What is the difference between a sphere and a ball?

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Discussion Overview

The discussion revolves around the distinction between a "sphere" and a "ball" in the context of mathematical definitions and dimensionality, particularly in relation to equations for surface areas in thermodynamics as presented in Schwabl's work. The scope includes conceptual clarification and technical reasoning.

Discussion Character

  • Conceptual clarification, Technical explanation

Main Points Raised

  • One participant references an equation for the surface area of a unit d-sphere and questions its validity for specific dimensions, suggesting a potential misunderstanding of the formula's application.
  • Another participant corrects the first by stating that the formula applies to a d-1 dimensional sphere, not a d dimensional sphere.
  • A subsequent reply acknowledges the correction and speculates that Schwabl may be considering the dimension in which the sphere is embedded.
  • Another participant clarifies the terminology, explaining that a "2-ball" refers to a two-dimensional disk while a "2-sphere" refers to the surface of a "3-ball," providing specific equations for both.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of "sphere" and "ball," but there is some uncertainty regarding the application of the surface area formula and its dimensional context.

Contextual Notes

The discussion highlights potential confusion regarding dimensionality and the definitions of mathematical objects, but does not resolve the implications of these definitions on the surface area formula.

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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