Note that ε ε 0 must be replaced with ε ε , permittivity, when charges are in non-empty media. When the charges q 0 and q 1 have the same sign this force is positive, directed away from the other charge, indicating the particles repel each other. When the charges have unlike signs the force is negative, indicating the particles attract. To make it easy to calculate the Coulomb force on any charge at position r 0 this expression can be divided by q 0 leaving an expression that only depends on the other charge (the source charge) E 1 ( r 0 ) = F 01 q 0 = q 1 4 π π ε ε 0 r ^ ^ 01 | r 01 | 2 = q 1 4 π π ε ε 0 r 01 | r 01 | 3 where