An electron of mass 9.11 x 10^-31 kg and charge 1.60 x 10^-19 C moves at 6.00 x 10^7 m/s perpendicular to a 0.500 T field. What is the radius of its path?

1. Use r = mv/(qB), which applies to motion perpendicular to a uniform field. 2. The numerator is mv = (9.11 x 10^-31 kg)(6.00 x 10^7 m/s) = 5.47 x 10^-23 kg m/s. 3. The denominator is qB = (1.60 x 10^-19 C)(0.500 T) = 8.00 x 10^-20 C T. 4. Divide: r = (5.47 x 10^-23)/(8.00 x 10^-20) = 6.83 x 10^-4 m. 5. Converting, 6.83 x 10^-4 m is 0.683 mm. 6. Such a tight circle is why a magnet held near an old picture tube badly distorts the image. 7. Answers of 6.83 mm and 68.3 mm come from slipping one or two decimal places in the division. _Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.5 Force on a Moving Charge in a Magnetic Field: Examples and Applications_