- 52

- 0

[the "model" answer is [tex]\frac{1}{4 \pi \epsilon_0} \frac{3 Q^2}{16 R^2}[/tex]]

my attempt:

regard two hemispheres as two point charges located at their center of mass, [tex]\frac{3 R}{8}[/tex] from the center.

so

[tex]

F = \frac{1}

{{4\pi \varepsilon _0 }}\frac{{\left( {Q/2} \right)^2 }}

{{\left( {2 \times \frac{3}

{8}R} \right)^2 }} = \frac{1}

{{4\pi \varepsilon _0 }}\frac{4}

{9}\frac{{Q^2 }}

{{R^2 }}

[/tex]

but I got it wrong....

so, can anyone tell me how should I start?