4.5 Magnetic Force on Moving Charged Particles

Physics 10 Grade MYP

SlidesVideoInteractive lessonWorksheet 1Worksheet 2

1. Determine the magnitude and direction of the magnetic force acting on the given charge in each of these situations.

Check the answer A

F = 2.6 x 10^-14 N, left
Use the magnetic force equation to find the magnitude of the force; use the Force-finding Right Hand Rule to determine its direction.

Check the answer B

F = 1.6 x 10^-13 N, into page
Use the magnetic force equation to find the magnitude of the force; use the Force-finding Left Hand Rule (since negative charge) to determine its direction.

Check the answer C

F = 8.0 x 10^-14 N, out of page
Use the magnetic force equation to find the magnitude of the force. To find the component of the magnetic field perpendicular velocity, the magnetic field should be multiplied by sine 30^o. Use the Force-finding Left Hand Rule (since negative charge) to determine its direction.

Solar activity can send protons toward the Earth with a large velocity. When such a proton enters the Earth’s magnetic field at an angle less than 90o, a component of the Earth’s field will be perpendicular to the velocity while another component will be parallel to the proton’s velocity. The result is that the proton travels in a helical spiral as shown. Assume a proton is traveling at 240 km/s when it enters the Earth’s 25 µT magnetic field at an angle of 75 with respect to the field.
(a) What magnetic force acts on the proton, and (b) why would this trajectory trace out a helix instead of just a circle?

Check the answer

Answer: F = 9.3 x 10^-19 N; only the component of B perpendicular to velocity exerts a magnetic force and causes circular motion, the parallel component does not exert a force on the proton thus the component of the velocity in direction of field is maintained resulting in a helical path.

Since the proton will spiral around a magnetic field line, and since these magnetic field lines point toward the pole of the Earth, the proton will eventually enter the atmosphere near one of the poles. This is why the Aurora Borealis (northern lights) and Aurora Australis (southern lights) are visible at high latitudes.

Source: https://www.physicsclassroom.com/class/Magnetic-Fields-and-Electromagnetism/Magnetism-and-Moving-Charges/Force-on-a-Moving-Charge

Answers