Electrostatics Tricks #1

1. While calculating the force between the two charges from Coulomb’s Law, never take into account, the sign of the two charges. The sign just indicates the nature of the force. 
2. ®F12 is the force on charge 1 due to 2. ®F21 is the force on charge 2 due to 1.®Fij is the force on charge i due to j. 
3. When we are to calculate the force on charge 1 due to charge 2, then we assume charge 2 to be fixed and vice versa, unless and otherwise stated. 
|®F12| = m1a1
®F21| = m2a2
i.e. m1a1 = m2a2 (by Newton’s Third Law) where m1 and m2 are the masses of charges q1and q2 respectively. 
4. If we are to calculate the force on charge q0 , due to assembly of charges q1 ,q2 , …. Qn, then we have
®F0 = ®F01 + ®F02 + ®F03 + ……. + ®F0n 
Where®F01 is calculated as if only q0 and q1 are present and all others are absent and so on for other combinations. This principle is called the Principle of Superposition. 
Note : When two identical bodies having charges q1 and q2 respectively are brought in contact and separated, then the charge on each body is ((q1+q2) /2) 

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