This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: Chapter 28 p.1 CHAPTER 28 Sources of Magnetic Field 1. The magnetic field of a long wire depends on the distance from the wire: B = ( /4p)2 I / r = (10 7 T m/A)2(65 A)/(0.075 m) = 1.7 10 4 T . When we compare this to the Earths field, we get B / B Earth = (1.7 10 4 T)/(5.5 10 5 T) = 3.1 . 2. We find the current from B = ( /4p)2 I / r ; 5.5 10 5 T = (10 7 T m/A)2 I /(0.25 m), which gives I = 69 A . 3. The two currents in the same direction will be attracted with a force of F = I 1 ( I 2 /2p d ) L = I 1 I 2 L /2p d = (4p 10 7 T m/A)(35 A)(35 A)(45 m)/2p(0.060 m) = 0.18 N attraction . 4. Because the force is attractive, the second current must be in the same direction as the first. We find the magnitude from F / L = I 1 I 2 /2p d 8.8 10 4 N/m = (4p 10 7 T m/A)(22 A) I 2 /2p(0.070 m), which gives I 2 = 14 A upward . 5. The magnetic field will be tangent to the circle with the current at the center. U s e W o r d 6 .0 c o r l a te r to v i e w M a c i n to s h p i c tu r e . 6. The magnetic field produced by the wire must be less than 1.0% of the magnetic field of the Earth. We find the current from B = ( /4p)2 I / r < 0.01 B Earth ; (10 7 T m/A)2 I /(1.00 m) < 0.010(5.5 10 5 T), which gives I < 2.8 A . Chapter 28 p.2 7. We find the direction of the field for each wire from the tangent to the circle around the wire, as shown. For their magnitudes, we have B 1 = ( /4p)2 I / r 1 = (10 7 T m/A)2(25 A)/(0.120 m) = 4.17 10 5 T. B 2 = ( /4p)2 I B / r 2 = (10 7 T m/A)2(25 A)/(0.050 m) = 1.00 10 4 T. We use the property of the triangle to find the angles shown: r 2 2 = r 1 2 + d 2 2 r 1 d cos 1 ; (5.0 cm) 2 = (12.0 cm) 2 + (15.0 cm) 2 2(12.0 cm)(15.0 cm) cos 1 , which gives cos 1 = 0.956, 1 = 17.1; r 1 2 = r 2 2 + d 2 2 r 2 d cos 2 ; (12.0 cm) 2 = (5.0 cm) 2 + (15.0 cm) 2 2(5.0 cm)(15.0 cm) cos 2 , which gives cos 2 = 0.707, 2 = 45.0; From the vector diagram, we have B = B 1 ( cos 1 i + sin 1 j ) + B 2 (cos 2 i + sin 2 j ) = (4.17 10 5 T)( cos 17.1 i + sin 17.1 j ) + (1.00 10 4 T)(cos 45.0 i + sin 45.0 j ) = (3.1 10 5 T) i + (8.3 10 5 T) j ....
View
Full
Document
 Spring '08
 ROSS

Click to edit the document details