5.9: Work Required to Assemble a Uniform Sphere
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Let us imagine a uniform solid sphere of mass M, density ρ and radius a. In this section we ask ourselves, how much work was done in order to assemble together all the atoms that make up the sphere if the atoms were initially all separated from each other by an infinite distance? Well, since massive bodies (such as atoms) attract each other by gravitational forces, they will naturally eventually congregate together, so in fact you would have to do work in dis-assembling the sphere and removing all the atoms to an infinite separation. To bring the atoms together from an infinite separation, the amount of work that you do is negative.
Let us suppose that we are part way through the process of building our sphere and that, at present, it is of radius r and of mass Mr=43πr3ρ. The potential at its surface is
−GMrr=−Gr⋅4πr3ρ3=−43πGρr2.
The amount of work required to add a layer of thickness δr and mass 4πρr2δr to this is
−43πGρr2×4πr2ρδr=−163π2Gρ2r4δr.
The work done in assembling the entire sphere is the integral of this from r=0 to a, which is
−16π2Gρ2a515=−3GM25a.