Step 1: Given
Three charges of charge q each.
Fourth charge q2
Step 2: Calculation of electric field at centre
We know that electric field is given by
∣∣→E∣∣=Kqr2
Electric field at centre O due to charges is
Enet=EA+EB+EC+ED
Now,
∣∣→EA∣∣=∣∣→EB∣∣=∣∣→EC∣∣=Kql2 (since chage on corners A, B and are equal)
∣∣→ED∣∣=kq2l
Since, the directions of the electric field →EAand→EC are opposite, therefore,
→EA+→EC=0
∣∣→Enet∣∣=∣∣→EB∣∣−∣∣→ED∣∣
∣∣→Enet∣∣=kql2−kq2l2
∣∣→Enet∣∣=kq2l2