Home / Topics / Magnetism & Induction
Helical motion
Advanced
Velocity component along B gives a helix.
If velocity has a component along B, the particle follows a helix: circular motion in the perpendicular plane plus uniform drift along B, pitch = v_∥·T.
Charged particles spiral along field lines in the aurora.
Parallel velocity is unaffected.
In more detail
When a charged particle has a velocity component along the magnetic field and a perpendicular component, it moves in a helix. The axial motion is unchanged, while the perpendicular motion circles the field lines.
Real-world uses
- Charged particles spiral along Earth's field lines in the Van Allen belts.
- Plasma confinement in fusion reactors relies on helical motion.
- Charged particles in the solar wind spiral along the Earth's magnetic field lines, following helical paths.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
What is the pitch of the helix?
The distance travelled along the field in one circular period.
Why do particles follow field lines?
Because the perpendicular motion is locked to the field, so the particle spirals around it.
What causes a helix?
The velocity component along the field is unchanged, while the perpendicular component produces the circular motion.
Related topics in Magnetism & Induction
Magnetic force on a moving charge · Circular motion in B field · Cyclotron frequency · Velocity selector · Mass spectrometer · Cyclotron