chapter 30 pt 2

30.28

28. | MRI (magnetic resonance imaging) is a medical technique that 3'0 produces detailed \"pictures\" of the interior of the body. The patient is placed into a solenoid that is 40 cm in diameter and 1.0 m long. A 100 A current creates a 5.0 T magnetic field inside the solenoid. To carry such a large current, the solenoid wires are cooled with liquid helium until they become superconducting (no electric resistance). a. How much magnetic energy is stored in the solenoid? Assume that the magnetic field is uniform within the solenoid and quickly drops to zero outside the solenoid. b. How many turns of wire does the solenoid have? 49. II An electric generator has an 18-cm-dian1eter, 120-turn coil that GM rotates at 60 Hz in a uniform magnetic field that is perpendicular to the rotation axis. What magnetic field strength is needed to generate a peak voltage of 170 V? 59. || FIGURE P3059 shows a U-shaped conducting rail that is oriented vertically in a horizontal magnetic field. The rail has no electric resistance and does not move. A slide wire with mass m and resistance R can slide up and down without friction while maintaining electrical contact with the rail. The slide wire is released from rest. a. Show that the slide wire reaches a terminal speed 12mm, and find an expression for vmm. b. Determine the value of vtcrm if 1= 20 cm, m = 10 g, R = 0.10 0., and B = 0.50 T. X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X FGX X X FIGURE P30.5965. || One possible concern with MRI (see Exercise 28) is turning the 3'0 magnetic field on or off too quickly. Bodily uids are conductors, and a changing magnetic field could cause electric currents to flow through the patient. Suppose a typical patient has a maximum cross- section area of 0.060 m2. What is the smallest time interval in which a 5.0 T magnetic field can be turned on or off if the induced emf around the patient's body must be kept to less than 0.10 V