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b. Two rings each of radius R and carrying an equal charge Q are separated by a distance d and rotating in opposite directions at an angular frequency Ew as shown below. R 3 Assuming that the rings are close together, so that d « R, calculate an expression for the magnetic force between the rings. Is this force attractive or repulsive? c. A disc of iron of radius 5 mm and thickness 1 mm is magnetised, and at a distance of 8 cm above the centre of the disk the measured magnetic field strength is B = 24.7T (i) Given that magnetic field from a magnetic dipole of strength y varies with distance z perpendicular to the disc as B HOM 27 23 calculate the magnetic dipole moment of the iron disc. (ii) The magnetic dipole moment of an iron atom is fefe = 2.1 x 10-23 JT-7, the density of iron is 5.87g em- and its molar mass is 55.8 g mol-!. Calculate the percentage of the atoms that are aligned in the iron disc.