What is the mass of a sphere with uniform charge?

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Discussion Overview

The discussion revolves around determining the mass of a uniformly charged sphere that levitates above an infinite sheet with uniform charge density. Participants explore the relationship between electrostatic and gravitational forces in this context, considering both theoretical and mathematical aspects.

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • One participant states that the sphere has a radius R and uniform volume charge density P, and questions how to calculate its mass while it levitates above a charged sheet.
  • Another participant suggests that the sphere can be treated as a point charge, implying that its charge and mass can be simplified for calculations.
  • A third participant notes that the weight of the sphere must equal the electrostatic force acting on it while it is levitating, proposing to write equations for both forces and rearrange them to isolate mass.
  • One participant cautions that the influence of the sheet must be considered, hinting at the method of mirror charges as a potential approach to account for this effect.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to calculate the mass of the sphere, with multiple viewpoints and methods being proposed without resolution.

Contextual Notes

Participants do not clarify certain assumptions, such as the specific definitions of charge density or the implications of treating the sphere as a point charge. The discussion also lacks resolution on the mathematical steps involved in the calculations.

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The sphere has radius R, and uniform volume charge density P. This sphere remains stationary (levitates) when placed above an infinite sheet of paper with a uniform surface charge density u. What is this sphere's mass?
 
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The sphere can be replaced with a point charge (the charge and mass of the sphere).
 
The weight of the sphere is equal to the electrostatic force between the sphere and the sheet when the sphere is levitating. Because the sheet is infinite the sphere is infinitessimal and is therefore a point. Write the equation for the electrostatic force and for the gravitational force and rearrange to give mass on one side of the equation and everything else on the other.
 
Be careful not to forget to take into account influence (hint: can be calculated by the method of mirror charges).
 

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