ch28-p081 - 81. (a) The textbook uses geomagnetic north to...

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81. (a) The textbook uses “geomagnetic north” to refer to Earth’s magnetic pole lying in the northern hemisphere. Thus, the electrons are traveling northward. The vertical component of the magnetic field is downward. The right-hand rule indicates that G G v B × is to the west, but since the electron is negatively charged (and G G G Fq vB ), the magnetic force on it is to the east. We combine F = m e a with F = evB sin φ . Here, B sin represents the downward component of Earth’s field (given in the problem). Thus, a = evB / m e . Now, the electron speed can be found from its kinetic energy. Since Km v = 1 2 2 , v K m e == ×× × 2 2120 10 160 10 911 10 649 10 31 9 31 7 .. . eV J eV kg ms c hc h Therefore, () ( ) 19 7 6 22 14 14 31 1.60 10 C 6.49 10 m s 55.0 10 T 6.27 10 m s 6.3 10 m s .
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