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922_Physics ProblemsTechnical Physics

# 922_Physics ProblemsTechnical Physics - 262 Inductance 0 NI...

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262 Inductance P32.79 (a) B NI = = × = × µ π 0 7 3 4 10 1 400 2 00 1 20 2 93 10 A T m A A m T upward e j b ga f b g . . . (b) u B = = × × = F H G I K J = = 2 0 3 2 7 2 2 93 10 2 4 10 3 42 1 3 42 3 42 µ π . . . . T T m A J m N m 1 J N m Pa 3 2 e j e j e j (c) To produce a downward magnetic field, the surface of the superconductor must carry a clockwise current. (d) The vertical component of the field of the solenoid exerts an inward force on the superconductor. The total horizontal force is zero. Over the top end of the solenoid, its field diverges and has a radially outward horizontal component. This component exerts upward force on the clockwise superconductor current. The total force on the core is upward . You can think of it as a force of repulsion between the solenoid with its north end pointing up, and the core, with its north end pointing down. (e) F PA = = × L N M O Q P = × 3 42 1 10 10 1 30 10 2 2 3 . . . Pa m N a f e j π Note that we have not proven that energy density is pressure. In fact, it is not in some cases; Equation 21.2 shows that the pressure is two-thirds of the translational energy density in an
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