4. Solenoid (6 points) A solenoid has R = 2 cm, length L = 0.9 m, and / = 400 turns. The figure below shows a cross section of the solenoid. Ad = 5 cm square coil of wire is placed around the solenoid, so that the axis of the solenoid is perpendicular to the plane of the coil. The current through the solenoid is 3.1A. R (a) (3 points) What is the magnetic flux through the square coil? (b) (3 points) If the current in the solenoid is decreased from 3.1A to zero in 0.4s, what is the magnitude of the average EMF generated in the coil