What is the difference between a sphere and a ball?

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SUMMARY

The discussion clarifies the distinction between a "sphere" and a "ball" in the context of Schwabl Thermodynamics. It highlights that the equation for the surface area of a unit d-sphere, $$ \int d\Omega_d = \frac{2 \pi^{d/2}}{\Gamma(d/2)} $$, applies to a d-1 dimensional sphere, not a d-dimensional sphere. For example, a "2-ball" represents a two-dimensional disk defined by the equation x² + y² ≤ r², while a "2-sphere" is the surface of a "3-ball" defined by x² + y² + z² = r². This distinction is crucial for accurate mathematical representation in thermodynamics.

PREREQUISITES
  • Understanding of dimensional geometry
  • Familiarity with the concept of spheres and balls in mathematics
  • Basic knowledge of thermodynamics as presented in Schwabl's texts
  • Proficiency in calculus, particularly integration and surface area calculations
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  • Study the properties of N-spheres and their applications in physics
  • Explore the mathematical derivation of surface area formulas for different dimensions
  • Learn about the implications of dimensionality in thermodynamic equations
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onanox
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I'm trying to follow Schwabl Thermodynamics, and I found the following equation for the surface area of a unit d-sphere:
$$ \int d\Omega_d = \frac{2 \pi^{d/2}}{\Gamma(d/2)} $$

But this formula clearly fails for d=1:
should be $$\pi$$
and d=2:
should be $$ 4 \pi $$. What gives?
 
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yea, you're right. Schwabl must be counting the dimension its embedded in or something.
 
It is the difference between a "sphere" and a "ball". A "2-ball" is the a two dimensional disk, which might have equation x^2+ y^2\le r^2, while the "2-sphere" is the surface of a "3-ball" and might have equation x^2+ y^2+ z^2= r^2
 

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