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(b) [8 marks] The figure shows an arrangement of charges on a square of side a. Each of the points has the same quantity of charge Q but the sign of the charge is as indicated, with two positive charges on the left of the square and two negative charges on the right of the square. i) Sketch this figure in your booklet. Draw an arrow showing the direction of the electric field at the centre of the square. If the electric field is zero, indicate that on the figure. ii) Calculate the electric potential at the centre P of the square. iii) What work is required to place a charged particle with charge +Q at the position P ? Show your working and explain any assumptions you make about symmetry.