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Unformatted text preview: CHAPTER 30SOURCES OF THE MAGNETIC FIELD ActivPhysics can help with these problems: Activities 13.113.3, 13.5 Section 30 1:The BiotSavart Law Problem 1. A wire carries 15 A. You form the wire into a singleturn circular loop with magnetic field 80 T at the loop center. What is the loop radius? Solution Equation 303, with x = 0, gives the magnetic field at the center of a circular loop, B I a = 2 = (with direction along the axis of the loop, consistent with the sense of circulation of the current and the righthand rule). Thus, the radius is a I B = = = 7 2 2 2 10 15 80 118 = = ( / )( ) ( ) . . N A A T cm Problem 2. A singleturn wire loop is 2.0 cm in diameter and carries a 650mA current. Find the magnetic field strength (a) at the loop center and (b) on the loop axis, 20 cm from the center. Solution Equation 303 gives: (a) at the center, x B I a = = = = 2 4 10 650 2 1 40 8 7 2 , ( / )( ) ( ) . = = N A mA cm T; (b) on the axis, x B Ia x a I = = + = + = 20 1 20 1 5 09 1 2 2 2 2 3 2 1 2 2 2 2 3 2 cm, cm cm cm nT ( ) ( ) [( ) ( ) ] . . = = Problem 3. A 2.2mlong wire carrying 3.5 A is wound into a tight, loopshaped coil 5.0 cm in diameter. What is the magnetic field at its center? Solution The magnetic field at the center of each of the tightlywound turns is the same (Equation 303 with x = 0), so the net field is B N I a = ( ), 2 = where N is the number of turns. If the wire has length L and the coil has diameter D , then N L D = = and B L D I D IL D = = = = ( )( ) ( )( . )( . ) ( ) . = = = = 2 7 2 2 4 10 3 5 2 2 5 123 N/A A m cm mT. Problem 4. What is the current in a long wire if the magnetic field strength 1.2 cm from the wires axis is 67 T? Solution Equation 305 (or Equation 308) gives the magnetic field strength near a long straight wire, B I r = 2 = , from which we find I rB = = = 2 12 67 2 10 4 02 7 2 = = ( . )( ) ( / ) . cm T N A A. Problem 5. Suppose Earths magnetic field arose from a single loop of current at the outer edge of the planets liquid core (core radius 3000 km), concentric with Earths center. What current would be necessary to give the observed field strength of 62 T at the north pole? (The currents responsible for Earths field are more complicated than this problem suggests.) Solution The north pole is on the axis of the hypothetical circular current loop, with radius a = 3 0 . Mm, at a distance x R E = = 6 37 . Mm from the earths center. Equation 303 gives a current of I B x a a = + = 2 2 2 3 2 2 ( ) = = 2 62 6 37 3 0 4 10 3 0 383 2 2 3 2 7 2 ( )[( . ) ( . ) ] ( / ( . ) . T Mm Mm T m A) Mm GA. + = = = 192 CHAPTER 30 Problem 6. Earths magnetic dipole moment is 8 0 10 22 2 . . A m What is the magnetic field strength on Earths surface at either pole?...
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 Spring '07
 L.ROSE
 Physics

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