How to Calculate Volume Thermal Expansion Coefficient?

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SUMMARY

The volume thermal expansion coefficient (B) of a solid is mathematically proven to equal the sum of its linear expansion coefficients (Ax, Ay, Az) in three dimensions, expressed as B = Ax + Ay + Az. The relationship is derived from the equation B = (dV/V)/dT and the linear expansion equation A = (dL/L)/dT. The discussion emphasizes that for isotropic solids, where expansion is uniform in all directions, B simplifies to B = 3A. Participants clarified the derivation of the volume change (dV) and the simplifications necessary for accurate calculations.

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  • Understanding of thermal expansion concepts
  • Familiarity with calculus, specifically derivatives
  • Knowledge of volume and linear measurements
  • Basic principles of isotropic materials
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  • Learn about isotropic vs. anisotropic materials in thermal expansion
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phrygian
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Homework Statement


Prove that the volume thermal expansion coefficient of a solid is equal to the sum of its linear expansion coefficients in the three directions: B = Ax + Ay + Az


Homework Equations



B = (dV/V)/dT
A = (dL/L)/dT

The Attempt at a Solution



My thought was using dV/V = ((dLx*dLy*dLz)/(Lx*Ly*Lz)) but when you use this it is clear that dL^3/L^3 does not equal 3* dL/L

What am I doing wrong?

Thanks for the help
 
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dV= ( Lx +dLx)( Ly +dLy)( Lz +dLz) - Lx*Ly*Lz
Simplify this and proceed.
 
Can you explain mathematically how you got there?

Also, the problem statement says at the end (So for an isotropic solid, which expands the same in all directions, B = 3A) if that makes the problem simpler.
 
Lx, Ly and Lz are the lengths of the block. When the temperature is raised through 1 degree C, the new lengths will be Lx + Ax, Ly + Ay and Lz+ Az.
New volume will be (Lx + Ax)( Ly + Ay)( Lz+ Az). Original volume is Lx*Ly*Lz.
Sp dV = (Lx + Ax)( Ly + Ay)( Lz+ Az) - Lx*Ly*Lz
Find dV/V. Neglect the terms like Ax*Ay and so on because they are very small quantities.
 
rl.bhat said:
dV= ( Lx +dLx)( Ly +dLy)( Lz +dLz) - Lx*Ly*Lz
Simplify this and proceed.

phrygian said:
Can you explain mathematically how you got there?

dV = (volume after expansion) - (initial volume)​

After expansion, the box dimensions have increased by dLx, dLy, and dLz, from their initial lengths Lx, Ly, and Lz.

EDIT:
rl responded faster than I. :smile:
 

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