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Unformatted text preview: PHYS 241 Ch. 28 Homework Solutions 28.3. I DENTIFY : A moving charge creates a magnetic field. S ET U P : The magnetic field due to a moving charge is 2 sin 4 qv B r = . E XECUTE : Substituting numbers into the above equation gives (a) 7 19 7 2 6 2 sin 4 10 T m/A (1.6 10 C)(3.0 10 m/s)sin30 . 4 4 (2.00 10 m) qv B r  = = B = 6.00 10 8 T, out of the paper, and it is the same at point B . (b) B = (1.00 10 7 T m/A)(1.60 10 19 C)(3.00 10 7 m/s)/(2.00 10 6 m) 2 B = 1.20 10 7 T, out of the page. (c) B = 0 T since sin(180) = 0. E VALUATE : Even at high speeds, these charges produce magnetic fields much less than the Earths magnetic field. 28.9.I DENTIFY : A current segment creates a magnetic field. S ET U P : The law of Biot and Savart gives 2 sin 4 Idl dB r = . E XECUTE : Applying the law of Biot and Savart gives (a) 7 2 4 10 T m/A (10.0 A)(0.00110 m) sin90 4 (0.0500 m) dB = = 4.40 10 7 T, out of the paper. (b) The same as above, except 2 2 (5.00 cm) (14.0 cm) r = + and = arctan(5/14) = 19.65, giving dB = 1.67 10 8 T, out of the page. (c) dB = 0 since = 0. E VALUATE : This is a very small field, but it comes from a very small segment of current. 28.13. I DENTIFY : A current segment creates a magnetic field. S ET U P : The law of Biot and Savart gives 2 sin . 4 Idl dB r = Both fields are into the page, so their magnitudes add. E XECUTE : Applying the Biot and Savart law, where 2 2 1 2 (3.00 cm) (3.00 cm) r = + = 2.121 cm, we have 7 2 4 10 T m/A (28.0 A)(0.00200 m)sin 45.0 2 4 (0.02121 m) dB = = 1.76 10 5 T, into the paper. E VALUATE : Even though the two wire segments are at right angles, the magnetic fields they create are in the same direction. 28.15. I DENTIFY : We can model the lightning bolt and the household current as very long currentcarrying wires. S ET U P : The magnetic field produced by a long wire is 2 I B r = . E XECUTE : Substituting the numerical values gives (a) B = 7 (4 10 T m/A)(20,000 A) 2(5.0 m) = 8 10 4 T (b) 7 (4 10 T m/A)(10 A) 2(0.050 m) B = = 4.0 10 5 T. E VALUATE : The field from the lightning bolt is about 20 times as strong as the field from the household current. 28.21. I DENTIFY : 2 I B r = . The direction of B r is given by the righthand rule in Section 20.7....
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This note was uploaded on 03/18/2010 for the course PHYS 241 taught by Professor Milsom during the Fall '08 term at University of Arizona Tucson.
 Fall '08
 Milsom
 Charge, Magnetism, Work

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