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Ch. 17 - Temperature, Thermal Expansion, and the Ideal Gas Law
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Non è quello che usi tu?Cambia libro di testo
Capitolo 17, Problema 21

If a fluid is contained in a long narrow vessel so it can expand in essentially one direction only, show that the effective coefficient of linear expansion α is approximately equal to the coefficient of volume expansion β.

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Start by recalling the definitions of the coefficients of linear expansion (α) and volume expansion (β). The coefficient of linear expansion (α) describes how the length of a material changes with temperature, while the coefficient of volume expansion (β) describes how the volume of a material changes with temperature.
For a long narrow vessel, assume the fluid expands primarily in one direction (let's call it the x-direction). The volume of the fluid can be expressed as \( V = A \cdot L \), where \( A \) is the cross-sectional area and \( L \) is the length of the vessel.
When the temperature changes, the volume expansion can be expressed as \( \Delta V = \beta V \Delta T \), where \( \beta \) is the coefficient of volume expansion and \( \Delta T \) is the temperature change. Substituting \( V = A \cdot L \), we get \( \Delta V = \beta (A \cdot L) \Delta T \).
Now consider the linear expansion of the vessel. The length \( L \) changes according to \( \Delta L = \alpha L \Delta T \), where \( \alpha \) is the coefficient of linear expansion. Since the fluid expands primarily in one direction, the change in volume is approximately due to the change in length, so \( \Delta V \approx A \cdot \Delta L \).
Substitute \( \Delta L = \alpha L \Delta T \) into \( \Delta V \approx A \cdot \Delta L \), giving \( \Delta V \approx A \cdot (\alpha L \Delta T) \). Comparing this with \( \Delta V = \beta (A \cdot L) \Delta T \), we see that \( \alpha \approx \beta \), as the cross-sectional area \( A \) and length \( L \) cancel out.

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Coefficient of Linear Expansion (α)

The coefficient of linear expansion (α) quantifies how much a material expands per unit length for each degree of temperature increase. It is defined as the change in length divided by the original length and the change in temperature. This concept is crucial for understanding how solids respond to temperature changes, particularly in one-dimensional expansion scenarios.
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Linear Thermal Expansion

Coefficient of Volume Expansion (β)

The coefficient of volume expansion (β) measures how much a material's volume changes per unit volume for each degree of temperature increase. It is defined as the change in volume divided by the original volume and the change in temperature. This concept is essential for analyzing how fluids behave under temperature variations, especially in confined spaces.
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Volume Thermal Expansion

Relationship Between Linear and Volume Expansion

In three-dimensional objects, the relationship between linear expansion and volume expansion is given by the equation β = 3α for isotropic materials. This means that if a material expands uniformly in all directions, its volume change is three times the linear change. In the context of a fluid in a narrow vessel, the effective linear expansion can be approximated to equal the volume expansion due to the constraints of the vessel allowing expansion primarily in one direction.
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Volume Thermal Expansion
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