Bfieldincreasesupwardasloopfalls

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Unformatted text preview: nge • the orientation of B and A does not change 50 40 30 20 10 0 BB Looking from right X X X X X X X X X X X X X X X X Preflight 4 X X X X X X X X X X X X X X X X X X X X X X X X • Motional emf is ZERO • There is no motion of conduction electrons ! Clockwise restores B 60 • HOWEVER: The flux IS changing • B decreases in time • current induced to oppose the flux change • clockwise current puts back B that was removed and opposes the decrease in flux 50 40 30 20 10 0 BB Preflight 6 Current changes direction every time the loop becomes perpendicular with the B field emf ~ dΦ/dt (B dA = max) d/dt (B dA ) = 0 70 60 50 40 30 20 10 B dA B dA 0 F O X B B Like poles repel Preflight 8 Ftotal < mg a < g (copper is not ferromagnetic) This one is hard ! B field increases upward as loop falls Clockwise current (viewed from top) is induced 50 40 30 20 10 0 BB HO W IT WO R KS Looking down B Preflight 8 (copper is not ferromagnetic) This one is hard ! B field increases upward as loop falls Clockwise current (viewed from top) is induced Main Field produces horizontal forces Fringe Field produces vertical force I I B IL X B points UP Ftotal < mg a < g Demo Demo ! Calculation Calculation A rectangular loop (height = a, length = b, resistance = R, mass = m) coasts with a constant velocity v0 in + x direction as shown. At t =0, the loop enters a region of constant magnetic field B directed in the –z direction. y a v0 B b xxxxxxx xxxxxxx xxxxxxx xxxxxxx What is the direction and the magnitude of the force on the loop when half of it is in the field? • Conceptual Analysis – – Once loop enters B field region, flux will be changing in time Faraday’s Law then says emf will be induced in loop, making a current flow that opposes the change in flux • Strategic Analysis – – – Find the emf Find the current in the loop Find the force on the current x dΦB emf = dt Calculation A rectangular loop (height = a, length = b, resistance = R, mass = m) coasts with a constant velocity v0 in + x direction as shown. At t =0, the loop enters a region of constant magnetic field B directed in the –z d...
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