Two point charges `q_(A)= 5mu C and q_(B)= -5muC` are located at A and B separated by 0.2m vacuum.
What is the electric field at the midpoint O of the line joining the charges? If a negative test charges of magnitude 2mC is placed at O, what is the force experienced by the test charge?

Explanation

`q_(A)= 5 xx 10^(-6)C, q_(B)= -5 xx 10^(6)C, AB= 0.2m, r= OA = OB= 0.1m`
Electric field at O due to charge at A `E= (1)/(4pi in_(0)) ((q_(A))/(r^(2)))`
`E_(A)= 9 xx 10^(9) [(5 xx 10^(-6))/(10^(-2))]= 45 xx 10^(5) NC^(-1)`
Electric field at O due to charge at B `E = (1)/(4pi in_(0)) ((q_(B))/(r^(2)))`
`E_(B)= 9 xx 10^(9) [(5 xx 10^(-6))/(10^(-2))] = 45 xx 10^(5)NC^(-1)`
Resultant electric field `E= E_(A) + E_(B)= 9 xx 10^(6)NC^(-1)`
`F=q E=2 xx 10^(-9) C xx 90 xx 10^(5) NC^(-1)`
`F= 18 xx 10^(-3)N`