Can a solid sphere only be considered a point for inverse-square forces?

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Discussion Overview

The discussion centers on whether a solid sphere can be treated as a point source for forces that do not follow an inverse-square law. Participants explore the implications of different force decay rates and their relationship to symmetry and spherical distributions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if the force from a solid sphere can be considered to originate entirely at its center for forces that are not of \(\frac{1}{r^2}\) nature, suggesting that conclusions about symmetry may not hold for other force types.
  • Another participant argues that if a force is not proportional to \(\frac{1}{r^2}\), the scenario may lack symmetry, complicating the treatment of the sphere as a point source.
  • It is noted that various forces, such as those involving electric dipoles, decay at rates like \(\frac{1}{r^3}\) or \(\frac{1}{r^8}\), prompting a question about the validity of the point-source approximation for these cases.
  • One participant proposes that as long as the force is proportional to \(r\) of any order, the point-source approximation should remain valid, provided that movements of the sphere's parts maintain symmetry.
  • Another participant raises the question of whether forces like dipole-dipole interactions exhibit spherical symmetry, noting that such forces fall off differently depending on their orientation.
  • It is acknowledged that while dipole-dipole forces have directional dependencies, certain intermolecular forces are spherically symmetrical and independent of direction.

Areas of Agreement / Disagreement

Participants express differing views on the implications of non-inverse-square forces and their symmetry properties. No consensus is reached regarding the treatment of solid spheres as point sources under these conditions.

Contextual Notes

Participants highlight the complexity introduced by non-inverse-square forces and the potential for varying symmetry, which may affect the applicability of certain conclusions drawn from inverse-square scenarios.

cyborg6060
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I am curious as to whether the force of solid sphere can be considered to be originating entirely at its center when the force is not of \frac{1}{r^2} nature.

It is said that the field inside a uniform spherical shell is zero for any \frac{1}{r^2} type force and not for any others. It would seem likely that the other such conclusions would not hold for any force that was not proportional to the inverse square of the distance.

If so, are there any similar conclusions or symmetries that are independent of the type of decay of the force?
 
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If you have a force that is NOT proportional to the inverse of r^2 then your dealing with some sort of crazy scenario where energy or space is not symmetrical.

So you can't treat the sphere as a point without taking into consideration the modifications you have made to the system to make it non-symmetrical.
 
To my knowledge there existed a fair amount of forces that are not inverse-square in nature.

When electric dipoles are involved, for instance, the decay becomes one of \frac{1}{r^3}. Also, the macroscopic functions for intermolecular forces such as those due to dipole-dipole and Van der Waals attractions are usually proportional to \frac{1}{r^8}.

Would the proof for considering the force from solid sphere to be from its center (from outside the sphere) fall apart without the inverse-square force decay?
 
Actually as long as it proportional to r of any order than it should not fall apart. Because if each part of the sphere that you move further away you also move another part the same distance closer when treating the sphere as a point at the center.
 
cyborg6060 said:
To my knowledge there existed a fair amount of forces that are not inverse-square in nature.

When electric dipoles are involved, for instance, the decay becomes one of \frac{1}{r^3}. Also, the macroscopic functions for intermolecular forces such as those due to dipole-dipole and Van der Waals attractions are usually proportional to \frac{1}{r^8}.
But do those things have spherical symmetry?
 
uart said:
But do those things have spherical symmetry?

You raise a very good point. The dipole-dipole force falls off as \frac{1}{r^3} only along the plane equidistant between the two poles. Doh.

As far as the LDF intermolecular forces go, the attraction is entirely independent of direction and therefore spherically symmetrical.
 

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