1)A magnetic field is set up with a North Pole at the sphere’s top. 2)A magnetic field is set up with a South Pole at the sphere’s top. 3)The magnetic field lines form closed concentric circles centered on the spin axis. 4)Since the charge stays in a fixed position, no magnetic field is generated. A positively charged sphere is set spinning as shown at left.
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If a N pole moves towards this loop, both flat in the plane of the page, in what direction is the induced current? 1) clockwise 2) counter-clockwise 3) no induced current SN
If a conducting coil is rotated one quarter-turn (90 o ) in the direction shown, while within the B field pointing right, in what direction will current be induced while turning? 1) clockwise 2) counter-clockwise 3) no induced current
Wire #1 (length L) forms a 1-turn loop, and a bar magnet is dropped through. Wire #2 (length 2L) forms a 2-turn loop, the same magnet is dropped through. Compare the induced EMF’s for both. 1) 1 2 Compare the magnitude of the induced currents in these two cases. 1) I 1 I 2 #1 #2
copper N S N S A bar magnet is held above the floor and dropped. In A, there is nothing between the magnet and the floor. In B, the magnet falls through a copper loop. In which case will the magnet fall faster? 1) A 2) B 3) same speed for both
2)down. Answers to Concept questions Top loop: current crosses field lines into the page. Fingers of right hand to the right, thumb following current across the lines (into the page);palm faces DOWN. 1)up. Exactly balancing the force from above. 3)into screen. 4)out. Thumb points up, palm faces into page. Fingers of right hand still to the right, thumb points down. 4) As the slide that follows this question shows. The angle between the B field lines and the normal to the loop is always 90 degrees. Since cos(90°) = 0 the flux is always zero and therefore not changing. 3) no induced current Flux will be increasing. Current will oppose increase, so induced B field will point to the left. The Right Hand Rule indicates a CW current. 1) clockwise 1) 1 < 2 = –N / t. Both loops have same / t but loop 2 has N = 2. I = V/R. Loop 2 has twice the EMF of loop 1, but also twice the resistance since it is twice as long. 2) I 1 = I 2 When the magnet is above, an induced current must produce an N-pole at the top of the loop, repelling the magnet. Once below, the induced current produces an N pole at the loop’s bottom, attracting the magnet’s S-pole. Or, think about energy. When an induced current flows its energy has to come from somewhere! It must come from the kinetic energy (½mv 2 ) of the falling magnet, i.e., it must fall more slowly. 1) A