Charges +Q, −Q, +Q, −Q at corners of a square (side 5 cm). Find field at centre. (Q = 1 μC)

With alternating charges, the two +Q charges are at diagonally opposite corners, and the two −Q charges are at diagonally opposite corners.

Field at centre from each pair:

Both +Q fields: exactly opposite directions ⇒ cancel

Both −Q fields: exactly opposite directions ⇒ cancel

Every field vector has an equal and opposite partner by symmetry.

Answer: Electric field at centre = 0 (zero)