An air-filled toroidal solenoid has a mean radius of 14.9 cm and a cross-sectional area of 4.95cm2 (see the attached figure). The current flowing through it is 12.1A, and it is desired that the energy stored within the solenoid be least 0.394J.
What is the least number of turns that the winding must have?? (A whole number to three significant figures.) See attached.