Example – 1 Atomic number of copper is 29, its atomic weight is 63.5 gm/mole. If two pieces of copper having weight 10 gm are taken and on one of the pieces 1 electron per 1000 atom is transferred to the other piece and there after this these pieces are placed 10 cm apart, then what will be the coulomb force between them.
Solution : In 1 mole copper (63.5 gm) there are 6 × 1023 atoms (NA = Avogadro number = 6 × 1023 atoms)
Number of atoms =  = 9.45 x 1022
For every 1000 atoms, 1 electron is transferred therefore total number of transferred electron
is
Therefore charge on one piece is 9.45 × 1019 e and on the other will be (9.45 × 1019e) Force when they are kept 10 cm apart
Example – 2 On an insulated rod having length L, Q charge is evenly distributed and a point charge
q is placed at a distance d from one of its ends as shown. What will be the total electric force on q.
Solution : Considering a small element dx of the rod at a distance x from q, force on q due to
this small element
 
where dQ is charge in small element dx and dQ = (Q/L) dx
Example – 3 Three equal charges (Q, each) are placed on the vertices of an equilateral triangle of side a. What is the resultant force on any one charge due to the other two?
Solution : The charges are shown in fig. The resultant force
 
From symmetry the direction is as shown along y-axis.
Example – 4 Five point charges, each of value +q are palced on five vertices of a regular hexagon
of side L m. What is the magnitude of the force on a point charge of value -q coloumb
placed at the centre of the hexagon?
Solution : If there had been a sixth charge +q at the remaining vertex of hexagon, force due to all the six charges on -q at O will be zero (as the forces due to individual charges will balance each other), i.e., ®FR = 0
Now If ®f is the force due to sixth charge and F® due to remaining five charges, ®F + ®f = 0
Example - 5 Two charged spheres of radius ‘R’ are kept at a distance ‘d’ (d > 2R). One has a charge +q and the other -q. The force between them will be-
[1] [2]  [3]  [4] None of these
Solution : [2] Redistribution of charge will take place due to mutual attraction and hence effective
distance will be less than d.
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