The electrostatic force on a small sphere of charge 0.4 `mu` c due to another small sphere of charge -0.8 `mu` c in air is 0.2 N (a) What is the distance between the two spheres (b) what is the force on the second sphere due to the first

Explanation

(a) Electrostatic force on the first sphere , `F=0.2N`
Charge on this sphere , `q_(1)=0.4muC=0.4xx10^(-6)C`
Charge on the second sphere,
`q_(2)=-0.8muC=-0.8xx10^(-6)C`
Electrostatic force between the spheres is given by the relation ,
`F=(q_(1)q_(2))/(4piepsilon_(0)r^(2))`
where, `epsilon_(0)`= permittivity of free space
and `(1)/(4piepsilon_(0))=9xx10^(9)Nm^(2)C^(-2)`
`r^(2)=(q_(1)q_(2))/(4piepsilon_(0)E)`
`=(0.4xx10^(6)xx8xx10^(6)xx9xx10^(9))/(0.2)`
`=144xx10^(4)`
`r=sqrt(144xx10^(4))=0.12m`
the distance between the two spheres is 0.12m.
(b) Both the spheres attract each other with the same force . therefore , the force on the second sphere due to the first is 0.2 N.