A plasmoid is a finite configuration of magnetic fields and plasma. With the virial theorem it is easy to see that any such configuration will expand if not contained by external forces. In a finite configuration without pressure-bearing walls or magnetic coils, the surface integral will vanish. Since all the other terms on the right hand side are positive, the acceleration of the moment of inertia will also be positive. It is also easy to estimate the expansion time τ. If a total mass M is confined within a radius R, then the moment of inertia is roughly MR, and the left hand side of the virial theorem is ⁠ MR / τ ⁠. The terms on the right hand side add up to about pR, where p is the larger of the plasma pressure or the magnetic pressure. Equating these two terms and solving for τ, we find