nagle_phys2020_fa07_lecture24

Nagle_phys2020_fa07_ - Exam 2 October 23 Exam#2(Tuesday 7:30-9:15 pm in room G1B30 Professor Nagle office hours today 1-2 pm Register your

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Unformatted text preview: Exam 2: October 23 : Exam #2 (Tuesday 7:30-9:15 pm in room G1B30). Professor Nagle office hours today 1-2 pm Register your iClickers (still many missing) Error in printing CAPA #9, fixed today. Current Loop v = iA v Area = A i Direction of A: Curl right hand fingers along current, thumb points along A. What are the forces and torques on the loop? "Voltage is like elevation." 62 63 Clicker Question A square loop of side length a of wire carrying current I is in a uniform magnetic field B. The loop is perpendicular to B (B out of the page). What is the magnitude of the net force on v v v the wire? F = iL B A: iaB B B: 4iaB C: 2iaB D: 0 E: None of these I Clicker Question The same loop is now in a non-uniform field. r $, B = Bz where B = B(y) = A y where A is a constant. The direction of the net force is? B stronger B A C D E: net force is zero y x B weaker 64 65 Clicker Question A long wire has a current moving as shown. What is the direction of the B-field created by the wire just above the wire? B=? A) Into the Page B) Out of the Page C)To the right D)Down E) None of the Above r L i I In a uniform B-field, regardless of the orientation between the B and the Magnetic Moment of the loop , the net force is always zero. v v v F = iL B B v v i L r ^ B = 0 4 r 2 However, that does not mean the net torque is zero! 66 67 1 v v v = r F v v v v = r (iL B ) = 2[( L / 2)iLB sin ] If is not parallel to B, then there is a net torque. Basic principle behind many motors! = B Torque wants to twist the loop so that and B align. Remember this point! v v v = iL2 B sin = iA B v v v v = B v v v 68 69 Interaction between two current carrying wires i i The two wires may exert forces on each other through Magnetic Interactions. 1. Think of the Red Wire as creating a B-field. 2. Then think of that B-field creating a force on the moving charges (current) in the Blue Wire. Clicker Question i i What is the direction of the Force acting on the Blue Wire? v v v F (total ) = iL B A) Up B) Right C)Left D)Into the Page E) Out of the Page 70 71 i1 i2 The B-field from the Red Wire at the location of the Blue Wire is into the page. Clicker Question i i What is the direction of the Force acting on the Red Wire? v i B1 = 0 1 (into the page) 2R X Then the Force on the Blue Wire is to the left. v v v F (total ) = iL B R v v v i F2 = i2 L B1 = i2 L 0 1 (left) 2R A) Up B) Right C)Left D)Into the Page E) Out of the Page 72 73 2 i1 i2 The B-field from the Blue Wire at the location of the Red Wire is out of the page. i i Wires with parallel currents attract each other. v i B2 = 0 2 (out of the page) 2R . Then the Force on the Red Wire is to the right. i What happens if we flip the direction of one current? Wires with antiparallel currents repel each other. Try following the procedure we just outlined to confirm this for yourselves. R v v v i F1 = i1 L B2 = i1 L 0 2 (right) 2R i 74 75 Demonstration Do not leave contact shut! Why? 800 Amps! 76 3 ...
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This note was uploaded on 02/27/2008 for the course PHYS 2020 taught by Professor Dubson during the Fall '06 term at Colorado.

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