Capacitor breakdown voltage..??? I get it, but I don't get it.

The first capacitance problem in the international edition text, # 3.40, asks in part B what the breakdown voltage of a coaxial capacitor is when inner radius (a) is 1 cm, outer radius (b) is 2 cm, and dielectric is mica, with ε_r = 6.

The only way I saw in the book and see elsewhere to calculate breakdown voltage in this context is to adapt the V=Ed formula by saying V_bd = E_ds*d, where V_bd is breakdown voltage, E_ds is dielectric strength, and of course d is distance. I have no idea how to calculate breakdown voltage using just the dielectric constant.

Looking in table 3-2 on page 93, mica has E_ds = 200 MV/m, or 200E6 V/m. The distance between inner and outer radii is .01m. So, 200E6*.01 = 2E6 V, or 2 MV.

The book says it's 1.39 MV. Wth???

I understood it eventually. I was using the wrong formula to determine breakdown. I needed to use the capacitor voltage formula for coaxial capacitors instead of for parallel plate capacitors. After that it worked fine. lol.