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9.2. Real Work
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Today, we're going to explore the concept of Real Work in energy methods. Can anyone recall the first law of thermodynamics?
Isn't it that energy cannot be created or destroyed, only converted?
Great! Yes. The first law relates to how the total energy changes as we apply work or heat. Can anyone explain what happens if we have an adiabatic system?
There’s no heat exchange involved, right?
Exactly! That leads to the key equation: External work equals internal strain energy. Can someone express that mathematically?
I think it’s W = U.
Correct! Always remember: W equals U is foundational in understanding these principles.
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Now, let’s examine internal work further. When we look at an infinitesimal structural element, how might we express internal strain energy density, given uniaxial stress?
Could we use the formula for strain energy density?
Exactly! It can be defined as dU = 1/2 * dx. How do we then find the total strain energy?
We integrate over the volume!
Right! So, the total might look like U = * σ * dVol. It's important to visualize these concepts with respect to different types of members.
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Let’s break down the strain energy for axial members. If the formula is U = L * P^2 / (2AE), what do each of these terms represent?
L is the length of the member, P is the force, A is the cross-section area, and E is the modulus of elasticity!
Excellent! And for torsional members, what about the energy formulation?
For torsional members, it involves the modulus of rigidity G and the polar moment of inertia J!
Exactly! Remembering these formulas will aid you greatly in your analyses.
Overview
Short Summary
The Real Work method illustrates the relationship between external work and internal strain energy in structural systems.
Medium Summary
This section delves into the Real Work energy method, rooted in the first law of thermodynamics, highlighting how external work equals internal strain energy in an adiabatic system without heat transfer. The discussion includes mathematical derivations for determining strain energy in various structural members under uniaxial stress.
Detailed Summary
Detailed Summary
The Real Work method is an essential framework in understanding energy methods in structural analysis, particularly in deriving the internal strain energy that structural elements undergo when subjected to external loads.
Key Concepts
-
First Law of Thermodynamics: The text begins by referencing the first law, stating that the time-rate change in total energy equals the sum of the external work done and the heat change. This relationship simplifies under adiabatic conditions (no heat exchange) and quasi-static loading where kinetic energy is negligible. Hence, the formulation simplifies to:
(where W is work done, and U is internal strain energy).
-
Internal Work: The section then shifts to consider the internal work done by a structural element experiencing uniaxial stress. Here, the net force, displacement, and strain energy density are discussed, leading to the formulation of total strain energy:
which can be elaborated into specific forms for various members:
- Axial Members:
- Torsional Members:
These formulations allow for the assessment of the energy associated with deformations in different structural contexts.
Reference YouTube Videos
Audio Book
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Create a free accountWe start by revisiting the first law of thermodynamics:
The time-rate of change of the total energy (i.e., sum of the kinetic energy and the internal energy) is equal to the sum of the rate of work done by the external forces and the change of heat content per unit time.
d(K + U) = W + H (9.1)
dt e where K is the kinetic energy, U the internal strain energy, W the external work, and H the heat input to the system.
Detailed Explanation
The first law of thermodynamics explains how energy is conserved in mechanical systems. It states that the change in total energy of a system is equal to the work done on the system plus any heat added to it. Here, total energy includes both kinetic energy (energy of motion) and internal energy (energy stored within the material). The equation d(K + U) = W + H captures this relationship by showing how energy enters or exits the system through work (W) and heat (H).
Examples & Analogies
Imagine a car engine. When you step on the gas, fuel combustion generates energy (heat), which does work moving the pistons (creating kinetic energy). If we analyze this system, we can use the first law of thermodynamics to understand how fuel (heat and work done) converts into movement (kinetic energy).
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Create a free accountFor an adiabatic system (no heat exchange) and if loads are applied in a quasi static manner (no kinetic energy), the above relation simplifies to:
W = U (9.2)
Simply stated, the first law stipulates that the external work must be equal to the internal strain energy due to the external load.
Detailed Explanation
In an adiabatic system, there is no heat transfer with the surroundings, simplifying our analysis. Under these conditions, if forces are applied very slowly (quasi-static), the work done on the system (W) is entirely converted into internal energy, specifically strain energy (U) in the material. This means that all the energy put into the system through external work is stored as internal strain energy.
Examples & Analogies
Think of a rubber band. If you slowly stretch the rubber band (quasi-static load) without letting it heat up (adiabatic), all the work you do in stretching it is stored as potential energy in the rubber band. In this scenario, the work you apply equals the stored strain energy.
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Create a free accountConsidering an infinitesimal element from an arbitrary structure subjected to uniaxial state of stress, the strain energy can be determined with reference to Fig. 9.2. The net force acting on the element while deformation is taking place is P = (σ) dydz. The element will undergo a displacement u =
Detailed Explanation
No detailed explanation available.
Examples & Analogies
No real-life example available.
Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- First Law of Thermodynamics:
The text begins by referencing the first law, stating that the time-rate change in total energy equals the sum of the external work done and the heat change. This relationship simplifies under adiabatic conditions (no heat exchange) and quasi-static loading where kinetic energy is negligible. Hence, the formulation simplifies to:
(where W is work done, and U is internal strain energy).
- Internal Work:
The section then shifts to consider the internal work done by a structural element experiencing uniaxial stress. Here, the net force, displacement, and strain energy density are discussed, leading to the formulation of total strain energy:
which can be elaborated into specific forms for various members:
- Axial Members:
- Torsional Members:
These formulations allow for the assessment of the energy associated with deformations in different structural contexts.
Examples
Memory aids
Imagine a bridge taking a load; as forces act, it stores energy, awaiting a time when it can unload.
Flash Cards
Glossary
Real Work
The energy method ensuring the external work done on a system equals its internal strain energy.
Strain Energy
The energy stored in a material due to deformation.
First Law of Thermodynamics
States that energy cannot be created or destroyed, only transformed.
Adiabatic System
A system where no heat exchange occurs with the environment.
Uniaxial Stress
Stress applied in one direction along a structural element.