BE2001 Mass and Energy Balance for Biosystem Module 3 Segment 2

Summary

This segment delves into mass and energy balance in biosystems, focusing on the counting of extensive properties such as total mass, energy, charge, and momentum. The lecture covers the steps involved in setting up accounting or conservation statements, including specifying the property to track, defining the system and its boundary, and selecting the appropriate time period. The discussion emphasizes the importance of understanding system boundaries and the conservation of certain properties in most systems, except nuclear reactions. The next segment will cover the conceptual framework for conservation equations.

Takeaways

• 😀 Extensive properties, such as mass, energy, and momentum, depend on the size of the system and are fundamental to conservation laws in biosystem engineering.
• 😀 The conservation of mass and energy is a key principle, and only certain extensive properties are always conserved (e.g., total mass, total energy, linear momentum, and angular momentum).
• 😀 Accounting for extensive properties involves counting the amount of a specific property in a system over a given time period.
• 😀 Systems are defined by boundaries that separate the system from its surroundings. These boundaries can be either real or arbitrarily defined by the problem solver.
• 😀 A system's boundary could enclose an entire object or capture relevant elements, such as the movement of a property of interest across the boundary.
• 😀 To set up an accounting or conservation statement, three steps are needed: specify the extensive property, define the system and its boundary, and specify the time period.
• 😀 The time period for accounting can be either discrete (with a clear start and end) or continuous (operating without a defined beginning or end).
• 😀 Accounting statements capture the mathematical process used to track extensive properties across a system over time.
• 😀 Only extensive properties (e.g., mass, energy, momentum) can be counted in an accounting statement, while intensive properties (e.g., temperature, pressure) cannot be tracked in this way.
• 😀 When specifying the system and its boundary, it's essential to ensure that the system's scope aligns with the specific properties being tracked and investigated.
• 😀 Understanding how to count and conserve extensive properties is crucial for solving engineering problems and applying fundamental laws like conservation of mass and energy.

Q & A

What are extensive properties, and why are they important in engineering systems?

Extensive properties are quantities that depend on the size or amount of matter in a system, such as mass, energy, and momentum. They are important because they can be counted and tracked using accounting and conservation equations, forming the basis of mass and energy balance in engineering systems.

How do extensive properties differ from intensive properties?

Extensive properties depend on the size of the system and can be added when systems are combined, while intensive properties do not depend on system size and cannot be counted using accounting systems, such as temperature and pressure.

What are some examples of extensive properties mentioned in the script?

Examples include total mass, mass of individual species, mass of individual elements, total moles, moles of species and elements, total energy (thermal, mechanical, electrical), net electrical charge, linear momentum, and angular momentum.

Which extensive properties are always conserved except in nuclear reactions?

The conserved extensive properties are total mass, mass of individual elements, moles of individual elements, total energy, net electrical charge, linear momentum, and angular momentum.

What is the purpose of an accounting or conservation statement?

An accounting or conservation statement mathematically tracks how an extensive property changes within a system over time by considering inputs, outputs, and accumulation within the system.

What are the three main steps in setting up an accounting or conservation statement?

The three steps are: specifying the extensive property of interest, defining the system and its boundary, and specifying the time period over which the accounting is performed.

Why is it important to define the system and its boundary clearly?

Defining the system and its boundary is important because it determines what is included in the analysis, affects assumptions, and controls how mass, energy, or other properties cross the boundary.

What are the two main types of system boundaries described in the script?

The two types are real boundaries, which are tangible and naturally exist, and arbitrarily defined boundaries, which are set by the problem solver to simplify or focus the analysis.

What is meant by specifying a time period in an accounting statement?

Specifying a time period means defining the duration over which the property is tracked, ensuring that all terms in the accounting equation are evaluated over the same time interval.

What is the difference between discrete and continuous time periods?

A discrete time period has a clear beginning and end, while a continuous time period has no defined start or end, with the system operating continuously.

Why must all terms in an accounting equation use the same units?

Using the same units ensures consistency, accuracy, and meaningful comparison of all terms in the accounting or conservation equation.

How does counting extensive properties help in solving engineering problems?

Counting extensive properties allows engineers to track material and energy flows, apply conservation laws, and analyze system performance, which is essential for designing and optimizing engineering systems.