# Coulomb’s Law

Two stationary electric charges repel or attract each other with a force proportional to the product of the magnitude of charge and inversely proportional to the square of the distance between them

$F\quad \alpha { \quad }{ q }_{ 1 }{ q }_{ 2 }$ ;

$F\quad \alpha { \quad }\frac { 1 }{ r^{ 2 } }$;

$F=C\frac { { q }_{ 1 }{ q }_{ 2 } }{ { r }^{ 2 } }$

$F=\frac { 1 }{ 4\pi \epsilon } \frac { { q }_{ 1 }{ q }_{ 2 } }{ { r }^{ 2 } }$

$\epsilon ={ \epsilon }_{ 0 }{ \epsilon }_{ r }$

For free space           $F=\frac { 1 }{ 4\pi {\epsilon }_{0}} \frac { { q }_{ 1 }{ q }_{ 2 } }{ { r }^{ 2 } }$

where C is the proportionality constant. The value of C depends upon the medium of the charges. Where ${ \epsilon }_{ 0 }$  is permittivity of the free space, ${ \epsilon }_{ r }$ called relative permittivity or dielectric constant of the medium the value is =8.85*10-12 coulomb2 / newton-meter2,

For the medium:      $F=\frac { 1 }{ 4\pi { \epsilon } } \frac { { q }_{ 1 }{ q }_{ 2 } }{ { r }^{ 2 } }$