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## Main Question or Discussion Point

1-capacitance of sphere with radius r : 4πεr

2-capacitance of 2 concentric spheres with inner radius a and outer radius b : 4πε(a.b/b-a)

3-when the outer the outer sphere is earthed it gives the same capacitance as in number 2

4-when the inner sphere is earthed it gives the sum of capacitances in number 1 and 2

my question : why does number 2 and 3 give the same capacitance while they are different capacitors (the inner sphere is earthed in one and not earthed in the other one) ? and when the inner sphere is earthed : why does it give 2 capacitances (sum of number 1 and 2) ?

2-capacitance of 2 concentric spheres with inner radius a and outer radius b : 4πε(a.b/b-a)

3-when the outer the outer sphere is earthed it gives the same capacitance as in number 2

4-when the inner sphere is earthed it gives the sum of capacitances in number 1 and 2

my question : why does number 2 and 3 give the same capacitance while they are different capacitors (the inner sphere is earthed in one and not earthed in the other one) ? and when the inner sphere is earthed : why does it give 2 capacitances (sum of number 1 and 2) ?