00:01
In this setup, we have four charges in these situations.
00:07
The qa, qb, qc, and qd.
00:11
Where qb, qc have the charge negative 1 column, and the qd have the charge positive 1 column.
00:20
And the question is, we want to know the qa, which makes the electric field at qd is 0.
00:30
So, in this problem, we can use the superposition principle.
00:36
From the principle, we know the electric field at qd, it's not depends on the qd.
00:43
It only depends on qa, qb, and qc.
00:47
So this situation, first we consider the electric field as the x component.
00:55
So we can write the contribution from the qc at x component, which is q.
01:03
E, cx, which is eq2, the q2 divided by the distance is a squared plus 2a square, which is a distance between c and d.
01:18
And we want to now the x component, so this method multiplied by this factor, and we can got that is the contribution from the qc at the x component.
01:33
We can do the same thing for the qb, which is abx.
01:41
It gets something but here is qb and the same distance, but here for the x component is this...