Two point charges of + 10 microcoulomb and + 40 microcoulomb respectively are placed 12 CM apart find the position of the point where electric field is zero
Let the point where electric field is 0, be at a distance x from the first charge and (12-x)cm from the second charge.
then, electric field due to first charge is E1=9*10^9*10*10^-6/x^2 =9*10^4/x^2
electric field due to second charge is E2=9*10^9*40*10^-6/(12-x)^2 =36*10^4/(12-x)^2
E1+E2=0 => E1=-E2
on solving the underlined equation, you will get the answer.
9*10^4/x^2 = 36*10^4/(12-x)^2
taking root on both sides
3*10^2/x = 6*10^2/(12-x)
3/x = 6/(12-x)
1/x = 2/(12-x)
12-x = 2x
3x = 12
x = 4
4 cm from the first charge the E will be zero.