Please explain equation ii and iii and f12=-f21

Please explain equation ii and iii and f12=-f21 charge elec b' s force on an e Coulomb •s force. F Vector Form of coulomb's Law S q I and separated by a distance (Figure 11), coulomb's force acting due to is given by, where is the unit vector directed from to qt Similarly, force acting on due to given by F 21 —k gggtL where r 12 is the unit vector directed from q I to = so eqn. (ii) can be written as From equations (ii) and (iii), we have - F21 Fn ¯ each otherareequala,idopposite. q, is positive and q, is negative then F12 and F21 are as shown in figure. (iii) Newton's third law of motion states that action and reaction are equal and opposite Clearly Coulomb's Law, giving F 12 - F is in accordance with third law. Due to this fact, the Coulomb 's HOT force is considered to be Newtonian in nature. Dielectric constant (K) Dielectric constant (K) of a mediumis defined as the ratio of the Coulomb's force F between two t charges placed in the air or vacuum to the Coulomb's force Fm between the same two point "s placed in the medium at the same distance from each Other. i.e. K F/F