A disk of radius R has positive charge uniformly distributed over an inner circular region of radius a and negative charge uniformly distributed over the outer annular (ring-shaped) region (Figure 1). The surface charge density on the inner region is +σ, and that on the annular region is −σ. The electrostatic potential at P, located on the central perpendicular axis a distance z=R from the disk center, is zero.
What is a?
Express your answer in terms of R. Express the coefficients using three significant digits.
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