inside the system. This means that mass can neither be created nor destroyed, only transformed from one form to another.

How do you apply the conservation of mass in chemical equations?

To apply the conservation of mass in chemical equations, you must ensure that the number of atoms of each element is the same on both the reactant and product sides of the equation. This often involves balancing the equation by adjusting the coefficients in front of the chemical formulas to achieve equal amounts of each element.

What are reactants and products in a chemical equation?

In a chemical equation, reactants are the substances that undergo a chemical change, found on the left side of the equation. Products are the new substances formed as a result of the reaction, located on the right side of the equation. The conservation of mass requires that the total mass of the reactants equals the total mass of the products.

Why is balancing chemical equations important in relation to conservation of mass?

Balancing chemical equations is crucial because it reflects the principle of conservation of mass. A balanced equation ensures that the same number of atoms of each element is present before and after the reaction, demonstrating that mass is conserved. Without balancing, the equation would inaccurately represent the chemical reaction and violate the law of conservation of mass.

Can you give an example of a balanced chemical equation that illustrates conservation of mass?

An example of a balanced chemical equation is the combustion of methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O. In this equation, there is one carbon atom, four hydrogen atoms, and four oxygen atoms on both sides, illustrating that the total mass of the reactants (methane and oxygen) equals the total mass of the products (carbon dioxide and water), thus adhering to the conservation of mass.

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