Coulomb’s Law- It states that force between two point charges is directly proportional to the product of the magnitude of two charges & inversely proportional to the square of distance between them . The direction of force is along the line joining two point charges.
Mathematically,
F µ q1 q2/r2 q1·----------------------------------·q2
F = k q1 q2/r2 ______________ r _________________
Case 1- When medium between two charges is air/vacuum/free space
k = 1/4Õeo
then F = 1/4Õeo´ q1 q2/r2 (eo = 8×854 ´10-12 C2N-1m-2 )
Case 2- When medium between two charges is other than air
k = 1/4Õe
Then F = 1/4Õe ´ q1 q2/r2
__________________________________________________________________________
QUESTION- A charge q is placed at the centre of line joining two equal charges Q. Show that the system of three charges will be in equilibrium if q = -Q/4 ?
SOLUTION- Let two equal charges Q each be held A and B , where AB = 2x & C is the centre of AB , where q is held ______________ 2x _________________
·-----------------·-----------------·
A C B
Net force on q is zero, so q is already is in equilibrium
Now total charge on Q at B
1/4Õe o´Qq/x2 + 1/4Õe o´QQ/(2x)2 = 0
1/4Õe o´Qq/x2 =-1/4Õe o´QQ/(2x)2
So q = -Q/4 (HENCE PROVED)
