A metal ball is surrounded by a thin ball shell. The metal ball is charged with a charge of 
. The shell is also charged with the same charge size but the opposite sign. The radius of the metal ball is 2 cm and the radius of the outer shell is 13 cm.
What is the electric field strength in the space between the ball and the shell at half the distance between them?
What is the capacitance of such a spherical capacitor?
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