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

Dear student,

f21  is  the force exerted by q1  on q2f12  is  the force exerted by q2 on q1These forces are equal in magnitude but will have opposite direction
Thus equation ii and iii are vectors with same magnitude but with opposite direction.

For vectors having opposite direction negative sign is used.

so f21 =-f12



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see when vectors are in the same direction the sign is positive to each other... but when they are in opposite direction with respect to each other they are -ve of each other keeping in mind that they have same magnitude.
here the forces21 and force 12 are equal in magnitude but opposite in direction. in other words f21=f12 in magnitude but in direction F21= -F12 or F12= -F21
hope it helps... if still doubt not cleared
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