a) The magnetic field of a proton is like that of a circular current loop 0.650 10-15 m in radius carrying 1.05 104 A. What is the magnetic field at the center of such a loop? b) An MRI is essentially a large solenoid. Consider a 500 turn circular coil of length 0.75 m. What is the current needed to create a 1.20 T magnetic field, typical of an MRI instrument, assuming the interior of the solenoid is just empty space/air? (This is a lot of current! To create such large B fields, the core is usually partially filled with iron to get the same B field for less current. Also, superconducting wires are generally used to make the current achievable with relatively normal amounts of power.)