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Today4/11

Today4/11. Current Loops “Phantom” magnets Force/torque on loops in a B-field B-Field due to many wires HW: 4/11 “Torque on Loops” Due Wednesday, 4/16 Magnetism HW delayed till Monday 4/14 I will go over it today Force on Loops and Torque on loops both due on Wednesday 4/16. S.

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Today4/11

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  1. Today4/11 • Current Loops • “Phantom” magnets • Force/torque on loops in a B-field • B-Field due to many wires • HW: 4/11 “Torque on Loops” Due Wednesday, 4/16 • Magnetism HW delayed till Monday 4/14I will go over it today • Force on Loops and Torque on loops both due on Wednesday 4/16

  2. S Magnetic Torque, Loops What happens to loop? Turn the loop into a magnet. Magnets line up with the field. (A compass needle is a magnet!) Loop is a magnet (needle) centered in the loop with the N pole facing away from you. The loop (magnet) will rotate so it lines up with the field. (S up and out of the page)

  3. Magnetic Torque, Loops What happens to loop? Turn the loop into a magnet. Magnets line up with the field. (A compass needle is a magnet!) Loop is a magnet (needle) centered in the loop with the N pole facing away from you. S N The loop (magnet) will rotate so it lines up with the field. (S up and out of the page) This is a torque on the loop.

  4. N S N S N S N S A bar magnet and current loop (powered by a battery) are shown in various orientations below. In each case the magnet if held in place and the current loop is released and allowed to rotate only about the axis shown. Determine if the loop will rotate. If so, find the direction of rotation and how far it rotates. If the loop is in unstable equilibrium, say so. (RHR #3) Clockwise (viewed from above) 90°North out Torque clockwise from above, can’t turnNorth out Clockwise (viewed from above) 90°North in Unstable equilibrium small tap will cause a 180° flip either direction. North right

  5. Ampere’s Law E field B field No point currents E  1/r2 point B  1/r wire“rod” of current E  1/r rod of charge B  constantsheet of current E  constantsheet of charge

  6. B r Note: B always perpendicular to the radius Ampere’s Law Sum of B times l around any path = 0 times current through that path. I Sum(B x l) = B x circ. = B2r = 0I B = 0I/2r ( 1/r ) Magnitude of the B-field given be this equationDirection by RHR #2 B  1/r wire“rod” of current

  7. I2 r I1 r 2 r I3 Magnetic Forces, Wires Find the force on wire 3 caused by wire 1 and 2.

  8. I2 I1 r B1 =  0 I/2 r B2 = B1/ 2 B1&2 add as vectors Magnetic Forces, Wires First find the direction of the B field at the X. All currents are the same Find Bx,net and By,net

  9. I2 I1 F = I l B I3 B1&2 Magnetic Forces, Wires Now find the force on wire 3.

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