Suppose we have a sphere of radius r and mass m and a negatively charged test particle at dist

Justine Webster

Justine Webster

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Suppose we have a sphere of radius r and mass m and a negatively charged test particle at distance d from its center, d r. If the sphere is electrically neutral, the particle will fall toward the sphere because of gravity. As we deposit electrons on the surface of the sphere, the Coulomb force will overcome gravity and the test particle will start to accelerate away. Now suppose we keep adding even more electrons to the sphere. If we have n electrons, the distribution of their pairwise distances has a mean proportional to r, and there are n ( n 1 ) / 2 such pairs, so the binding energy is about n 2 / r. If this term is included in the total mass-energy of the sphere, the gravitational force on the test particle would seem to be increasing quadratically with n, and therefore eventually overcomes the linearly-increasing Coulomb force. The particle slows down, turns around, and starts falling again. This seems absurd; what is wrong with this analysis?

Answer & Explanation



Beginner2022-05-20Added 12 answers

Suppose we have N electron in the sphere, the electrostatic energy to bring a new electron into the sphere is N e / R. If we add a new electron, only the net electrostatic potential N e / R.
the net potential on far-away electrons is
N ( e m e G ) / R
what is actually quadratic in N is the energy required to bind together N electrons in the sphere, it is actually
e / R + 2 e / R + . . . N e / R = N ( N 1 ) e / 2 R
This also contributes to gravitational weight, but there will be a maximum capacity where the electrons will escape the sphere
the capacitance of a sphere is given by 4 π ϵ R. So in this case the binding energy is bounded by the capacitance of the conductor used for your sphere
Marco Villanueva

Marco Villanueva

Beginner2022-05-21Added 6 answers

The statement that the gravitational attraction will eventually dominate the coulomb repulsion as n increases is correct. You probably think the restmass of the electrons will invoke gravitational attraction, but that part is neglible for high electrondensities on the sphere. The gravitational attraction by (the curvature of spacetime caused by) bindingenergy is far greater for high densities.
I don't know where the breakpoint is, but suppose I look way past that limit, i.e. an incredible dense sphere(or shell) of electrons. If I make it dense enough, that could very well become a black hole. Note that this would be a strange black hole, since the 'mass' of such a black hole consists almost entirely of the bindingenergy, and not the restmasses of the electrons. From this point of view it might be more easily imaginable that the gravitional attraction will dominate eventually.

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