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Unformatted text preview: Reading : Magnetostatics 2 Key concepts: Currents create B-felds; Biot-Savart law; Amperes law. 1. Two long wires carrying current are at right angles to each other. Wire 1 runs perpendicular to the page with the current out of the page as indicated. Wire 2 is in the plane of the page, with current to the right. a. Draw a typical Feld line for the B-Feld created by I 1 , showing carefully how it intersects wire 2. b. Argue that the total force exerted by wire 1 on wire 2 is zero. c. What direction is the torque exerted by wire 1 on wire 2? [Does it tend to twist wire 2 so its current is parallel or anti-parallel to that in wire 1?] 2. A triangular wire loop carrying current I is in a uniform B- Feld into the page as shown. a. What is the torque on the loop? Explain. b. What is the net force on the loop? c. If the Feld is not uniform but increases as you move into the page, what direction is the net force on the loop? 3. A proton enters the earths B-Feld above the equator, moving as shown. a. Sketch the path followed by the proton, treating the Feld as uniform in the plane of the page. b. What direction is the magnetic moment of the current loop represented by the protons trajectory? c. If we are looking north in the diagram, the B-Feld is increasing into the page. What direction is the force on the protons current loop? 4*. The two wires connecting a lamp to the wall outlet are parallel and close together, protected by insulation from each other. Suppose they are 4 mm apart and carrying current to a lamp that consumes 60 W at 120 V. a. What is the B-Feld midway between the wires? [Use the long straight wire approximation.] b. What is the B-Feld at distance 10 cm from the midpoint? c. Compare this Feld to the earths magnetic Feld, about 5.5 10 5 T. I 1 I 2 a 2 a I B B PHY 54 Assignment #5 Summer 2011 5. A very large number of closely spaced insulated wires carry identical parallel currents along the surface of a large Fat sheet. Shown are two views, from above the sheet and from the edge of the sheet with the current coming out of the page. Each wire carries current I , and there are n wires per unit length across the sheet....
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