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A charged particle moves in a circle of radius 2 cm in a 1 T magnetic field with speed 4 × 10⁶ m/s. Mass-to-charge ratio m/q:

A5 × 10⁻⁹ kg/C
B5 × 10⁻⁸ kg/C
C2.5 × 10⁻⁸ kg/C
D8 × 10⁻⁹ kg/C
Answer & Solution
Correct answer: A. 5 × 10⁻⁹ kg/C
r = mv/(qB). So m/q = rB/v = (0.02)(1)/(4 × 10⁶) = 5 × 10⁻⁹ kg/C. For comparison, electron m/e ≈ 5.7 × 10⁻¹² kg/C (much smaller, meaning electron mass is light).
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