These belts were discovered by James Van Allen while trying to measure the flux of cosmic rays on Earth (high-energy particles that come from outside the solar system) to see whether this was similar to the flux measured on Earth. Trapped particles in magnetic fields are found in the Van Allen radiation belts around Earth, which are part of Earth’s magnetic field. If the reflection happens at both ends, the particle is trapped in a so-called magnetic bottle. This is similar to a wave on a string traveling from a very light, thin string to a hard wall and reflecting backward. The particle may reflect back before entering the stronger magnetic field region.

In particular, suppose a particle travels from a region of strong magnetic field to a region of weaker field, then back to a region of stronger field. While the charged particle travels in a helical path, it may enter a region where the magnetic field is not uniform. The pitch is the horizontal distance between two consecutive circles. The velocity component perpendicular to the magnetic field creates circular motion, whereas the component of the velocity parallel to the field moves the particle along a straight line. The direction of motion is affected but not the speed.įigure 11.8 A charged particle moving with a velocity not in the same direction as the magnetic field. The particle’s kinetic energy and speed thus remain constant. Another way to look at this is that the magnetic force is always perpendicular to velocity, so that it does no work on the charged particle. The particle continues to follow this curved path until it forms a complete circle. Since the magnetic force is perpendicular to the direction of travel, a charged particle follows a curved path in a magnetic field. If the field is in a vacuum, the magnetic field is the dominant factor determining the motion.

The simplest case occurs when a charged particle moves perpendicular to a uniform B-field ( Figure 11.7). What happens if this field is uniform over the motion of the charged particle? What path does the particle follow? In this section, we discuss the circular motion of the charged particle as well as other motion that results from a charged particle entering a magnetic field. Describe how to determine the radius of the circular motion of a charged particle in a magnetic fieldĪ charged particle experiences a force when moving through a magnetic field.Explain how a charged particle in an external magnetic field undergoes circular motion.

By the end of this section, you will be able to:

Particle world speed after