Two insulated copper sphere of charges `+10 mu C and -20 mu C ` have their centres seprated by a distance of 10 cm . What is the force of attraction if they placed.
a in free space b. in water of dielectric constant 81.

Explanation

Data supplied
` r=10 xx 10 ^(-2) m.`
` q_1 =10 mu C =10 xx 10^(-6) C , q_2 = 20 mu C =20 xx 10 ^(-6 ) C `
a. ` " " F_("air") =(1)/( 4pi in _0) (q_1q_2)/( r^(2)) `
` F_(air) =( 9xx 10^(9) xx10 xx 10 ^(-6)xx-20xx10 ^(-6))/( (10xx 10 ^(-2))^(2)) =-180 N`
b. ` F_("water") =(1)/( 4piin _0) =(q_1q_2)/( r^(2)) =(air )/(in_r) =(-180)/(81) =-2.22N`
Negative sign indicates that the force is attractive.