Two point charges `q_(A) = 3 mu C` and `q_(B) = -3 muC` are located 20 cm apart in vaccum (a) what is the electric field at the mid point O of the line AB joining the two charges ? (b) If a negative test charge of magnitude `1.5xx10^(-9) C` is placed at the point, what is the force experienced by the test charge ?

Explanation

Put the unit positive charge at O
(a)Electric field at point O
`barE_0=barE_A+barE_B`
`=E_A -E_B (because E_A=E_B=1/(4piepsilon_0)q/(10xx10^(-2)))`
(b)
If we put negative test charge at the centre of AB, The net force
`vecF_0=vecF_A+vecF_B=F_A-F_B=0`
`because |vecF_A|=|vecF_B|=1/(4piepsilon_0)(q q_0)/r^2`