Point charges Q1 nC = 1, Q2 = 1 nC, Q3 = -1 nC, Q4 = -1 nC disposed on a plane in the lattice sites with the cell in the form of a square with a side of 0.1 m. The lattice nodes in which these charges are, given radius vectors r1 = (a, 0), r2 = (a, a), r3 = (-a, a), r4 = (-a, 0) The remaining nodes are no charges. Determine the strength and potential of the electric field at the point with radius vector r = (0, - a)

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